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9
The Electroweak
Synthesis and Beyond
Occasionally in the history of science, a new unifying principle has
emerged that joins two separate bodies of knowledge whose connec-
tion at some deep level had not previously been recognized. The first
great unification in physics was probably Newton's demonstration that
gravity acts on the heavenly bodies in the same way that it acts on
objects in our own world. Later, in the nineteenth century, Maxwell
unified electric and magnetic forces by showing that they are just two
different manifestations of a single force-electromagnetism. In our
own century, Einstein unified the concepts of space and time surely
one of the greatest single intellectual achievements in physics-and of
matter and energy, through relativity.
After the mid-1930s, the four fundamental forces of nature were
considered to be gravitation, electromagnetism' the strong force, and
the weak force. In 1967, however, the work of S. Weinberg, A. Salam,
and S. Glashow led to a remarkable synthesis of electromagnetism and
the weak nuclear force into a single electroweak force. This achieve-
ment, one of the triumphs of modern science, has had a profound effect
on the development of nuclear physics and particle physics during the
last decade. In this chapter we examine a few of the directions in which
the electroweak synthesis appears to lead.
THE STANDARD MODEL
The value of great unifying syntheses comes not only from the ways
in which they illuminate the underlying simplicity of nature in a very
160
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THE ELECTROWEAK SYNTHESIS AND BEYOND 161
real sense, they change our view of the world-but also from the
predictive power of their logical consequences. Maxwell's unification
of electricity and magnetism, for example, required the existence of
electromagnetic waves moving through a vacuum with the speed of
light, and we know that this requirement is fulfilled.
Similarly, the electroweak synthesis already has an impressive list of
successful predictions to its credit. One of these is that the weak force
should be mediated not only by the exchange of massive charged
particles (the W+ and W- bosons) but also by the exchange of a
massive neutral particle (the Z° bosom. All three of these particles
were discovered at CERN in 1983. Furthermore, the electroweak
theory makes detailed predictions about nuclear processes. For exam-
ple, the weak-interaction decay of a neutral kaon into a positive muon
and a negative muon is permitted by the exchange of a neutral particle,
such as the Z°, but this process occurs only very rarely. The
electroweak theory explains this result correctly on the basis of subtle
effects pertaining to the strange and down quarks. Consideration of this
problem led to the postulation of a new type of quark called charm (so
named because it made the theory "work like a charmed. The charm
quark was subsequently shown to exist-another triumph of the
theory. It is because the present theories of the electroweak force and
the strong force are so successfill that together then Burp roll the
Standard Model.
Every known fact about nuclear and particle physics is consistent
with the Standard Model. This does not mean, however, that the
Standard Model explains everything that we know-far from it!
Despite its spectacular successes, physicists are certain that the
Standard Model is incomplete. It does not, for example, include the
gravitational force; it does not tell us why there are three lepton
families; and it does not explain some important conservation laws or
their violations. Parity violation, for example, is a dominant charac-
teristic of the weak force, yet it must be built into the electroweak
theory arbitrarily. Similarly, time-reversal-invariance violation is
known to occur, but among several possible ways of incorporating it
into the theory, it is not clear which way is correct. As for the
conservation laws for certain other properties, such as lepton family
number, we do not know whether an underlying symmetry principle is
at work or whether the law seems to hold only because present
experiments are insufficiently sensitive to detect possible violations of
it.
The mathematical form of the electroweak theory inspires confi-
dence, however, because it is the only known theory of the weak
interaction that is renormalizable. In a renormalizable theory, of which
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162 NUCLEAR PHYSICS
quantum electrodynamics is the archetype, observable quantities can
be calculated to apparently any desired degree of accuracy. Quantum
chromodynamics (QCD) is also a renormalizable theory, but its math-
ematical complexities are so great that reliable QCD calculations are
very difficult, except near the limit of asymptotic freedom.
PHYSICS WITH NEUTRINO BEAMS
The advent of very intense beams of protons at meson factories has
opened up the possibility of making neutrinos from the nuclear debris
created when these beams are brought to rest in matter. Neutrinos
interact only through the weak interaction and can penetrate vast
amounts of matter without stopping. However, if copious numbers of
neutrinos are present and detectors weighing many tons are used, a few
neutrino interactions can be observed. Such experiments permit the
study of the weak part of the electroweak force and, by comparison
with the much more easily studied electromagnetic part, can test the
fundamental unity of the electroweak interaction.
An experiment now under way at the Los Alamos National Labo-
ratory is designed to measure the scattering of electron neutrinos from
electrons in an advanced detector. According to electroweak theory,
this scattering can happen in two ways: the neutrino and the electron
can exchange a W- boson, thereby also exchanging their identities (the
neutrino turns into an electron, and vice versa), or they can exchange
a Z° boson and retain their original identities. There is no way an
observer can tell which process actually happened in any given
scattering, so quantum mechanics predicts that these processes can
interfere with each other: the total probability for the event is not just
the simple sum of the individual probabilities. Demonstrating this
interference and measuring its sign will be a key test of electroweak
theory.
With even more-intense and more-energetic neutrino beams, such as
might be produced by the next generation of accelerators, one can hope
to carry out experiments in which neutrinos scatter from nuclei,
sometimes leaving them in excited states. Because the nuclear states
have specific quantum numbers, experiments of this sort will be able to
dissect electroweak theory into its parts, each corresponding to these
different quantum numbers. Such tests have never been performed and
would provide a far more searching evaluation of electroweak theory
than can be made at present.
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THE ELECTROWEAK SYNTHESIS AND BEYOND 163
TESTING THE GRAND UNIFIED THEORIES
With two powerful theories of nuclear matter at our disposal the
electroweak theory and QC~the scientific imperative is obvious: we
must try to unify the electroweak and strong forces within a Grand
Unified Theory that would include them both in one self-consistent
mathematical framework. In the previous unifications, the main diffi-
culty was in constructing a viable theory having all the required
properties. Now, however, we are faced with an unprecedented and
most peculiar problem: there is already a glut of Grand Unified
Theories, which turn out to be rather easy to construct. Each reduces
correctly to QCD and electroweak theory at low (terrestrial) energies;
the catch is that at cosmological energies, such as must have existed
briefly after the big bang, they predict a bewildering variety of
phenomena that are as bizarre as they are different.
These differences between contending Grand Unified Theories be-
come evident only at particle energies estimated to be about 1O'5 GeV,
which is hopelessly beyond the reach of any currently conceivable
terrestrial accelerator and far above even the energies of cosmic rays.
How, then, can such stupendous energies possibly be achieved so that
the correct Grand Unified Theory can be recognized from among the
welter of alternatives? The answer may lie in the Heisenberg uncer-
tainty principle, which allows a particle of any arbitrary energy to
emerge out of a vacuum as a virtual particle, as long as it disappears
back into the vacuum within a certain time, i.e., as long as its lifetime
falls within a prescribed limit. The higher the energy, the shorter the
allowed lifetime. Thus, ultrahigh-energy virtual particles can enable
us- if we are clever enough to study interactions that would other-
wise be inaccessible.
A virtual particle of mass 1O~s GeV would have some astounding
properties, even by the standards of particle physics. In terms of
conventional units, its free mass would be about 10-9 gram (equivalent
to 10'4 carbon atoms, or about the mass of a typical bacterium!), and it
might exist for a fleeting 10-39 second, long enough for it to move only
10-~6 of a nucleon diameter at the speed of light. This incredibly brief
virtual existence of such a supermassive unification particle means that
any effect it may have in a laboratory experiment will be extremely
tiny. Experimentalists may have to sift through staggering numbers of
nuclear events to find the precious few that reveal the signature of a
unification particle. Nevertheless, a number of technically feasible
experiments have been designed that bear on the unification of the
strong and electroweak forces. A few of these experiments are
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164 NUCLEAR PHYSICS
described in the following sections; some of them are already in
progress, while others await the construction of specialized new
accelerators.
Time-Reversal-Invariance Violation
The origin of time-reversal-invariance violation is unknown. At
present, the only known instance of this phenomenon is in the decay of
neutral K mesons (kaons). A neutral kaon and its antikaon are exactly
alike except for the quantum number called strangeness, which is
related to the strong interaction. The weak interaction does not respect
strangeness and "mixes" the pure kaon and its pure antikaon; the two
kaons that are actually observed can be thought of (roughly) as two
different hybrids of the pure kaon states.
Now that tentative Grand Unified Theories are available, it appears
to be possible to incorporate time-reversal-invariance violation into
their framework, based on certain details of the decay properties of
these kaons. Experiments to measure the neutral kaon decay precisely
and to search for evidence of time-reversal-invariance violation in
another possible decay mode may be crucial in finding the correct way
to account for the violation in the context of grand unification.
However, kaon beams 10 to 100 times more intense than those
currently available will be needed for these experiments.
The Electric Dipole Moment of the Neutron
Finding a second example of time-reversal-invariance violation
would be a major event in physics. Such an example might conceivably
be found in the neutron if it can be shown to have an electric dipole
moment. An electrically neutral particle can possess a measurable
electric dipole moment (internal separation of positive and negative
charge) only if both parity and time-reversal invariance are violated.
Very sensitive experiments have been carried out over the past three
decades to try to measure the electric dipole moment of the neutron.
When a neutron is between the poles of a magnet, the interaction with
the neutron's intrinsic magnetism produces two possible energy levels,
depending on whether the neutron's axis is aligned parallel or antiparal-
lel to the applied magnetic field. An observable change from one level
to the other can be induced by bathing the neutrons in an oscillating
radio-frequency field having just the right frequency; a representative
value is 60 megahertz (60 million cycles per second) in a strong magnet.
The principle is just the same as in the nuclear-magnetic-resonance
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THE ELECTROWEAK S YNTHESIS AND BEYOND 165
equipment routinely used by chemists to detect protons in molecules.
However, a beam of free protons is not suitable for the electric dipole
moment search, because protons are charged and would be deflected
out of the magnetic field. Neutrons, on the other hand, are uncharged
and can be obtained as a slow-moving beam; the experimental sensi-
tivity is thus enhanced because of the increased length of time that they
remain in the magnetic field.
In the experiment, a strong electric field is applied simultaneously
with the magnetic field. If the neutron has an electric dipole moment,
the energy added by the electric interaction will slightly shift the
difference between the neutron's energy levels in the magnetic field.
Current experiments are sensitive to shifts as small as 0.001 hertz.
With the present sensitivities, no electric dipole moment has yet
been observed in the neutron. If the neutron does have an electric
dipole moment, it must be smaller than that which would be due to a
positive electron and a negative electron separated by only 6 x 10-25
cm (roughly 10-~ times the radius of the neutron). Thus, if a neutron
were expanded to the size of the Earth, the "bulge" of electric charge
in one hemisphere represented by this maximum value of the dipole
moment would be only about the thickness of a human hair! This
infinitesimal limit has ruled out a number of theories that predict an
observably large moment, leaving only theories that predict either an
extremely small moment or no observable time-reversal-invariance
violations outside the kaon system.
To increase further the sensitivity of the experiments, very-slow-
moving (cold) neutrons will be needed, because they will remain longer
in the magnetic field of the detector, allowing a more sharply defined
measurement. Present experiments have reached the limits imposed by
the two major reactor facilities (in France and the Soviet Union) that
produce cold neutrons. Further progress will require specialized tech-
niques, such as spallation neutron sources and cold moderators at
accelerators.
Rare Muon and Kaon Decays
According to the quark model, the six quark flavors fall into three
distinct families of two each. It has been known for many years that the
weak interaction "mixes" the quark families, so that a quark from one
family can change into a quark from another. The lambda hyperon
(quark structure ads), for example, has a rare decay mode in which it
transforms to a proton (uadD, an electron, and an antineutrino; this
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166 NUCLEAR PHYSICS
decay mode evidently requires a strange quark from one family to
become an up quark from another.
It is interesting, but not necessarily significant, that leptons also
come in three families of two each, and many Grand Unified Theories
allow mixing between lepton families, in analogy with the mixing
between quark families. Such mixing would, in turn, allow the occur-
rence of decay modes in which lepton family number was not con-
served- for instance, the decay of a muon into an electron and a
gamma ray (see Figure 9.11. The observation of this decay would be
both an indication of such mixing and a much-needed signpost pointing
toward the correct Grand Unified Theory.
Intensive effort at all three of the world's meson factories the Los
Alamos Meson Physics Facility, the Tri-University Meson Facility
(Vancouver, British Columbia), and the Swiss Institute of Nuclear
Research (Villigen) has been put into the search for the electron mode
of muon decay. The lowest limit to date, established at Los Alamos,
shows that this mode occurs no more frequently than once in every 6
x 109 muon decays. This is a very small limit, but a more-intense muon
source would allow even lower limits (greater experimental sensitivity)
to be achieved. Failure to see one distinctive electron-mode decay in
every 1Ois muon decays might eliminate all but a few of the currently
conceived Grand Unified Theories from further consideration.
Rare decays of kaons offer a cornucopia of opportunities for looking
at the electroweak synthesis and beyond. Present theory predicts that
a positive kaon should decay into a positive pion and a neutrino-
antineutrino pair somewhere between 1 and 30 times in every 10~° kaon
decays. Agreement of experiment with this prediction would confirm
the number of quark families, including the existence of the hitherto
unobserved top quark, and would even provide a value for the latter's
mass. Experiments to search for this decay are planned for existing
accelerators and will require large detectors and long measurement
times. If the decay probability is significantly less than one event in
10~°, then its detection is out of reach at present. Accelerators capable
of producing kaon or muon beams of far greater intensity are needed
for the study of electroweak interactions through rare decay modes.
Together, the theories of the electroweak and strong interactions
explain most of what we know about atomic nuclei. Those things that
we know but are unable to explain as well as many of the innumerable
things that we do not yet know at all may have their origins in levels
of understanding that can arise only from a grand unification of these
two interactions. Direct tests of grand unification are at present
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THE ELECTROWEAK SYNTHESIS AND BEYOND 167
FIGURE 9.1 The Crystal Box spectrometer, an advanced particle and radiation
detector currently under construction at the Los Alamos Meson Physics Facility.
Consisting of several hundred specially shaped sodium iodide crystals with associated
electronics packages, it will be used in searching for the decay of muons to electrons and
gamma rays. (Courtesy of the Los Alamos National Laboratory.)
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168 NUCLEAR PHYSICS
impossible, of course, because no conceivable accelerator could even
approach the necessary 10~5-GeV energies.
Instead, the current emphasis is on extremely rare but profoundly
significant processes that can be observed at accessible energies. In
addition to high experimental selectivity and sensitivity, this search
requires the maximum possible beam intensities, in order to produce
the huge numbers of events among which the occasional rare ones may
be found. These invaluable bits of information from nuclear physics
may ultimately prove essential for weaving together our fragmentary
knowledge into a Grand Unified Theory of the fundamental
interactions.
a
Representative terms from entire chapter:
electroweak theory
