Review of the GWS Theory
Following pioneering work by Sheldon
Salam independently constructed
unified theories of the electromagnetic and weak interactions (Weinberg in 1967, Salam in 1968). These models
are essentially the same as those used today. Developments in the 1970s merely added to them. The models
in 1967 contained all the elementary particles known at the time.
The matter particles in the original GWS model are the electron and the muon along with their neutrinos.
For each matter particle there is
also a corresponding antimatter particle as well. The forces between these matter particles are
mediated by four force carriers. At low energies the weak force is
carried by the electrically charged W+ and W- bosons,
and the neutral Z boson, and the electromagnetic force is carried
by the photon. The W and Z bosons are very massive -- about 160 000 times more massive
than the electron --, while the photon is massless.
The low energy properties of the theory are controlled by something known as the
Roughly speaking, at low energy the effect of the Higgs mechanism is to create a sort of
"mud" through which the other particles move. This "mud", called the Higgs vacuum,
causes various particles to acquire masses depending on their strengths of interacting.
This mass gain happens for the W and Z bosons, as well as the muon and electron. The issue of neutrino
masses is more subtle, and will be covered in a future column.
At high energies, the GWS theory looks very different. The masses generated by the
Higgs vacuum field become negligible, instead, at this energy, Higgs particles can
be created. Higgs particles have no quantum spin and are roughly 240 000 times the mass of the
electron. A major task in experimental particle
physics over the next 15 years will be determining the properties of the Higgs
't Hooft, Veltman, the GIM Mechanism, the November Revolution, and Neutral Currents
The period from 1967 until 1972 saw very little activity on the GWS model. For example, Weinberg
cited his own paper on the subject only once during this period. While the GWS model reproduced
what was known about the weak force at the time it had several serious problems. The most
major problem was the problem that nobody had proved that the theory was
We discussed renormalization in the context of Quantum Chromodynamics in part 2 of
this column. Briefly, if a theory is not renormalizable, then it has much less
In 1967, it was not known if theories of the GWS type were renormalizable.
Many people suspected that they should be, however, all efforts to prove this
had stalled by the mid sixties, despite efforts from some of the field's best
Feynman. Without a convincing proof people tried
to explore alternative avenues. However, none of the alternatives offered the same
simple structure that the GWS theory had.
Just as for quantum Chromodynamics, the situation for the GWS theory changed drastically
in 1972. This was the year that
proved that the class of theories to which the GWS model belonged
renormalizable. This was a huge development in particle physics because it showed
how truly precise predictions could be extracted from the GWS theory.
Renormalizability wasn't the only problem with the GWS theory. As formulated by
Weinberg and Salam the theory had another significant difficulty -- neutral flavour changing weak
interactions. Recall the radioactive decay of the neutron into a proton, electron
and anti-neutrino. In this process, what happens, according to the GWS theory, is one of the
down quarks inside the neutron, which is made up of two downs and an up quark, emits a
W- boson, and changes into
an up quark which combines with the remaining up and down quarks to make a proton. The
W boson quickly disintegrates into an electron and anti-neutrino. This type of process,
where a quark of one flavour changes into another is known as a flavour changing
interaction. In this case, it's a charged interaction, since a charged W boson is involved.
The trouble with the GWS theory was with the strange quark. The strange quark had a charged
flavour changing interaction, in which the strange quark decayed into an up quark and
a W+ boson. This interaction was observed in the decays of charged
K mesons, for example when a K+ decayed into a neutral pion, plus a positron
and a neutrino. However the GWS theory, as it stood in 1967, also permitted a second type of
interaction for the strange quark. In this interaction the strange quark could
change into a down quark and emit a neutral Z boson. This would
lead to decays in which a K- decayed into a negatively charged
pion, along with an electron--positron pair.
The problem, of course, was that such a decay was not observed, and the rate for this decay predicted
by the GWS theory was way above the upper limits that had been measured.
This problem was solved in 1970 by
Glashow, Iliopoulos and
Their solution was to add a fourth quark to the theory, the charm quark. This was proposed
on two grounds. The first was that while there were four non-quark matter particles, there were only three
quarks. The second was that in a natural way the strange quark to down quark
neutral interaction got cancelled by the new charm quark to up quark neutral interaction.
This cancellation is known as the GIM mechanism after its three authors.
Of course, this could be regarded as jury-rigging a theory that didn't agree with
data. The proposed solution certainly worked, however apart from aesthetics there
wasn't much justification. It's worth pointing out that the charm quark did fit
very nicely into the framework of the theory and in fact, the theory with the charm seemed
to hang together better. The other nice thing about this solution was that
it provided a new prediction, that ought to be easy to test, which could quickly rule it out.
With the addition of a new quark, Glashow and company predicted would come new bound
states. New mesons and baryons which contained charm quarks. The simplest such
state would be a meson made up of a charm and anti-charm quark, dubbed charmonium, in analogy
to positronium, the bound state of an electron and a positron.
The nice thing about charmonium was that, at the correct energy, it ought to be copiously
produced in electron-positron collisions. Glashow gave several talks at conferences
encouraging experimentalists to look for these new states.
It took four years for the energies of particle accelerators to reach the point where
charmonium could be produced. In November 1974 the discovery of charmonium was
announced by two teams working at separate
accelerators. At the Stanford Linear accelerator
by Burton Richter's
team and at Brookhaven National Laboratory by
Samual Ting's team.
Experimentally the electron-positron collider at Stanford was an excellent tool for
studying these new bound states. The reason was that the events involving them were
very clean. This
image is a very good example of that. The image is a reconstruction of the particle
tracks from the Stanford detectors. The incoming electron and positron are clearly visible, and easily
distinguished from the outgoing positive and negative pions.
The discovery of Charmonium was perhaps the biggest event in particle physics
since the discovery of parity violation. Among physicists it is known as the November
Revolution. However, rather than opening up new fields,
it had the opposite effect of picking out one model above all others. Of all
the competing models available in 1974 only Quantum Chromodynamics and the GWS electroweak theory, with the addition
of the charm quark, accounted for all the data. The discovery of charmonium was the key
that tied together the proton scattering experiments, which favoured quarks and quantum Chromodynamics, and
the large body of weak interaction phenomenology that was explained by the GWS model.
When combined with the still-recent, 't Hooft and Veltman proof of renormalizability, it is clear
why this event turned the vast majority of particle physicists into believers of the standard model of
particle physics. Another gauge of the importance of the charmonium discovery
was the rapidity with which Richter and Ting were awarded the Nobel prize. They announced their
discovery in 1974 and were awarded the prize in 1976, which is extremely rapid by Nobel standards.
There was one more major piece of evidence that was found in the mid-Seventies to support
the standard model of particle physics, the observation of weak neutral currents.
We've seen how the introduction of the charm quark eliminated an effect of the
neutral Z boson that had not been observed. However, the Z boson was predicted
to have many observable effects as well. For example, when an electron
and a positron collide, they annihilate. At low energy the result of this annihilation
is a photon. As the collision energy is raised, there is a growing chance
that they will annihilate into a Z boson instead. Now unlike photons, Z bosons
interact with neutrinos, so the Z boson can decay into a neutrino anti-neutrino pair.
Since neutrinos are very hard to detect, in a particle detector what is observed is an
electron positron collision, then nothing.
It is also possible to attempt to make a beam of neutrinos and try to to have them hit
something in a detector. This is the the experiment that was carried out at
CERN, which culminated in
the discovery of weak neutral currents in 1973. In the CERN experiment, a huge
Gargamelle -- the last of
the bubble chambers -- was bombarded by a beam of neutrinos. If the weak neutral current
was present, occasionally one of the neutrinos would hit one of the atoms in the bubble chamber,
causing a reaction. Such an event is illustrated in
this photograph. Notice
the spray of particles on the left hand side, which appear to emerge from a single point.
There were many developments in the late seventies relating to the electroweak theory.
The most important was the discovery of the third generation of matter.
The third generation or family of matter consists of the tau lepton, which is a heavy
copy of the muon -- itself a heavy copy of the electron --, the associated neutrino, and
two new quarks, named top and bottom quarks. Both
the tau lepton and the bottom quark were discovered rather quickly in the late Seventies.
The top quark and the tau neutrino weren't directly observed until much later, but
because of the success of the GWS model, most physicists assumed that they existed.
The postulating of these particles was strongly motivated by more than aesthetics this time.
Theoretical work in the mid-Seventies on something known as the axial anomaly showed
that the standard model of particle physics would only be renormalizable if the matter
particles it contained came in the observed family structure. That is, the standard model
gives no way to predict the total number of matter particles, but it does say
that they must come in groups of four, a lepton, its neutrino, and two quarks.
The Direct Observation of the W and Z Bosons
With the standard model theoretically established, and observationally confirmed, people
set about to verify one of it's most important features. Although there effects had been
indirectly observed, no actual W and Z bosons had been directly produced in the laboratory.
The standard model made precise predictions about the conditions in which these particles should
be producable, so the accelerator designers set about trying to build a particle
accelerator which could reach the required energy goal.
Thanks to a technical achievement by
Simon van Der Meer, known
as stochastic cooling, it was possible to build the
Super proton synchrotron.
This machine collided protons and anti-protons with enough energy to produce the elusive W and
Z bosons. In 1983 a team lead by Carlo
observed the first W and Z events. The observed masses of the W and Z were
right in the range predicted by the standard model. This work had two primary impacts,
the first was the confirmation of the standard model, which had been expected, and the
second was the technical triumph. The SPS was the first proton anti-proton collider and
paved the way for the
The Number of Light Neutrinos
Throughout the 1990's the main centre for experimental work on electroweak physics was
Electron Positron collider, or LEP. Like all particle physics experiments,
LEP was very simple in principle. The idea was to smash electrons and positrons together,
right at the energy needed to make Z bosons. This proved to be very fruitful.
Until its shutdown in 2001 LEP produced vast numbers of Z bosons and measured their properties
very precisely. The electroweak theory makes lots of predictions
about the Z boson and LEP confirmed a great number of them to very high precision. Most of these
predictions had to do with the decays of the Z. A typical Z event at LEP went something like
this. The electron and positron smash into each other, at an extremely high energy. Because
the energy is so high, it is possible to produce a Z boson. In fact, the chance of producing
a Z boson is much higher than the chance of producing a photon. This Z boson zips along for
a (very) short distance then decays in about 10-25 seconds. Being very heavy
the Z can decay into lots of different stuff, exactly what depends on how much energy it has.
At the original LEP energy the Z primarily decayed in three different ways. The first
was into a quark-anti-quark pair where the quark type was anything but the top, which is
too heavy. As we discussed in the second instalment, these quarks will quickly turn
into jets of hadrons, which is what the LEP teams observed. Another possibility with the quarks
was that they were created in a bound state. In this case, you might see some specific types of
hadrons in your detector. If you lump all of these decays together you find that they
account for about 77% of all of the Z decays.
The Z could also decay into
a lepton-anti-lepton pair. That is it could decay to electron and a positron, a muon and an
anti-muon, or a tau and an anti-tau. These types of decays are really easy to see because
you see the lepton and anti-lepton emerge back to back, and you can easily track their
charges. Decays of this type account for about 3.5% of all Z decays.
We've already discussed the final way that the Z can decay, into a neutrino-anti-neutrino pair.
Now, you don't actually
see the neutrinos in your detectors, so what you see is a collision event, with no final
products. This is referred to as an invisible event. These types of events account for about 20%
of all Z decays. These events proved to be very useful in making a remarkable observation.
You can use the electroweak theory to compute the decay rate of the Z into neutrinos. Since the
neutrinos are nearly massless, to a (very good) first approximation you can treat them all as the same.
Then you get some decay rate, A, for each neutrino type. Since each type is being treated the same,
the total decay rate is just A+A+A=3*A, since there are three types.
But what if there was four types? Or six? Well, then we'd have 4*A or 6*A. Rather than
guessing let's just say that there are X types of neutrinos, so the decay rate is X*A. By
measuring the total invisible decay rate of the Z it is possible to determine X to a very
high precision. This then tells you how many different types of neutrinos there are. Recall
the discussion above about the structure of the matter families in the standard model. Matter
comes in "generations" of four, a lepton, its neutrino, and two quarks. So a measurement
of the number of neutrino types is, within the context of the standard model, a measurement
of the number of matter families there are.
Data for this measurement was taken over the entire ten year LEP run. As of 1998 the best value, with
no extra input from theory, was X=3.00 plus or minus 0.06. This is strong confirmation that
within the standard model, the three generations of matter we've discovered are all that there are.
Put in a different and more exciting way, this measurement indicates that any new form of matter that we discover
will likely not fit within the framework of the standard model.
The Top Quark
While LEP may have been the centre for precision electroweak physics, for sheer power the
Fermilab Tevatron had it beat. Like the SPS before it the Tevatron collides protons
and anti-protons, however the Tevatron has much higher energy. By the early 1990s it
was clear that the Tevatron was the machine that was needed to find the top quark.
With the discovery of the the bottom quark, the existence of a sixth quark
was postulated in order to preserve the structure of the standard model. This quark was named "top". Unfortunately, the
mass of the top quark was not predicted by the standard model, so it was difficult
to know where to find the top.
Through the eighties and early nineties observations of other electroweak processes
constrained the mass of the top (the top quark can indirectly intrude into the calculations of
processes) however these constraints were not strong. The other major constraint came
from the ever increasing energies being probed by particle accelerators.
By the early nineties, the lower, direct, limit from accelerators, and the upper, indirectly limit
had almost converged on a value. It was almost certain that the top was very massive, and
that meant that the only machine that stood a chance of finding it directly was the
Tevatron. After a number of hints the
CDF collaboration announced the discovery of the top quark.
Their result was confirmed shortly thereafter by the other major Fermilab group,
D0. The observed mass was consistent
with the theoretical expectations.
The Final Mystery of the Standard Model
With the observation of the top quark, all three generations of matter
particles in the standard model have now been directly seen in experiments.
In addition all the force carriers have been observed. The only
remaining particle that needs to be measured is the Higgs boson.
Unfortunately it's not easy to detect Higgs bosons. The major problem is that they are
very massive, more massive than the Z boson, which is 180 000 times as massive as the
electron, but somewhat less massive than the top quark. Incidentally these
limits are empirical. The standard model itself does not predict the mass of the Higgs boson.
Direct limits, from the LEP collider, just before it shut down, seem
to indicate that the Higgs boson is around 220 000 times the mass of the electron.
The other trouble with the Higgs boson is that it's not entirely clear what it's
going to be. The standard model only requires a massive spin zero particle, which
at low energy creates the Higgs vacuum field. At higher energies there
could be a multitude of things. For example people have suggested triplets of
Higgs bosons, or Higgs bosons which are bound states of new particles, much like
mesons are bound states of quarks.
The reason for these proposals is that the properties of the Higgs boson
may depend strongly on whatever theory of particle physics comes
beyond the standard model. All of the current proposals, most
affect what will be seen in the region where the Higgs boson is expected.
In 2001, right at the end of its ten year run, the LEP accelerator saw what might have
been the first events involving
the Higgs boson. The teams at LEP observed several events which seemed to be
in line with a Higgs boson decay, however, there were not enough of them
to be considered statistically significant. After some discussion about
extending the run, LEP was shut down for good in late 2001, with the Higgs
particle left undiscovered.
This instalment of the column has reviewed the modern history of the unified electroweak
force, from 1967 until the present. With the discovery of the charm quark, the
proof of renormalizability, and the observation of weak neutral currents, it
was the early Seventies which saw the true rise to prominence
of the standard model of particle physics. The data amassed during these
few years was enough to rule out any competing theory, and focus
all theoretical work on the standard model.
A crucial, if expected, observation occurred in 1983, with the direct observation
of the W and Z bosons. Additionally this experiment was a technical triumph
in particle accelerator design, which paved the way for the Fermilab
Tevatron, and the future
The Tevatron, of course, was the accelerator which had energies
sufficient to observe the top quark in 1995.
Among the thousands of precision measurements preformed at LEP over its run
the most remarkable was the measurement of the total number
of standard model families of matter. The LEP experiments measured
the number of neutrino types present, then, using the theoretical
constraints of the standard model one can conclude that there are only
3 families of matter. This remarkable measurement gives hope to the
idea that the next set of major discoveries in particle physics will
not be simply extensions of the standard model.
Finally, among a number of open problems, the mystery of the Higgs boson
has yet to be solved. With the shutdown of LEP, this mystery will likely
remain unsolved until the LHC starts its run (around 2009). With the discovery
of the Higgs an almost certain first result, the LHC promises to usher
in a brand new era in particle physics.
Thanks to all the people who have commented on the other instalments and participated in the discussions. I would also like to thank Peter Whysall for extensive editorial assistance with this instalment.
Matthew Nobes is a PhD student in theoretical particle physics. He studies at
Simon Fraser University, in British Columbia, Canada. He has been working on
a PhD for about three years, prior to that he spent two years doing a masters
degree. He has a web page here
where you can go to find some links relating
to particle physics.