Observation:
In the field of engineering, the inner workings of physical processes are
usually, with effort, observable, so the theory, or simulation, can indeed be
verified. This is not the case in highly abstruse particle physics, where theories may
be developed which depend for their predictive value on unobservable things
happening, such as unobservable ‘intermediate particles’.
notes on
Constructing Quarks: A Sociological
History of Particle Physics
by
Andrew Pickering is a sociologist, philosopher and historian of science
at the
from the
book jacket:
“Widely
regarded as a classic in its field, Constructing Quarks recounts the
history of the post-war conceptual development of elementary-particle physics.
Inviting a reappraisal of the status of scientific knowledge, Andrew Pickering
suggests that scientists are not mere passive observers and reporters of
nature. Rather they are social beings as well as active constructors of natural
phenomena who engage in both experimental and theoretical practice.”
"A prodigious piece of scholarship that I can heartily
recommend."—Michael Riordan, New Scientist
“An admirable history…. Because his account is so detailed and
accurate, and because it makes clear why physicists did what they did, it is
eminently suited to be required reading for all young physicists entering or
contemplating entering the practice of elementary particle physics.” – Hugh N.
Pendleton, Physics Today
Preface
Etched into the history of twentieth-century physics are cycles of
atomism. First came atomic physics, then nuclear
physics and finally elementary particle physics. Each was thought of as a
journey deeper into the fine structure of matter.
Atomic physics was the study of the outer layer of the atom, the
electron cloud. Nuclear physics concentrated ….upon the nucleus, which itself
was regarded as composite…protons and neutrons…Protons, neutron, and electrons
were the original “elementary particles”. ….In the post world war II period, many other particles were discovered
which appeared to be just as elementary as the proton, neutron and electron,
and a new specialty devoted to their study grew up within physics. The new
specialty was known as elementary particle physics, after its subject matter,
or as high energy physics, after it’s primary tool,
the high energy particle accelerator.
In the 1960s and 70s, physicists became increasingly confident that
they had plumed a new stratum of matter: quarks. The aim of this book is to
document and analyse this latest step into the
material world.
p. ix
The view taken here is that the reality of quarks was the upshot of
particle physicist’s practice, and not the reverse.
……The main object of my account is not to explain technical matters per
se, but to explain how knowledge is developed and transformed in the course of
scientific practice. My hope is to give the reader some feeling for what
scientists do and how science develops, not to equip him or her as a particle
physicist.
There remain… sections of the account which may prove difficult for
outsiders to physics…. especially the development of “gauge theory”, which
provided the theoretical framework within which quark physics was eventually
set. In this context, gauge theory was modeled on the highly complex
mathematically sophisticated Quantum Electro Dynamics theory.
p. x
Part I: Introduction, the Prehistory of HEP and its
Material Constraints
Chapter 1:
Introduction
The archetypical scientists account begins in the 1960’s. At that time
particle physicists recognized the four forces: Strong, electromagnetic, weak,
and gravitational. Gravitational effects were considered to be negligible in
the world of elementary particles.
p. 3
Associated with these forces was a classification of elementary
particles. Particles which experienced the strong force were called hadrons,
which include the proton and neutron. Particles which were immune to the strong
force are called leptons, and include the electron.
In 1964 it was proposed that hadrons were made up of more fundamental
particles known as quarks. Although it left many questions unanswered,
the quark model did account for certain experimentally observed regularities of
the hadron mass spectrum and of hadronic decay
processes. Quarks could also explain the phenomena of scaling, which had
recently been discovered in experiments on the interaction of hadrons and
leptons. To the scientist, the quark represented the fundamental entities of a
new layer of matter. Initially the existence of quarks was not regarded as
firmly established, mostly because experimental searches had failed to detect
any particles with the prescribed characteristic of fractional electric charge.
In the early 1970s, it was realized that the weak and electromagnetic
interactions could be seen as the manifestation of a single electroweak force
within the context of gauge theory. This
unification carried with it the prediction of the weak neutral current,
verified in 1973, and of charmed particles, verified in 1974. It was
recognized that a particular gauge theory, known as quantum chromodynamics, or QCD, was a possible theory of the strong
interaction of quarks. it explained scaling, and observed deviations from scaling.
QCD became the accepted theory of the strong interactions. Quarks had not yet
been observed, but both electroweak and QCD theory assumed the validity of the
quark identity. It was noticed that since the unified electroweak and QCD were
both gauge theories,
they could be unified with one another. This unification brought
with it more fascinating predictions which were not immediately verified. Thus,
with quarks came the unification of three of the four forces; strong, EM, and
weak.
It is important to keep in mind that it is impossible to understand the
establishment of the quark theory without understanding the perceived virtues
of gauge theory.
p. 4-5
In the scientist’s account, experiment is seen as the supreme arbiter
of theory. Experimental results determine which theories are accepted and which
are rejected. There are however, two well know objections to this view.
p. 5
First, even if it is accepted that the result of experiment is
unequivocal fact, it is always possible to invent an unlimited set of theories,
each able to explain a given set of facts. Many of these theories may seem
implausible, but plausibility implies a role for scientific judgment. The
plausibility cannot be sen to reside in the data.
While in principle one might imagine a given theory to be in perfect agreement
with the relevant facts, historically this seems never to be the case. There
are always misfits between theoretical predictions and experimental data.
Second, the idea that experiment produces unequivocal fact is deeply
problematic. Experimental results are fallible in two ways: scientists
understanding of any experiment is dependent on theories of how the test
apparatus performs, and if those theories change, then so will the data
produced. More importantly, experimental results necessarily rest upon
incomplete foundations.
For example, much effort goes into minimizing “background”: physical
processes which are uninteresting in themselves, but which can mimic the
phenomenon under investigation. A judgment is required that background effects
cannot explain the reported signal.
[background can also mean processes that do
not jive with the theory of interest]
The scientist’s account is a reference to the judgments made in
developing said theories.
p. 6
Theoretical entities like quarks, and
conceptualizations of natural phenomena like weak neutral current, are
theoretical constructs. However, scientists typically claim these constructs are “real”
and then use these constructs to legitimize
their judgments.
For example, experiments which discovered the
weak neutral current are now represented as closed systems just because the weak
neutral current is seen as real. Conversely, other observation reports which
were once taken to imply the non-existence of the neutral current are now
represented as being erroneous.
Similarly, by interpreting quarks etc as
real, the choice of quark models and gauge theories is made to seem
unproblematic.
In the scientist’s account, do not appear as
active agents; they are represented as passive observers. The facts of natural
reality are revealed thru experiment. The experimenter’s duty is simply to
report what he sees.
p. 7
The view of this book is that scientists make
their own history; they are not the passive mouthpieces of nature. The historian deals with texts, which give him access not to
natural reality, but to the actions of scientists; scientific practice.
My goal is to interpret the historical
development of particle physics, including the pattern of scientific judgments
in research.
The quark-gauge theory view of elementary
particles was a product of the founding and growth of a whole constellation of
experimental and theoretical research traditions.
p. 8
Opportunism in context is the theme that runs
through my historical account. I seek to explain the dynamics of practice in
terms of the contexts within which researchers find themselves, and the
resources they have available for the exploitation of those contexts.
p. 11
the most striking feature of the conceptual
development of HEP is that it proceeded through a process of medelling or analogy. Two key analogies were crucial in the
establishment of the quark/ gauge theory picture: the theorists had to se
hadrons as quark composites, just as they had already learned to see nuclei as
composites of neutrons and protons, and to see atoms as composites of nuclei
and atoms. The gauge theories of quark and lepton interaction were modeled on
QED. The analysis of composite systems is part of the training of all
theoretical physicists. During this period, the methods of QED were part of the
common theoretical culture of HEP.
p. 12
Part of the assessment of any experimental
technique is an assessment of whether it “works”- of whether it contributes to
the production of data which are significant within the frame work of
contemporary practice. And this implies the possibility of the “tuning” of
experimental techniques: their pragmatic adjustment and development according
to their success in displaying the phenomena of interest.
If one assumes that the contents of theories
represent the “reality” of natural phenomena, such tuning is simply development
of a necessary skill of the experimenter. Otherwise tuning is much more
interesting. “Natural phenomena” are then seen to serve a dual purpose: as
theoretical constructs they mediate the symbiosis of theoretical and
experimental practice, and they sustain and legitimate the particular experimental practices
inherent in their own production.
p. 14
Two constellations of symbiotic research traditions characterize
the historic period of development of the quark as reality concept; the old and
new physics.
The “old physics” dominated HEP practice
throughout the 1960s. Experimenters concentrated upon phenomena most commonly
encountered in the lab, and theorists sought to explain the data produced. Among the theories developed
were the early quark model, as well as theories related to the so-called
“bootstrap” conjecture which explicitly disavowed the existence of quarks.
Gauge theory did not figure in any of the dominate theoretical traditions of old physics. By the
end of the 1970s the old physics had been replaced by the new physics, which
was “theory dominated”, and focused not on the most conspicuous phenomena, but
upon very rare processes. The new physics was the physics of quarks and gauge
theory. Once more, theoretical and experimental research traditions reinforced
one another, since the rare phenomena on which experimenters focused their
attention were just those for which gauge theorists could offer a constructive explanation.
Experimental practice in HEP was almost entirely restructured to explore the
rare phenomena at the heart of the new physics. The bootstrap approach largely
disappeared from sight. The evolution from old to new physics involved much
more than conceptual innovation. It was intrinsic to the transformation of the
physicist’s way of interrogating the world thru their experimental practice.
p. 15
Chapter 9 discusses a crucial episode in the history of
the new physics: the discovery of “new
particles” and their theoretical explanation in terms of “charm” The discovery
of the first new particle was announced in November 74, and by mid 76 the charm
explanation had become generally accepted. The existence of quarks was
established, despite the continuing failure of experimenters to observe
isolated quarks in the lab. The new interest in gauge theory provided a context
in which all of the traditions of new physics experiment could flourish,
eventually to the exclusion of all else.
Chapter 10 more new particles are discovered;
detailed experiment on on the weak neutral current culminated in
the “standard model”, the simplest and prototypical electroweak theory.
Chapter 12 shows by the late 1970s, the
phenomenal world of the new physics had been built into both the present and
future of experimental HEP: the world was defined to be one of quarks and
leptons interacting as gauge theorists said they would.
Chapter 13 discusses the synthesis of
electroweak theory with QCD in the so called Grand Unified Theories. We shall
see gauge theory permeating the cosmos in a symbiosis between HEP theorists,
cosmologists, and astrophysicists.
Chapter 2:
Men and Machines.
Gives some statistics on the size and
composition of the HEP community, discusses the general features of HEP
experiment, and outlines the post 1945 development of experimental facilities
in HEP
Chapter 3:
The Old Physics: HEP 1945-1964: Sketches the major traditions in HEP theory and
experiment. From 1945 to 1964, the year in which quarks were
invented. It thus sets the scene for the intervention of quarks into old
physics.
Experimenters explored high cross section
processes, and theorists constructed models of what they reported.
By the early 1960s, old physics hadron beam
experiments had isolated two broad classes of phenomena: At low energies, cross
sections were bumpy- they varied rapidly with beam energy and momentum
transfer, At high energies, cross sections were
“soft”: they varied smoothly with beam energy and decreased very rapidly with
momentum transfer. ???
The bumpy and soft cross sections were
assigned different theoretical meanings. The low energy bumps were interpreted
in terms of production and decay of unstable hadrons. As more bumps were
isolated, the list of hadrons grew. This was the “population explosion” of
elementary particles.
p. 46
S matrix theorizing was the growing list of
hadrons, but in the early 1960s, one strand of S matrix development, ‘Regge theory’ came to dominate the analysis of high energy
soft scattering regime.
In the 1960s and 1970s, the Regge oriented traditions of theory and experiment were a
major component of old physics research, but led nowhere, in that they
contributed little to the new physics of quarks and gauge theory. They
enshrined a world view quite different from the new physics.
3.1: theoretical attempts to account for the
population explosion of particles.
In 1951, Robert Marshak
noted that: In 1932, with the discovery of the neutron, it appeared that the
physical universe could be explained in terms of just three elementary
particles: the proton and neutron in the nucleus, and the electron. these were seen as the basic building materials of the 92
kinds of atoms, the elements.
In 1951, Marshak
counted 15 elementary particles. as years passed, the
list grew longer.
p. 47
Many new hadrons were discovered in the weak
interaction (radioactive decay of particles)
and had lifetimes of 10 -8 to 10 -10 seconds
low energy accelerators showed production of many new
particles and decay in much shorter times; 10-23 seconds, which is
characteristic of strong force interactions.
- 48
by 1964, 90 to 100 subatomic objects (mostly hadrons)
had been discovered.
Dirac’s equation predicts antiparticles for
all particles. The anti- electron or positron was discovered first. The
anti-proton was detected in 1955.
This swelled the list even more.
p. 50
3.2 Conservation laws and quantum
numbers: from spin to the eightfold way.
section 2 outlines the way in which conservation laws were
used to sharpen the distinction between the strong, weak, and EM interactions,
and to classify the hadrons into families. This approach resulted in the
eightfold way classification scheme, a precursor to the quark.
energy-momentum and angular momentum conservation laws were well
rooted in fundamental beliefs about time. According to quantum mechanics, which
prevails here, where orbital angular momentum is not a continuous arbitrary
quantity, but is restricted to certain values, which are integral multiples of
h bar, or quantized.
Physicists also concluded that it made sense
to ascribe angular momentum or ‘spin’ to the elementary particles, in addition
to their orbital angular momentum. This spin is also supposed to be quantized;
integral or half integral multiples of h bar. Each species of particle was
supposed to carry a fixed spin which could not be changed. Half integral spin
particles as the electron and proton were known as fermions; integral spin
particles were called bosons. Since angular momentum, both orbital and spin is
a vector, and since vector is magnitude and direction, the
direction of the particle is also quantized; a spin 2
particle must have either +2 direction, a parallel alignment of the spin with a
chosen axis, or -2 direction, in an anti-parallel alignment.
p. 50-51
Three other conservation laws have been
established: conservation of electric charge, conservation of baryon number,
and conservation of lepton number. conservation of
charge is based on electrodynamics, while baryon and lepton numbers are
analogical extensions of the charge concept to explain empirical regularities.
In the late 1970s the conservation of baryon and lepton number came under
theoretical challenge.
p 52-53
The above conservation laws were regarded by
article physicists as having unlimited and universal scope. a
second class of laws were believed to
apply in some interactions but not in others. Parity was regarded as
absolute, since it was based on the mirror symmetry of physical processes.
Then, in 1956 it was discovered that in certain processes parity conservation
was violated: in the weak interactions. it was
maintained in EM and strong interactions.
A similar comment applies to two other
quantum numbers: ‘strangeness’ and ‘isospin’.
Strangeness can be thought of as a conserved
charge, like electric charge, baryon number, and lepton number. Isospin on the other hand was a vector quantity like spin.
Isospin was an extension of the pre-war ‘charge
independence’ hypothesis, and asserted that hadrons having similar masses, the
same spin, parity and strangeness, but different electric charges, were
identical as far as strong interactions were concerned. Thus in their strong
interaction, neutron and proton appeared to be indistinguishable. This was
formalized by assigning each group of particles an isospin
quantum number: The neutron and proton, now collectively called neucleons, were assigned isospin
˝; the pion, in all of it’s charge states, isospin 1, etc.
Because isospin is
supposed to be a vector, it’s ‘3-component’ I3 was also supposed to
be quantized, and different values of I3 were taken to distinguish
between different members of each isospin family of multiplet.
p. 53-54
Use of conservation laws resulted in the
Eightfold Way classification scheme, of which the quark was the direct
descendent.
SU(3): The Eightfold Way:
Isospin brought economy as well as order by grouping
particles into multiplets. In the 1950s, many theroists attempted to build on this, looking for ways to achieve greater
economy by grouping particles into
larger families. These efforts culminated in 1961 with the ‘Eightfold Way’, or
SU(3) classification of hadrons.
In theoretical physics, there is a direct
connection between conserved quantities and ‘symmetries’ of the underlying
interaction. Thus the strong interaction, which conserves isospin, is said to possess isospin symmetry. This corresponds to the fact that the
choice of axis against which to measure the e-component of isospin
is arbitrary. The strong interaction is said to be invarient
under transformations of I3
Such changes of axes are called ‘symmetry operations’, and form the basis of a branch of math called
‘group theory.’ Different symmetries correspond to different symmetry
groups, and the group associated with arbitrariness of the orientation of the isospin axis is denoted ‘SU(2)’
By making the connection with group theory,
physicists translated the search for a classification wider than isospin into a search for a more comprehensive group
structure; a higher symmetry of the strong interaction with representations
suitable to accommodate particles of different isospin
and strangeness.
In 1961, Gell Mann
and Israeli theorist Yuval Ne’eman proposed the
Eightfold Way, which mathematicians denote as ‘SU(3)’.They were trying to set
up a detailed quantum field theory of the strong interaction. However, the
rapid development of the quark theory
alternative to field theory approach to the strong interaction caused field
theory to fall out of fashion, so SU(3) was divorced from its roots in gauge
theory and used as a classification system.
Agreement that the SU(3)
multiplet classification was found in nature was only
reached after several years debate within the HEP community.
One problem was that early experiments
indicated the lambda and sigma baryons were of opposite parity, which made the SU(3) assignment of
the lambda and sigma to the same family impossible, and favored alternative
classification schemes. In 1963, CERN
experimenters reported that lambda and sigma had the same parity, thus favoring
the SU(3) assignment.
By 1964, the predictive and explanatory
successes of SU(3) were so great that there was little
argument in the HEP community that SU(3) was not appropriate classification
scheme for hadrons.
3.3 Quantum Field Theory
p. 60- 73.
Discusses how HEP theorists attempted to
extend the use of quantum field theory from EM to the weak and strong
interactions. this attempt bore fruit in the early
1970s with the elaboration of gauge theory. This approach was pragmatically
useful for the weak interaction, but failed miserably for the strong
interaction.
The use of conservation laws, symmetry
principles and group theory brought some order into the proliferation of
particles. Besides a classification scheme for hadrons, the eightfold path (Su(3)) also broadly predicted some relationships. In pursuit
of a more detailed dynamical scheme, HEP theorists inherited quantum field
theory: the QM version of classical field theory; ie,
the field theory of EM developed by Maxwell.
QED
The QED Lagrangian p. 61
L(x) = Ψ(x)D Ψ(x) +m Ψ(x)Ψ(x) +(DA(x))2 + e A(x) Ψ(x)Ψ(x)
Where
L(x)
is the Lagrangian density at space-time point
x
Ψ(x) and Ψ(x)
represent the electron and positron fields at point x
A(x) is the electromagnetic field
D is the differential operator so that D Ψ and DA
represent field gradients in space time
e and m
represent the charge and mass of the electron
Each term of this equation can be represented
by a diagram
The first term of the equation corresponds to
a straight line with an arrow, and represents an electron or positron of zero
mass traveling through space. The second term adds mass to the electron or
positron. The third term represents a massless photon, the quantum of the EM
field.
According to the uncertainty principle, the
range over which forces act is limited by the mass of the particle responsible
for them, so only zero mass particles can give rise to forces that travel
macroscopic distances.
If only the first three terms are included in
the Lagrangian, QED is an exactly soluble theory. The dynamics of any collection
of electrons, positrons, and photons can be exactly described. Whatever
particles are present simply propagate through the space; a trivial result. The
fourth term is ‘trilinear’, containing two electrons
and a photon, and represents the fundamental interaction of coupling between
electrons and photons.
In quantum field theory, all forces are
mediated by particle exchange: In QED the force transmitting particle is the
photon.
Exchanged particles in the Feynman diagrams
are not observable thru recording of their tracks, (say using a bubble chamber)
This is because there is a difference between ‘real’
and ‘virtual’ particles. All ‘real’ particles, as the incoming and out going
particles in a Feynman diagram, obey the relationship E2 =p2 +
m2, where E=energy, p=momentum, and m=mass. Exchanged particles do
not obey this relationship, and are said to be ‘virtual’, or ‘off mass-shell’
particles.
In quantum physics, as a result of the
Uncertainty Principle, virtual particles can exist, but only for an
experimentally undetectable length of time.
Pre WWII QED suffered from a theoretical
‘disease’: calculation involved infinite integrals.
The cure was renormalization, which
transformed it into the most powerful and accurate dynamic theory ever
constructed.
It was found that if one puts up with the infinities, but at the end of
the calculation set the apparent infinite mass and charge of the electron equal
to their measured values, one arrives at sensible results. This absorption of
the infinities into
the values of physical constants appeared intuitively suspect,
but it immediately gave a quantitative explanation of the Lamb shift. In
renormalized QED, calculations of EM processes could be pushed to arbitrarily
high orders of approximation and always seemed to come out right.
The renormalization idea had been suggested
before the war by Weisskopf and Kramers,
and was carried through in the late 1940s by Sin-Itiro
Tomonga et al in
Because of the success of the renormalized
QED, HEP theorists in the early 1950s tried to apply the same methodology to
the strong and weak interactions; by quantizing appropriate fields. This did
not bear fruit however.
The quantum field theory of weak interaction,
despite its theoretical shortcomings, offered a coherent organizing principle
for weak interaction research.
The quantum field theory of the strong force
met severe difficulties which were not overcome.
Interest in quantum field theory lost favor
until 1971, when there was an explosion of interest in one particular class of
quantum field theories: gauge theories.
3.4: The S-Matrix: p. 73-78
Reviews the
struggle to salvage something from the wreckage of applying quantum field
theory to the strong interaction. What
emerged was the ‘S matrix’ approach to strong interactions. the
S matrix was founded in quantum field theory but achieved independence, and was
seen, in the ‘bootstrap’ formulation, as
an explicitly anti-field-theory approach.
The entire array probabilities for transition
between all possible initial and final states became known as the S-matrix, or
scattering matrix. The S-matrix approach was taken up in response to the infinities problem in QED, but then dropped with the
adoption of the renormalization solution.
All that can be observed in the strong
interaction is that, in the course of an experiment, an initial collection of
particles with specific quantum numbers and momentum is transformed into a
different final state. One cannot observe nucleons exchange pions.
Thus Fenyman type diagrams, and the corresponding
equations, are not really applicable, so physicists tried applying quantum
field theory and diagrammatic techniques to revisit the S-matrix.
In the 1960’s, work by Geoffrey Chew, Murry Gell Mann et al established
that the S-matrix could be regarded as an ‘analytic’ function of the relevant
variables, based on functions of complex variables. Thus S-matrix theory could
be pursued as an autonomous research project independent of field theory. Many
physicists abandoned the traditional field theory approach to interactions in
favor of exploring this option.
Chew expounded on the explicitly anti-field
theory ‘bootstrap’ philosophy: S-matrix led to an infinite set of coupled
differential equations by which all properties of all hadrons could be
determined. Each particle does not have to be assigned it’s
own quantum field. By truncating the infinite sets of equations, it was
possible to calculate the properties of certain particles.
Italian Tullio Regge used complex energy and momentum variables to study
non-relativistic potential scattering. Chew et al translated Regge’s
ideas to a relativistic S-matrix format. For several types of particles at high
energies and small momentum transfer rates, experimental results supported the
theory.
Regge theory also predicted cross sections that varied smoothly with the square of energy, and
softly with respect to time for the high energy regime above the resonance
region, characteristics
Observed for high
energy hadron scattering. Enormous sets of data were generated at
accelerator labs in the 1960s. But these developments were not related to the
‘new physics’ of the low energy quark concept.
Part II: Constructing Quarks and the Founding of
the New Physics.
Chapter 4:
The Quark Model
In the 1960s, the old physics split into 2
branches: the high energy focused on soft scattering via Regge
models, and at low energies, resonance was studied. In the early 1960s, group
theory was the most popular framework for resonance analysis, but buy the mid
1960’s, quark models took over this role.
4.1 The Genesis of Quarks
In 1964, Murray Gell-Mann proposed that
hadrons were composite, built up from more fundamental entities which themselves manifested the SU(3)
symmetry. Gell-Mann remarked that “a …more elegant scheme can be constructed if
we allow non-integral values for the charges [of four basic components]. This
was the quark model.
George Zweig also proposed a quark model, and
both were elaborated into a distinctive tradition of experimental practice.
Gell-Mann abstracted the name ‘quark’ from
James Joyce’s Finnegan’s Wake.
There were some similarities in their models,
including the use of fractionalized spin. Apart from the 1/3 integral baryon
number, the oddest thing about quarks was their electrical charge: +2/3 for the
u(up) quark, and -1/3 for the d(down) and s(strange)
quarks.
the group theory approach to hadronic
symmetries invited the quark viewpoint.
Gell-Mann’s and Zweig’s quark formulations were
both open to the same empirical objection: although the properties of quarks
were quite distinctive, no such objects had ever been detected.
Gell-Mann notes:
“…A search for stable quarks….at the highest
energy accelerators would help to reassure us of the non-existence of real
quarks.”
Gell-Mann’s statement neatly exemplifies the ambiguities felt
about the status of quarks.
Gell-Mann was particularly prone to
suspicions concerning their status.
Although experimenters continued to look for
quarks throughout the 60’s, 70’s, and 80’s, an acknowledged quark was never
found.
The lack of direct evidence did much to
undermine the credibility of the quark model in it’s early years, but variants
of the quark model
showed growing success in explaining a wide range of hadrionc phenomena. the two
principle variants could be traced back to Gell-Mann’s model and Zweig’s model,
called the Constituent Quark Model.
4.2 The Constituent Quark Model (CQM)
Zweig treated quarks as physical constituents
of hadrons, and thus derived all of the predictions of SU(3).
In 1963 he concluded: “In view of the extremely crude manner in which we have
approached the problem, the results we have obtained seem somewhat miraculous.”
Zweig further wrote: “The reaction of the
theoretical physics community to the ace [quark] model was generally not
benign…”
In the old physics, there were two principle
frameworks for theorizing about the strong interaction:
quantum field theory and S-matrix theory. The CQM was
unacceptable to protagonists of both of these frameworks.
To explain why free quarks were not experimentally
observed, the obvious strategy was that they were too massive, so that the
accelerators of the day (mid 1960’s) had insufficient energy. this implied quarks had masses of at least a vew GeV. However, in combination
with one another, quarks were supposed to make up much lighter hadrons; the 140
MeV pion being the most extreme example.
The idea that the mass of the bound state
should be less than the total mass of the constituents was familiar from
nuclear physics, where the difference was known as the ‘binding energy’ of the
nucleus.
However, that meant that the binding energy of in hadrons
was of the same order as the masses of
the quarks themselves, as opposed to the situation in nuclear physics, where
binding energies were small fractions of the nuclear mass. This implied very
strong binding, which field theorists did not know how to calculate. Field
theorists were left cold by the quark theory.
On the other hand, at the heart of S-matrix
theory, especially in the bootstrap version, there were no fundamental
entities.
Although CQM
failed experimentally, and was disreputable,
it had great heuristic value.
it became a resource for explaining data.
It brought resonance physics down to earth.
The group theory properties of hadrons were reformulated in terms of quarks.
Hadron spectroscopy
The straightforward CQM conjecture was that hadrons
constituted the spectrum of energy levels of composite quark systems.
in the late 1960’s, CQM analysis of hadron mass
spectra grewinto a sophisticated phenomenological
tradition.
It was
concluded that the simplest quark model could reproduce qualitatively the gross
features of the established
hadron resonance spectra.
Besides classifying hadrons and explaining
their mass spectra, CQM enthusiasts analyzed hadron production and decay
mechanisms. Again theorists drew their inspiration from the standard treatment
of composite systems in atomic and nuclear physics. Again the procedure was to load the
properties of hadrons onto their constituent quarks.
The agreement between theory and data for EM
decay of the positively charged delta resonance was one of the first
successes of the application of the CQM to hadronic
couplings.
Models were also constructed of the strong
and weak hadronic couplings, which sometimes led to
paradoxical results.
From the theorist’s perspective, CQM was an
explanatory resource for the interpretation of data.
However, just as the theorist’s practice was
structured by the products of experiment, so the experimenter’s practice was
structured by the products of the theorist’s research. Through the medium of CQM,
theorists and experimenters maintained a mutually supportive symbiotic
relationship.
With the advent of quarks, the symbiosis
between theory and experiment became more intimate. This is because
experimenters began to move on from the low mass classic hadrons to higher mass
hadrons which were more difficult to identify. Zweig observed, even in 1963,
that “particle classification was difficult because many [resonance] peaks
…were spurious”, and that of 26 meson resonances listed in an authoritative
compilation, 19 subsequently disappeared. Kokkedee
noted: “because of the unstable experimental situation many detailed statements
of the model are not guaranteed against the passage of time.”
CQM theory analysis offered experimenters
specific targets to aim at. CQM also made exploration of the low energy
resonance regime interesting: something worth the time, money, and effort. The
experimental data on resonances made available encouraged more work on CQM.
4.3 Quarks and Current Algebra
Above all else, Gell-Mann was a field
theorist. Characteristic of his approach was idea that one should take
traditional perturbative field theory modeled on QED
and extract whatever might be useful. He sought that is, to use field theory
heuristically to arrive at new formulations. For example his exploration of the
analytic properties of the S-matrix using perturbative
quantum field theory as a guide. The
current algebra approach is another example. Gell-Mann proposed thst in discussing the weak and EM properties of hadrons,
one should forget about traditional QFT, and regard weak and EM currents as the
primary variables. Hadronic currents were postulated
to obey an SU(3)XSU(3) algebra; hence, “current
algebra” is the term for the work which grew up around his proposal. Current
algebra had an immediate phenomenological applications, since currents were
directly related to observable quantities via standard theoretical techniques.
Although Gell-Mann was clearly working within the field theory idiom, the
algebraic structure relating the currents could not be said to derive from
field theory [so what?] Gell-Mann then observed that the relationships between
the weak currents required to explain experimental data were the same as those
between currents constructed
from free quark fields.
4.4 The Reality of Quarks
Although the CQM provided the best available
tools for the study of low energy strong interaction hadronic
resonances, and current-algebra tradition, which frequently called on quark
concepts was at the cutting edge of research into the weak and EM interaction
of hadrons, throughout the 1960s and 1970s, papers, books and reviews were
filled with caveats concerning quark reality.
British quark modeler Frank Close noted in
1978: “there was much argument …[in the 1960s] as to
whether these quarks were real physical entities or just an artifact that was a
useful Mnemonic aid when calculating with unitary symmetry groups.
George Zweig: ‘the quark model gives us an
excellent description of half the world’: In low energy resonance physics,
hadrons appeared to be quark composites, but in high –energy soft scattering,
they were Regge poles. Hadronic structure differed according to who was analyzing it.
For a
while, quark and Regge modelers co-existed
peacefully, with no one group claiming priority.
The image of hadrons and quarks differed
between the CQM and the current-algebra tradition.
In CQM, theorists used ‘constituent quarks’
to represent hadrons of definite spin which were observed in experiment.
In Current algebra, theorists built
hadrons out of ‘current quarks’, but these were not the individual hadrons
observed in experiment: they were combinations of different hadrons having
different spin, so in some sense the two types were different objects.
Gell-Mann’s view was that the two forms were
somehow related. Lots of work was done to explore the possible relationship
which led to new insights into the classification and properties of hadrons,
but a full understanding was never achieved. The CQM and current-algebra
traditions remained separate, with their own quark concept.
The theoretical wing of the old physics in
the mid 1960s consisted of Regge theory, in which I
was possible to discuss hadron dynamics without reference to qurks, CQM theory, in which QM.
Chapter 5:
Scaling, Hard Scattering and the Quark-Parton
model. (p. 125-158)
5.1 ScaIing at Stanford Linear
n elastic electron-proton
scattering, an incoming electron exchanges a photon with the target proton. The
beam and target particles retain their identity and no new particles are
produced. The process is primarily EM
and assumed to be understood by QED. Electrons are assumed to be structureless
points, while protons are assumed to occupy a finite volume and structure.
Measurements of electron-proton scattering could therefore be considered to
probe the proton’s structure, and in particular explore the spatial
distribution of the proton’s electric charge.
Electrons frequently scattered off one another at large angles, while electron proton
scattering was normally at small angles. Ie,
electrons seemed “hard” while protons seemed “soft” and served to diffract
incoming electrons, at both low and high energies.
In optical physics, the dimensions of a diffracting object was a
function of the diffraction pattern it produced, and based on this pattern the
proton had a diameter of about 10-13 cm.
To study inelastic electron-proton scattering, in 1997 a Stanford and
MIT study group fired the electron beam at a proton target (liquid hydrogen) , and counted how many ‘system’ particles emerged with
given energies and at given angles to the beam axis. (inclusive
measurements)
For the small angle scattering of low energy electrons, experimenters
found what they expected: the most important process was resonance production:
resonance peaks are typically seen at three different values of mass of the
produced hadronic systems.
However, for larger scattering angles
and higher beam energies, they found that although the resonance peaks
disappeared, the measured cross sections remained “large”, suggesting that the
proton contained hard point like scattering centers, which resembled electrons.
however, the experimenters did not know what these
results meant.
In the 1960s, James Bjorken concluded, based
on a variety of non-rigorous perspectives, that the inelastic cross sections for larger
scattering angles and higher beam energies would have to be large. He also
inferred that in the limit where q2 (momentum transfer) and v (a
measure of the energy lost by the electron in collision) become large, but with
a fixed ratio (the Bjorken limit) W1 and vW2 would
not depend on upon v and q2 independently, but rather, they
would only be functions of the ratio v/q2,
where W1 and W2 are ‘structure functions’. Drell and Walecka had previously
determined that the inelastic electron-proton scattering cross section could be
expressed in terms of these ‘structure functions’. Curves of W1 and
vW2 became known as scaling curves. To experimenters and many
theorists these
relationships were esoteric to the point of
incomprehensibility. A few years later,
in 199, the response of the HEP community was more enthusiastic.
5.2 The
Parton Model
In 1968, Feynman formulated the ‘parton model’
explanation of scaling on a visit to SLAC.
Feynman Diagram for:
non interacting electron;
a single point in space
emission and reabsorption of a
photon by an electron;
(an EM reaction; a first order approximation to
QED)
photon conversion into an
electron-positron pair
(an EM reaction; a second order approximation
to QED)
At higher orders of approximation to QED, ever more photons and
electron-positron pairs appear.
when the higher terms are
taken into account, the electron appears not as a single point, but as an
extended cloud of electrons, positrons, and photons, which together carry the
quantum numbers, energy and momentum of the physically observed electron.
There are an indefinite number of possible
orders.
However, because of the weakness of the EM interaction, the higher order
QED corrections
(successive perturbations) are smaller .
This means that for most practical purposes, electrons may be regarded
as point-like particles.
in an
analogous fashion for a proton, or any hadron:
For a proton, the strong as well as EM interactions need to be
considered
Feynman Diagram for:
non interacting proton;
emission and reabsorption of a
pion by a proton;
pion conversion into a
nucleon-antinucleon pair
up to this point the analog between
electrons and protons is valid, but the strong interaction is large, and
successive terms in the perturbation are larger.
From the field theory perspective, since there are
also an indefinite number of orders, the proton has a complex structure.
The field theory perspective was welcome because the finite size
explained why the elastic electron –proton scattering cross sections decreased
diffractively with momentum transfer, but unwelcome because having to deal with
a proton as a particle cloud rather than as a single point
made the task too difficult.
Fenyman took the view that the
swarms of particles within hadrons; whether quarks and anti quarks, or nucleons
and anti nucleons, were of indefinite quantum numbers, and called them ‘partons’. He reasoned that for high energy collisions
between protons, each
proton will see the other as relativistically
contracted into a flat disc, and have very short time to react to one another,
so the interaction of partons within an individual
disc would be negligible, so the partons of each
proton would act as a unit.
Scaling emerged as an exact prediction of the parton
model, which became central to the practice of increasing numbers of HEP
scientists.
Popularity of the parton model could not lie
in it’s success at explaining scaling per se; Bjorken and others had explanations that worked just as
well. Neither can it’s popularity be ascribed to the
validity of it’s predictions. Straight forward extensions often led to
conflicts with observation.
Bjorken’s
calculations led first to the prediction of scaling, then to the observation
that scaling corresponded to point like scattering. Feynman started with point partons,
and predicted scaling. The parton model provided an
analog between the new phenomena of scaling and the idea of composite systems
known and comfortable to physicists.
A well known analogy existed between
Feynman’s parton model effectively bracketed
off the still intransigent strong interactions: all the effects of the strong
interactions were contained in the parton momentum
distributions. Knowing or guessing these distributions, calculations of
electron hadron scattering could be reduced to simple first order QED
calculations. The only difference was that the quantum numbers for the partons were unknown. One popular speculation held that the
partons had the same quantum numbers as quarks, and
this quickly came to be accepted.
The parton approach and the constituent quark
model approach were both composite model approaches. However, to explain
scaling, one had to treat partons as free particles,
while the existence of strong interactions binding quarks into hadrons was at
the heart of the CQM. Further, the CQM had its greatest success in low energy
resonance physics, while the parton model was applied
to the newly discovered high energy high momentum transfer scaling regime.
Partons became quickly identified
with quarks for most particle physicists. This means that if the quark remains
a free particle, it should shoot out of the proton and appear as debris of the
collision. However, quarks have never been observed. It was then assumed that
after it’s initial hard interaction, it subsequently undergoes a series of
soft, low momentum strong interactions with it’s fellow partons.
Such assumptions were unavoidable and theoretically unjustifiable.
5.3 Partons, Quarks, and Electron Scattering
Although the parton model was Feynman’s
creation, it was taken up at SLAC. Field theorists, following Feyneman’ lead, began to practice
their art in the realm of the strong interaction, leading to the formulation of
QCD.
The parton model was use phenomenologically
to give structure and coherence to the experimental program at SLAC. The
identification of quarks with partons was grounded in
the phenomenological analysis of
scaling. Besides their scaling property, the magnitudes and shapes of the
structure functions could be measured. Attempts were made to try to back out
attributes of the parton based on data fitting of the
structure functions.
More information on parton quantum numbers was
sought from an exam of the individual structure functions. According to QED,
each species of parton was supposed to contribute to
the total structure function of the proton in proportion to the square of its
electric charge. Hence, by making assumptions about the parton
composition of the proton, estimates of the proton structure functions could be
made.
Such estimates proved to be in error, even when, to conform with parton requirements, a
quark-antiquark cloud (indefinite number of quark-antiquarks) was adopted.
Still, theorists elaborated the quark-parton
model, and introduced ‘glue’ into the make up of the proton. The argument was
that IF the nucleon was simply a
composite of non-interacting quarks, THEN
it would fall apart unless some ‘glue’ held it together; thus the ‘gluon’ was
hypothesized, which was incorporated into the structure function model; the
gluons were assumed to be electrically neutral, but would have momentum. Like
the quark-antiquark cloud, the gluon component was another free parameter which
a critic might argue could be tweaked to help reconcile the expected properties of
quarks with experimental findings. Many physicists were skeptical of such an
approach. Something more was required to persuade the HEP community to
Accept the parton model, and the equation of partons to quarks. Neutrino scattering experiments provided
that something more.
5.4 Neutrino
Physics.
Theorists quickly extended the parton model to
the neutrino-antineutrino scattering from nucleons. The weak neutrino-parton interaction could be seen as very similar to the EM
electron-parton interaction. In this case neutrino-parton scattering is said to be mediated by exchange of a
W-particle, as the EM electron-parton interaction is
mediated by a photon. Although HEP neutrino physics was bolstered by
Brookhaven AGS and CERN studies in the early 1960s, no W-particles
(intermediate vector bosons; IVBS) were found in the “first round experiments”
Nevertheless, by 1973 there was general agreement that the parton model, with quarks identified as partons,
was a promising approach to the analysis of both electron and neutrino
scattering. Theoretical arguments could still be brought against the quark-parton idea- why were quarks not produced as free particles
in deep inelastic scattering; what was the mysterious gluon component?? But Feyneman remarked that “there is a great deal of evidence
for, and no experimental evidence against, the idea that hadrons consist of
quarks….. let us assume it is true.” Many physicists
did just that.
5.5
Lepton-Pair Production, Electron-Positron Annihilation and Hadronic
Hard Scattering
The quark-parton model was extended to certain
other processes; notably Lepton-Pair Production, Electron-Positron Annihilation
and Hadronic Hard Scattering.
Lepton pair production could be visualized as a quark and antiquark
being produced by the collision of protons made of partons;
the [never observed] quark-antiquark combine to emit an electron which then
produces a lepton pair. Again, another Brookhaven AGS study conducted in 1968
bolstered lepton physics, yet again failed to find any W-particles.
Electron-positron annihilation was visualized as electron and positron
annihilate to form a photon which materializes as a [never observed]
quark-antiquark pair which would then somehow rearrange themselves into hadrons.
Collider data from Frascati
With the advent of the proton-proton collider, the Intersecting Storage
Rings (ISR) at CERN, several groups of experimenters again set out to hunt for
IVBS (the W particle), but were unsuccessful. It was not clear how the parton model should be extended to hadronic
hard scattering, yet an exciting new phenomenon was discovered: the
excess production of high transverse momentum (pt) hadrons.
Chapter 6:
Gauge
Theory, Electroweak Unification and the Weak Neutral Current
New physics theory incorporated two sets of resources: quark models of
hadron structure, and a class of quantum field theories known as gauge
theories.
Gauge theory was invented in 1954 by CN Yang and RL Mills. Modeled
closely on QED, gauge theory initially enjoyed some popularity, and was one of
the resources leading to the development of the Eightfold Way symmetry
classification of hadrons. Soon however, quantum field theory
went into decline, and with it gauge theory.
Electroweak unification amounted to the representation of the EM and
weak interactions in terms of a single gauge theory. There was a drawn out
struggle to show that gauge theory, like QED, renormalizable.
Once this was established, gauge theory became a major theoretical industry,
and one important branch of that industry was the construction of unified gauge
theory models of the electroweak interaction. Such models predicted the
existence of a new phenomena in neutrino experiments,
and one such phenomena, the weak neutral current, was reported from the Gargamelle neutrino experiment at CERN in 1973 and
confirmed at Fermilab the following year. Thus the
gauge theory proposed by Yang and Mills in 1954 finally made contact with
experiment 19 years later.
6.1
Yang-Mills Gauge Theory p 160-165
Yang became interested in the strong interactions, and studied quantum
field theory, specifically referring to articles published by Wolfgang Pauli
which emphasized the importance of gauge
invariance in QED.
There is a certain arbitrariness in classical
EM as represented by Maxwell’s equations, which are formulated in terms of
electric and magnetic fields, which can be expressed as derivatives of vector
and scalar potentials. The arbitrariness arises because
one can modify the potentials in space and time, without changing the
associated fields. Thus, classical EM is said to exhibit gauge
invariance. This invariance carries over into the quantized version of
electromagnetism, QED.
The QED Lagrangian p. 61:
L(x) = Ψ(x)D Ψ(x) +m Ψ(x)Ψ(x) +(DA(x))2 + e A(x) Ψ(x)Ψ(x)
Where
L(x) is the Lagrangian density at space-time
point x
Ψ(x) and Ψ(x) represent the electron and positron
fields at point x
A(x) is the electromagnetic field
D is the differential operator so that D Ψ and DA represent field
gradients in space time
e and m represent the charge and
mass of the electron
is invariant under the
following transformations:
Ψ(x)
= Ψ(x) e iθ(x)
A(x) =
A(x) + Dθ(x)
ie, L(x) remains unchanged with the substitution
of the above quantities into the above equation, which means L(x) remains
unchanged under these gauge transformations.
The existence of photons interacting with electrons is a formal
requirement of a gauge invariant theory of EM, or QED.
Yang and Mills tried to model a theory of the strong interaction based
on QED. They then attempted to follow a standard route to get physical predictions from it by
Feynman rules and diagrams, but failed.
The mathematical complexities were such that Yang later recalled ‘We completely
bogged down…’ but nevertheless published
a paper on the topic. This was the starting point of the gauge theory tradition
in HEP.
Theorists then began developing different versions of this gauge theory and tried to bring
the predictions of gauge theory to the data.
A major obstacle was the ‘zero mass problem’. The Yang Mills Lagrangian, which was developed from the QED (EM
interaction) Lagrangian for the strong interaction, as the QED
version, contains no mass terms. So, according to the uncertainty principle,
any forces mediated by the exchange of the massless W particles of Yang Mills
(gauge theory) would be long range. However, the strong and weak interactions
are short range. Gauge theory thus appeared inapplicable to these short range interactions.
This situation met with various responses; for example, agreeing that
gauge theory indeed had nothing to do with elementary particles; arguing that
gauge theory was incompletely understood, and that with more study the W
particles may be found to have mass. Another response was to just give the
gauge particles mass by inserting ‘by hand’ an appropriate mass term in the
Yang Mills Lagrangian. This disturbed the analogy
with QED and destroyed the gauge invariance of the theory, but did make gauge
theory a candidate for description of the short range weak and strong
interactions.
Later it was proposed that in addition to the triplet of W vector
particles there existed two other vector meson particles, which were identified
experimentally shortly afterwards. But this predictive success was not unique
to gauge theory: the disregarded S-matrix bootstrap theory could also produce
vector mesons.
Gauge theory was a central concern of Gell-Mann in the construction of
the Eightfold Way of Hadrons. However, in Gauge theory, the vector mesons were
fundamental entities; the quanta of the gauge field, but as the Eightfold Way
prospered and transformed into the quark model, vector mesons came to be
regarded as ordinary hadrons; quark-antiquark composites just like all the
other mesons. This signaled the downfall
of gauge theory as an approach to the strong interaction.
In the 1970s another set of candidate gauge particles emerged in the
study of the strong interactions; the gluons.
6.2
Electroweak Unification and Spontaneous Symmetry Breaking
The Fermi and ‘V minus A’ theories of the weak interaction envisioned
the weak interaction as taking place between two currents at a point in space. Both were subject to
two theoretical objections: they were non-renormalisable,
and violated the physically reasonable ‘unitary limit’ at ultra high energies.
Theorists conjectured these objections might be overcome if the weak force was
represented as mediated
by particle exchange. To reproduce the space-time structure of the
‘V minus A’ theory, the exchanged particles; the carrier of the weak force,
should be intermediate vector bosons (IVBS). two such
particles of opposite polarity were sufficient to recover the ‘V minus A’
phenomenology. Another conjecture was that the vector gauge particles of the
Yang Mills Gauge theory, in a suitable formulation, could be identified with
the IVBS of the weak interaction.
Further, since gauge theory was closely related to QED, there was speculation that in some sense the weak and EM interactions
were manifestations of a single underlying ‘electorweak’
force.
Sheldon Glashow, and Abdus Salam working with
JC Ward, developed unified electro weak gauge theories in the 1960s. In both
theories, the intermediate particles were given a suitable mass ‘by hand’,
making them non-renormalisable. In 1967, the
‘Weinberg-Salam’ model appeared. It’s distinctive
feature was that the IVBS acquired masses by ‘slight of hand’, with no explicit
IVBS mass terms appearing in the Lagrangian.
Spontaneous
Symmetry Breaking and the Higgs Mechanism
In the perturbative approach to quantum field
theory of the 1940s and 1950s, a direct correspondence was assumed between the
terms in the Lagrangian and physically observable
particles. In QED for example, the propagation of real massed electrons and
real massless photons (remembering of course that the electrons and photon
particles are NOT directly observable) However, in the 1960s, this assumption
was challenged. The impetus for the challenge came from solid state physics, where
all sorts of quasi particles were used to explain experimental observation, and
these particles did not map onto the fundamental fields of a field theory
approach. There was a considerable conceptual gulf between solid state and HEP
physics, and how the ideas of one might be transformed to apply to the other
was not obvious.
One of the first to try transforming the ideas of one to the other was Yoichiro Nambu, who had worked in
both superconductivity and HEP. In 1961 two papers were published with the
title
A
Dynamical Model of Elementary Particles Based Upon an Analogy with
Superconductivity.
These papers introduced the new concept of “Spontaneous Symmetry
breaking” (SSB)
The thrust of SSB was that it is possible for a field theory Lagrangian to possess a symmetry which is not manifest in
the physical system which the theory describes.
The concept of SSB may be explained by analogy with ferromagnetic
material. Magnetism is produced by the mutual interaction of atomic spins; each
spin behaving like a little magnet. The Lagrangian
for a system of interacting spins shows no particular direction in space; it is
rotationally invariant. Yet in the actual physical system, the spins of a ferromagnet line up to produce macroscopic magnetism. So
physical ferromagnetism is an example of SSB, and superconductivity can be explained
likewise.
The SU(3) Eightfold Way symmetry of the (strong
interaction) hadrons seemed to be only approximate, and it was conjectured that
SSB had something to do with that.
Jeffery Goldstone however, concluded that SSB must be accompanied by the
appearance of massless spin zero particles- Goldstone Bosons. This result
became the object of increasingly forma proofs, which implied that a theory of
SSB of the SU(3) symmetry was out of the question;
since the strong interaction was of short range, massless particles were out of
the question.
Theorists then proposed that the pion, which was much lighter than all
other hadrons, could be regarded as a “pseud-Goldstone boson”
The original inspiration for Goldstone’s particle physics SSB was
superconductivity, but there are no massless particles in superconductivity;
even the photon acquires an effective non-zero mass. A controversy arose in the
HEP community over whether phenomena seen in superconductivity persisted in
relativistic situations. The upshot of the resolution was that there existed a
class of relativistic field theories where the Goldstone theorem could be evaded:
those theories having a local gauge symmetry: QED and
Yang Mills theories.
Peter Higgs exhibited and analyzed the evasion of the Goldstone theorem,
introducing a model, and a ‘Higg’s mechanism’
.
The model consisted of the standard QED Lagrangian
augmented by a pair of scalar (spin zero) fields which were coupled to the
photon and to one another in such a way as to preserve gauge invariance of EM.
Higgs found that if he gave the scalar fields a negative mass term in the Lagrangian, the physical result of the model would be a
massive photon and one massive scalar particle; a ‘Higgs particle’. The
physical interpretation of the Higgs mechanism was that massless photons can
only be polarized in two directions, while massive vector particles have three
possible axes of polarization. The massless photon can be seen as ‘eating up’
one of the scalar particles.
In 1967, Steven Weinberg and Abdus Salam,
working independently, adapted the unified electroweak gauge theory model
proposed by Glashow, Salam and Ward, and replaced the IVB mass terms previously
generated by hand with masses generated by the Higgs mechanism. The result was
a unified electro weak Weinberg Salam model.
Weinberg’s model was essentially that proposed 6 years earlier by
Glashow, except that certain mass relationships between the IVBS were
determined in Weinberg’s model in terms of a single free patrameter,
the ‘Weinberg angle’.
As early as 1962,
Salam had discussed the possibility of mass generation in gauge
theories by means of SSB.
The Weinberg Salam model was initially ignored: Field theory was in
decline, the model was focused on the weak interaction of leptons, and except
as an exercise in virtuosity, the model had no special
appeal. When applied to hadrons, it led
to predictions in conflict with experimental results.
When theorists found that Weinberg and Salam’s theory of weak
interactions was renormalizable, interest in unified gauge theories
exploded.
6.3 The Renormalisation of Gauge Theory
In 1971, through the efforts of Dutch HEP theorist Martin Veltman and his student, ‘t Hooft, it was demonstrated that electroweak gauge theories
were renormalizable. Proof of the renormalizability
of QED had been difficult, and for gauge theory had been formidable. Any
attempt to go into this proof would be futile. The author focuses instead on the
activities of Veltman and his student, Gerard ‘t Hooft.
In 1968, Veltman investigated the renormalizability. He decided to look at massive Yang Mills
theory, the version in which masses were inserted by hand, since pure gauge-invarient theory with massless gauge particles was
unrealistic. He encountered the first of the infinite integrals, which were
commonly assumed to make the theory non- renormalizable.
But he quickly convinced himself that many, if not all of the infinite
integrals were of opposite sign and cancelled. Their contribution to the
physical process was zero. He concluded that it was conceivable that massive
gauge theory might be renormalizable, contrary to the conventional wisdom.
Looking for a more elegant approach, he reformulated the Feynman rules of the
theory in such a way that the cancellation between infinite integrals was
manifest by inspection. But to do this he had to include in the set of diagrams
the interaction of a ‘ghost’ particle, which appeared only in closed loops and
not as a physical incoming or outgoing particle. He found that work done of
massless gauge theories included ‘ghosts’, and resulted in a set of Feynman
diagrams similar to his. Further, analysis of the massless theory had been
carried through arbitrarily many loops, and the theory was shown to be renormalizable. Veltman then
investigated a two loop diagram in the massive theory, but established a
puzzling result, which forced him to conclude that at the two loop level,
massive gauge theory was non-renormalizable. He then
began to entertain the possibility that perhaps appropriate scalar fields could
be introduced into the massive Yang Mills Lagrangian
in such a way as to cancel the two loop divergences.
In 1971, ‘t Hooft
published the first detailed argument that massless gauge theory was renormalizable. Veltman told ‘t Hooft that what was needed
was a realistic theory involving massive vector particles; ie
a massive Yang Mills Lagrangian. ‘t
Hooft’s resulting paper ushered in the ‘new physics’,
though that was not apparent at the time. By adding multilets
of scalar particles into the massless YM Lagrangian,
t’ Hooft in effect re-invented the Higg’s mechanism.
Like pure massless theory, but unlike the massie-b-hand
theories explored by Veltman, t’ Hooft
found that gauge theories in which vectors acquired mass by SSB were renormalizable.
However, Veltman’s tools for analyzing gauge
theory were unfamiliar to many physicists, and the
path integral formalism t’ Hooft inherited from him
was widely regarded as being dubious.
t’ Hooft’s
work needed support. Enter the influential
In a very short time period; late 1971 to early 19722, Yang Mills gauge
theory ceased to be regarded as a mathematical curiosity, and seen instead as a
respectable, even profound field theory.
The next question was, did it work? Veltman always had the weak interaction in mind, and the spontaneously
broken theories investigated by ‘t Hooft
and Lee had precisely the from of the Weinberg-Salam unified electroweak model.
How did the renormalized model agree with data?
6.4
Electroweak Models and the Discovery of the Weak Neutral Current
Weinberg had based his model on the simplest choice of gauge group, the
minimal set of scalar particles, and made a choice concerning the multiplet structure of the known leptons. Alternative
models could easily be constructed by varying the initial choices. Constructing
such models at random was not very interesting. The question arose as to why
one particular model should be preferred over another. Could support be found
for any of these new unified models from the data.
The massive IVBS predicted as intermediaries for the weak force had
never been detected, but this was not surprising, since the energy required to
produce such massive particles was unattainable.
Standard V-A theory predicted the weak interactions were mediated by two
electrically charged IVBS, W+ and W-, leading to a charged current situation.
However, the WS model also predicted a massive Z0 particle. The Z0, being
electrically neutral, could mediate ‘neutral current’ processes: weak
interactions in which no change in charge occurred between incoming and
outgoing particles. No data was available as long as the WS model was applied
only to leptons.
In the 1970s, the obvious extension to
hadrons was through quarks. The idea that quarks were vehicles of the hadrionic weak and EM currents was central to current
algebra and CQM traditions, and furthermore, the success of the parton model suggestd that quarks
were point like entities- just like the leptons of the Weinberg Salam model.
However, when theorists incorporated hadrons
into the WS model by assuming the IVBS coupled to quarks in the same way they
coupled to leptons, a conflict with accepted data arose.
The GIM Mechanism and Alternative Models…
6.5 Neutral Currents and Neutron Background
In the archetypical ‘scientist’s account’,
the experimental discovery of the weak neutral current would be an independent
verification of unified electroweak gauge theory. The author however, suggests
this view cannot withstand historical scrutiny based on two observations:
1) reports of
observation of neutral currents from CERN in the 1960s and 1970s were based on
questionable interpretive procedures. Physicists had to choose whether to
accept or reject these procedures and the reports which went with them, and 2) the communal
decision to accept one set of interpretative procedures in the 1960s, and
another set in the 1970s can best be understood in terms of the symbiosis of
theoretical and experimental practice.
The Symbiosis of Theory and Experiment
The history of the neutral current can be divided into two periods of
stability: in the period from the 1960s to 1971 communal agreement
was that the neutral current did not exist. In the period from 1974 onwards, communal agreement
was that the neutral current did not exist. In each period, experimental
practice generated both justification and subject matter for theoretical practice, and vice
versa.
Experimenters reappraised their interpretive procedure in the early
1970s, and succeeded in finding a new set of practices which made the neutral
current manifest. The new procedures remained pragmatic, and were, in
principle, as open to question as the earlier ones, but like the earlier ones,
the new procedures
were then sustained within a symbiosis of theory and experiment.
Acceptance of the neutral current theory left theorists with a problem.
The GIM mechanism created a significant difference between kaon
decay and neutrino scattering experiments, making it possible that neutral
currents should be observed in the later but not the former. it
did this at the expense of the introduction of new and unobserved charmed
particles.
Chapter 7
Quantum Chromodynamics: A Gauge Theory of the Strong interaction
7.1 From
Scale Invariance to Asymptotic Freedom
Field theorists work culminated in the 1973 discovery that gauge theory was ‘asymptotically free’. The implication of
this discovery appeared to be that gauge theories were the only field theories
capable of underwriting the phenomenological success of the quark-parton model, and hence of giving an explanation of
scaling.
7.2
Quantum Chromodynamics (QCD)
Although the construction of QCD was to have far reaching implications
for HEP development, in 1973-1974 it remained of interest only to field
theorists.
Once asymptotic freedom was discovered, given that gauge theory reproduced the
predictions of the quark-parton model, it was
conjectured that quark fields were the fundamental fields of the strong
interaction. In gauge theory, the quark fields would interact via the exchange
of gauge vector fields, conjectured to be gluons.
For a gauge group it was tempting to conjecture that the theory should
be invariant under local transformations of the SU(3),
Eightfold Way symmetry group, but the consequences of this choice intermixed the
weak and strong interactions, spelling disaster. To get around this, theorists
argued that quarks carried not one but two sets of quantum numbers, which they
referred to as ‘flavors’ and ‘colors’.
In the mid 1960s, color appeared to have little phenomenological
relevance, and was generally regarded as being a theorist’s trick.
However, in the late 1960s, two sources of empirical support for color
appeared.
7.3 The
Failings of QCD
A central obstacle to further development was the lack of any
constructive approach to the problem of quark confinement. Although QCD
predicted deviations from the exact scaling behavior predicted by the parton model, there was no prospect of immediate
experimental investigation of these deviations, and thus little incentive for
theorists to explore them in depth.
QCD theorists were unable to appropriate the Higgs trick to give masses
to the gluons, as had been done for the unified electroweak models.
Because quarks and gluons appeared in the QCD Laganagian,
a naive reading of the physical particle states led directly to a world
populated by real colored quarks and gluons. Furthermore,
the gauge theories shown to be
asymptotically free were pure gauge
theories, in which the gauge vector particles , the gluons, were massless. Many
years of experimental effort had been extended without success in searches for
quarks (colored or not) and strongly interacting massless gluons.
By 1974, gauge theorists were arguing that imputing to QCD an unrealistic
particle spectrum (ie, assuming that quarks and massless gluons would
exist as real particles) was not legitimate, because it was based on perturbative activity, so made no sense. However, gauge
theorists had talked themselves into a corner: perturbative
arguments could not be applied to the long distance properties of QCD, but pertubative arguments were what field theorists were
acquainted with. They had no math tools to use, so used the verbal doctrine of
confinement instead. Gauge theorists admitted that once the effective coupling
constant approached unity they could no longer compute its evolution as one
considered larger and larger distance scales. Insead
they simply asserted that the coupling continued to grow, or at least remain
large. They also stated their faith that because of this, color was “confined”:
all colored particles, quarks and gluons, would forever be confined to hadrons.
Although several variations on the QCD theme arose; the 1/N expansion, ‘monopoles’, ‘instantons’,
and ‘lattice gauge’ theories, none provided an agreed upon solution to the
confinement problem.
In 1973, it was unclear to theorists what to do next for QCD, except to
point out that it was the only field theory of strong interactions not
obviously wrong.
Chapter 8
HEP in
1974; The State of
1974 was the year of the “November Revolution”. In November 1974, the
discovery of the first of a series of highly unusual elementary particles was
announced. within 5 years, the “old
physics’ was eclipsed by the field
theory oriented “new physics” traditions of current algebra, the parton model, unified electro-weak, and QCD.
By 1974 it was possible to represent each of the fundamental forces of
HEP; the strong, EM and weak interactions, in terms of gauge theory.
Gauge theories are a class of quantum field theories, wherein all forces
are ascribed to particle exchange. There
is no place for “action at a distance”. the exchanged
particles are massless. unless the theory is suitably
modified.
The gauge theory of electromagnetism is QED.
The “new orthodoxy” which SLAC theorist James Bjorken
references in his 1979 presentation
was the belief that the
Weinberg-Salam electroweak model and QCD could explain all the phenomena of
elementary particle physics. The smallest feasible gauge-theory-structure
(SU(2) X U(1) for electroweak, SU(3) for
strong, and Su(5) for grand unification accounts very well for the
observations. Bjorken
continued: While searches for what is predicted by the orthodoxy will proceed,
searches for phenomena outside the orthodoxy will suffer… marginally significant
data supporting orthodoxy will tend to be presented to and accepted by the
community, while data of comparable or even superior quality which disagrees
with orthodoxy will tend to be suppressed…and even if presented, not taken
seriously.
The key phase in the communal establishment of the new physics centered
on the debate over the newly discovered particles; the seeds of revolution were
planted by those who advocated charm and gauge theory. Charm advocacy was
sustained by two groups: the old guard of gauge theorists; Gell-Mann, Weinberg,
and Glashow on one hand, but also younger theorists who were not trained in
gauge theory on the other. The question is why this second group aligned
themselves at a crucial time with the gauge theory camp.
He provides biographies for three of these younger non gauge theory
trained theorists: Mary Gaillard, Alvaro De Rujula
and John Ellis. All three began their research within the current-algebra
tradition. Unlike the dominant CMQ and Regge
traditions, current algebra never lost touch with its field- theory ancestry.
He shows that career development and the and the parallel acquisition of new
expertise were structured by the local context of day-to-day practice rather
than by the overall public culture of HEP, such as professional literature.
De Rujula learned gauge theory calculation and was
familiar with sophisticated perturbative techniques
and the parton model. By 1974, he was presenting a
review of “Lepton Physics and Gauge Theories” at the London Conference. When
the revolution came, the entire HEP theory group at Harvard was on the side of
gauge theory and charm.
Ellis recalled
that theoretical research at
Ellis supported gauge theory and believed that QCD was ‘the only
sensible theory’. in 1973-4 a major HEP issue was the
data on electron-positron annihilation emerging from SLAC. These data were in
conflict with the ideas of scale invariance, and Ellis recalled that for many
they were seen as ‘total nemesis’ for field theory.
Because of his expertise, he was asked to give reviews on theoretical ideas concerning electron-positron
annihilation. It then became ‘quite obvious to me that there was only one
solution’ ie, the existence of a new hadrionic degree of freedom such as charm. The difference
between Ellis’ reaction and that of other theorists to the electron positron data
illustrates the role of prior experience in determining responses in particular
contexts.
Ellis recalled that many of his colleagues
attitude was “who believed in that parton light-cone
crap anyway?” He however argued that the idea of partons
worked in so many different cases that they must be in some sense correct, so that if there was a
violation of those predictions for electron positron annihilation, it could not
be that the ideas of light-cone expansion were breaking down. It had to be that
there was some new parton coming in, or some new
degree of hadronic freedom. By autumn 1974, it was
clear to Ellis that something like charm must exist.
Thus, when the first of the new particles was discovered (in
electron-positron annihilation), Ellis had an explanation: charm, and the tools
with which to construct arguments in it’s favor:
asymptotically free gauge theory.
Gaillard attained
the status of an expert in weak interaction phenomenology by 1972. In 1973, she
spent a year at Fermilab, where she found herself in
the local context where the phenomenological
implications of the electroweak force was an important topic, and she
was in daily contact with Benjamin Lee, leader of the theory group at Fermilab,
and a leading gauge theorist. She began the investigation of the detailed
phenomenology of electroweak theories, learning gauge theory techniques with
the help of Lee. She subsequently investigated, with Lee, the consequences of
the existence of GIM charm. This resulted in a major article ‘Search for
Charm”, co-authored by Gaillard. She also reviewed prospective experimental
searches for charm at an international conference in
In a 1978 interview by
Part III
Establishing
the New Physics: The November Revolution and Beyond
Chapter
9: Charm: The Lever that Turned the World
The author’s aim is to set the revolution in the context of the
traditions described in Part II, and to show how, in turn, the revolutionary
developments defined the context for subsequent developments of HEP.
9.4 Charm
The peculiarity of the new particles were the
combination of large masses with long lifetimes. Normally the
heavier a hadron, the shorter its lifetime. The J-psi, discovered in
1974, was the
heaviest particle known, but also the longest lived.
In the Search for Charm paper
by Gaillard, Lee, and Rosner, one proposed mechanism
for this result was the production of ‘hidden charm’ states in the electron
positron annihilation: mesons composed of a charmed quark plus a charmed
antiquark in which the net charm cancelled out. G,L,
and R observed that these hidden charm states would be expected to be
relatively long lived via application of the empirically successful Zweig rule.
This rule offered a qualitative explanation of the longevity or the psi,
but quantitatively it failed: even when the Zweig rule was applied, the
measured lifetimes were 40 times longer than expected. This discrepancy was, as
Gaillard, Lee, and Rosner put it, ‘either a serious
problem or an important result, depending on one’s point of view’.
A number of theorists chose to see the Zweig discrepant lifetime of the
J-psi as an important result rather than a serious problem. These were the
gauge theorists, and in justification o f their position they rolled out the ‘charmonium model’, which explained the properties of hidden
charm states. This model explained the properties of the hidden charm states as
an analog of the ‘positronium’, the name given to the atomic
bound states of an electron positron pair. So, just as positronium
atoms decay to photons, so too the hidden charm hadrons decay could be supposed
to proceed via charmed quark charmed anti-quark annihilation to two or three
gluons, which then rematerialize as conventional quarks.
It is clear that after 1975, experimenters at SPEAR and elsewhere
oriented their research strategy around the investigation of the phenomena
expected in the charmonium model. If the experimenters had
oriented their strategy around some other model, the pattern of experiments
around the world would have been quite different, and the outcome might well
have been changed.
The experimental techniques which were adapted allowed discovery of five
“intermediate hidden charm particles”. Some considered that this discovery
constituted a direct verification of the charmonium
model. However, two of the new particles had reported masses and widths in
conflict with the predictions of the charmonium
model. By 1979, further experimentation convinced most physicists that these
two particles did not exist at all, at least not with the masses originally
reported. Thus the empirical basis of the charmonium
model, a key element I the November revolution, was retrospectively destroyed.
Chapter
10
The
Standard Model of Electroweak Interactions
This chapter reviews the development of electroweak physics in the late
1970s. By 1979, consensus had been established on the validity of the standard
Weinberg-Salam electroweak model. However, data on both neutrino scattering and
neutral-current electron interactions at times was not in line with this
theory.
An authoritative review of the experimental data on neutral currents was
given by CERN experimenter F. Dydak at the European
Physical Society Conference held at CERN summer of 1979. It was noted that
several experiments were in disagreement with the standard model.
10.1 More
New Quarks and Leptons
10.2 The
Slaying of Mutants
A so called “high-y”
anomaly reported in
The HPWF team observed six “trimuon” events,
and Caltech two “trimuon” events at Fermiland in
The third major anomaly which challenged the standard model was atomic
parity violating electron-hadron interactions. Atomic physics experiments at
the Universities of Oxford and Washington (
Steven Weinberg was quoted as saying that he was willing to embrace ‘an
attractive class of theories which are not radical departures from the original
model’.
There were two ways of looking for neutral-current effects in the weak
interaction of electrons. One way was the atomic physicist’s bench top approach,. The other was the particle physicist’s use of high energy
electron beams and HEP equipment. Using the HEP approach appeared to be
technically impossible, but Charles Prescott of SLAC pursued this technique
anyway. Using precise techniques, the SLAC team found what they were looking
for, which supported the Weinberg-Salam model.
In mid 1978, Soviet atomic physicists reported results (E122) which agreed with
the W-S model. Support for the
10.3 The
Standard Model Established: Unification, Social and Conceptual
By summer 1979, all anomalies threatening the W-S model had been
dispelled to the satisfaction of the HEP community at large. Weinberg, Salam,
and Glashow shared the 1979 Nobel Prize for their work on establishing the
unified electroweak interactions. It is easy to claim that the W-S model, with
an appropriate complement of quarks and leptons, make predictions which were
verified by the facts. However, in asserting the validity of the W-S
electroweak model, particle physicists chose to accept certain experimental
reports and reject others. The Soviet E122 results were just as novel as the
Washington-Oxford results, and therefore just as open to challenge. But
particle physicists chose to accept the results of the SLAC experiment, chose
to interpret them as supporting the W-S model, chose to reject the Washington-Oxford
results, and chose to accept the Soviet results. Experimental results did
not exert a decisive influence
on theory. The standard electroweak model unified no only the weak and EM interactions;
it also unified research practice in HEP.
Chapter
11 QCD in Practice
Phenomenological
QCD- An Overview
There were two QCD based phenomenological traditions; relating to hadron
spectroscopy and hard scattering phenomena, and were extensions of the charmodium and parton models
respectively. Within all three major
areas of HEP experiment: electron-positron; lepton-hadron; and hadron-hadron
physics, traditions existed for the exploration of spectroscopic and hard
scattering phenomena. Data from these traditions were seen as both
justification and subjet matter for phenomenological
QCD analyses. In turn, these analyses generated a context for further growth of
the relevant experimental programs. The upshot was that, by the end of the
1970s, experimental practice in HEP was almost entirely centered on phenomena
of interest to QCD theorists. Data on other phenomena, for example hadronic soft scattering, for which QCD theorists could not
supply a ready made analysis, were no longer generated in significant
quantities. Proponents of alternative theories – principally Regge theorists, were starved of data.
HEP experimenters had come to inhabit the phenomenal world of QCD, and
in effect, obliged
non-QCD theorists to do likewise.
Never, during the 1970s, was QCD alone brought to bear on existing data.
Thus the story does not even begin to resemble the confrontation of theory with
experimental facts. Instead we will be dealing with what can best be described
as the construction of the QCD world view.
Within this world view, a particular set of natural phenomena were seen to be
especially meaningful and informative, and these were phenomena conceptualized
by the conjunction of perturbative QCD calculations
and a variety of models and assumptions (different models and assumptions being
appropriate to different phenomena within the set.)
In a given phenomenal context, mismatched arose between data and the
prediction of particular QCD based models. But the overall context was such
that these mismatches could be represented more readily as subject matter for
future research, rather than as counterarguments to QCD itself.
QCD spectroscopy covered only a limited low energy domain of
experimental practice. However, in parallel with these, new traditions based on QCD hard scattering developed
from the parton model traditions. through the 1970s
these traditions grew to dominate high energy research at major HEP labs.,
completing the gauge theory takeover of experimental practice.
Initially, in the early 1970s, QCD only underwrote a limited subset of parton model predictions. By the late 1970s, with a change
of QCD calculation, QCD was seen to be applicable to all processes described by
the parton model. The new calculation approach was
called ‘intuitive perturbation theory’ (IPT). Its departure from the original
‘formal’ approach can be characterized by how theorists coped with gluons. Gluons having specified interactions with one
another were the distinguishing feature of QCD. In the formal approach, gluons appeared in simple perturbative diagrams of only a few closed loops. IPT
computed infinite sets of diagrams in which quarks emitted an arbitrary number
of gluons, an approach made possible by importing and adapting techniques used
by QED in representing an arbitrary number of photons.
This resulted in the acceptance of three types of gluon: soft, emitted
with low energy; collinear, of high momentum traveling parallel to the
direction of said quark; and hard, of high momentum traveling transverse to the
direction of said quark.
The status of QCD in its many applications to hard scattering was
reviewed in a plenary session by a Caltech theorist in
While HEP experimenters were busy collecting data in support of QCD,
theorists cutting the ground from beneath their own feet. This is because early
predictions of scaling violations were truly asymptotic; they were expected to
apply only at infinitely high energy.
Perhaps the most accurate statement of the theoretical position in 1978
is that it was confused.
Calculations of higher order corrections to scaling violations were
complex, and experts disagreed on their results. In 1978-79 these differences
were sorted out. Although QCD predictions continued to the data, said
predictions were an amalgam of non-asymptotic contributions, perturbative calculation, and nonperturbative
model contributions. Thus it was possible that the agreement between QCD
prediction and scaling violation data was accidental.
The arguments for and against QCD from the data became highly technical.
Thus, even as HEP entered the 80s, particle physicists were unable to convince
themselves that the scaling violations seen in deep inelastic scattering
demonstrated the validity of QCD, but nonetheless QCD was solidly entrenched as
te central resource in HEP physics.
In the late 1970s, the situation in high energy electron positron
annihilation experiment resembled that of deep inelastic lepton-hadron
scattering. QCD alone could not be used to make detailed and well defined
predictions. However, in conjunction with various other models and assumptions,
perturbation QCD could be used to motivate a world view, centering on “three
jet” phenomena.
Chapter
12: Gauge Theory and Experiment: 1970-90
By 1980, HEP experimenters had effectively defined the elementary
particle world to be one of quarks and leptons interacting according to the
twin gauge theories electroweak and QCD.
Chapter
13: Grand Unification
We have seen how in the late 70s, electroweak and QCD dominated HEP. But
the “new orthodoxy” also included an SU(5) Grand
Unification theory.
Since the basic building blocks of nature had been identified, there was
no further need for HEP theorists, so they turned to “Grand Unified Theories”,
which had little impact on HEP experiment.
GUTss embraced, within a single
gauge theory, the weak, electromagnetic, and strong interactions. Weak and
electromagnetic had been unified in electroweak. It was only a mater of time
until a unification of QCD and electroweak in an extended group structure was
attempted.
GUTs were modeled on electroweak theories. In electroweak, an exact gauge invariance was spontaneously broken by a set
of Higgs scalars, suitably chosen to give large masses to the IVBS while
leaving the photon massless. GUTs were based on a larger gauge group, broken in
such a way as to eave the photon and 8 gluons massless, whlle
giving appropriate masses to the IVBS, W, and Z0 . Because of the group structure
however, GUTs
had to incorporate more than just these 12 gauge vector bosons. Su(5) would
require another 12 “X-bosons” The Higgs sector had to be suitably chosen to
give these bosons extremely high masses, to account for the observed decoupling
of strong and electroweak at currently accessible energies.
As far as mainstream HEP was concerned, the single major
prediction of GUTs involved the electroweak mixing angle, θw In subsequent work, a two standard
deviation discrepancy was found between theory and experiment. As usual, this
discrepancy was considered an important result, rather than a serious problem.
Aside from this, GUTs offered little to the experimenter, largely
because th X-bosons were
located at unattainable high energies to provide the needed masses. There were
two approaches to this problem; one was to go to the Big Bang for the high
energies. his led to a social and conceptual
unification of HEP and cosmology. The other approach was to look for evidence
of X-boson exchange in proton
decay.
Although HEP theorists had thoroughly explored the SU(5)
gauge model, and although it is the lowest level in gauge theory compatible
with grand unification, SU(5) was seen by some to be a danger to the future of
HEP. The danger, stressed from 1978 onwards by Bjorken,
Glashow and others, was that it implied a vast ‘desert’, stretching in energy
from102 GEV to 1015 GEV. At 100 GEV, new physics would
emerge in the shape of the intermediate vector bosons of electroweak.
Thereafter, according to SU(5), increasing energy
would reveal no new phenomena until the X-boson region of 1015 GEV,
which was the unattainable energy. For these reasons, the HEP community was
more then happy to look at more complicated gauge structures, and experimenters
were happy to coordinate with them.
Proton decay experiments were all conceived along the same lines. If the
proton had a lifetime of 1031 years, and if one monitored a total of
1031 protons for a year,
then one could expect to see a single decay. Various proton decay detection
systems were implemented around the world. The first positive data came from
the Kolar gold mine in
Chapter
14 Producing a World
“The world is not directly given to us, we have
to catch it through the medium of traditions”
Paul
Feyerabend 1978
Judgments were crucial in the development of the “new physics”. The potential for legitimate dissent exists
in a discussion of what may be considered key experimental discoveries.
A plurality of theories can be advanced for any set of data held to be
factual. No theory has ever fitted the experimental data or the “facts”
exactly. Particle physicists were continually obliged to choose which theories
to elaborate on and which to abandon, and the choices which were made produced
the world of the ‘new physics’; it’s phenomenon and its theoretical entities.
The existence or nonexistence of pertinent natural phenomena was a
product of scientific judgments. Judgments of which
theory is ‘valid’ results in the view that misfits between prediction and data
be viewed as grounds for further study and elaboration of the theory rather
than its rejection. Within HEP, judgments displayed a social or group
coherence. The world view produced by HEP was socially produced.
The author argues that the construction and elaboration of new physics
theories depends on the recycling of theoretical resources from established
scientific areas. Two major analogies were at the heart of the conceptual
development of the new physics: analogs from the well developed area of atomic
and nuclear physics, whereby hadrons were represented as composites of more fundamental
entities: quarks, and analogs of QED applied to the
weak and strong forces. How valid are the analogs ???
Thomas Kuhn’s argument was that if scientific knowledge were a cultural
product, then scientific knowledge of different cultures would create different
worlds (world views) for the different scientific communities. Each world view
would recognize different natural phenomena and explain it with different
theories. Therefore the theories of different cultures would be immune to
testing from one another. They would be ‘incommensurable. Thus the world of ‘new
physics’ is incommensurable with the world of the physics preceding it.
To summarise, the quark-gauge theory picture
of elementary particles should be seen as a culturally specific product. The
theoretical entities of the new physics, and the natural phenomena which
pointed to their existence, were the joint products of a historical process- a
process which culminated in a communally congenial representation of reality….
There is no obligation to take account of what 20th century
science has to say about a world view. Particle physicists of the late 1970s
were themselves quite happy to abandon much of the world view constructed by
the science of the previous decade. There is no reason for outsiders to show
the present HEP world view any more respect.
World views are cultural products. One scientist put it this way: ‘The
lofty simplicity of nature [presented to us by scientists] all too often rests
upon the un-lofty simplicity of the one who thinks he sees it.”