The Higgs Boson: Searching for the God Particle (26 page)

Theorists' prescriptions for tying up such untidy edges usually entail the prediction of yet more exotic and massive particles. One kind of extension of the Standard Model, for instance, is “grand unification.” We have good reason to believe that at a very high energy the strong force (which holds the nucleus together) becomes unified with the electroweak. These forces become equally strong, joining to form a grand unified force. In that case, leptons become relatives of the quarks, and several
parameters relating to the strong forces become the same as those of the weak. The overall structure of a grand unified model is much simpler, and more rational, than that of the Standard Model. But it also requires the existence of ultraheavy particles, called grand unified particles, that have a mass of about 10
¹⁶
GeV (1 GeV, roughly the mass of a proton, is a billion electron volts).

Among other interactions, these ultra-heavy particles allow quarks to change into leptons—and the proton to decay. Physicists have looked for proton decays for more than a decade, and the searches are now becoming more definitive. With Carlo Rubbia of CERN and
others in Italy, I am working on the ICARUS proton decay experiment at the Gran Sasso Laboratory in Italy. Giant detectors are being constructed at Gran Sasso and in Japan.

But there is a problem with the grand unified model. Its ultraheavy particles, by interacting with particles of the known world, would increase the masses of the latter. Quarks and leptons would then also have masses of about 10
¹⁶
GeV. In that case, not only would humans not have observed them, but also they would not exist—at least in their current form.

The only solution known to this “hierarchy problem” is supersymmetry, or SUSY. Supersymmetry postulates that
each known particle is one of a super-symmetric pair. The superpartner of a quark, for example, would have a heavier mass and a different spin, or angular momentum. It would in effect cancel the interaction between the heavy grand unified particles and the quarks and leptons of the world, solving the hierarchy problem.

Many theorists are convinced that supersymmetric partners must exist. But none have been found. Maurice Goldhaber of Brookhaven National Laboratory sometimes jokes that the situation is not that bad: we at least have one half of all supersymmetric particles in the universe—the quarks and leptons!
One necessary consequence of supersymmetry
is the existence of flavor-changing neutral currents. For example, supersymmetric particles would provide a pathway for bottom quarks to change into strange quarks. In fact, the FCNCs might be so large that they would have to be suppressed somehow.

The FCNCs mediated by SUSY particles can be reduced if the partners in a supersymmetric pair have rather similar masses. The similarity implies that SUSY particles have low masses, like those already known. But because experimenters have seen none of these particles in accelerators, their masses must actually be much heavier. They are supposed to range from 100 GeV to 10 TeV (1 TeV is a trillion electron volts). These contradictory requirements for the masses have put most versions of supersymmetry in trouble.

A more straightforward way in which the Standard Model may be extended is by additional quarks. Physicists have speculated on the possibility of a fourth family of quarks for years.

Because grand unification suggests that the quark families are also related to leptons, electrons and neutrinos are cousins of the up and down. If physicists were to find an additional, fourth neutrino, it would indicate the presence of a fourth quark family. Data taken at the Large Electron Positron collider at CERN indicate that only three light neutrinos exist. Still, there may well be a fourth, massive neutrino.

The massive quark family that would come along with a massive neutrino would almost certainly induce flavor-changing processes. As noted, GIM mechanisms, which cancel FCNCs for low-mass quarks, would not work so well with the heavier quarks. Flavor-changing events would take place most often in reactions involving the third family, into which the fourth family would preferentially decay.

Another theory has recently been put forward by Weinberg and Lawrence J. Hall of the University of California at Berkeley, as well as by some other theorists. They argue that there is no theoretical constraint on the number of Higgs particles that exist in nature. Whereas the Standard Model requires only one Higgs, it does not rule out the presence of many.

These extra Higgs particles could exist even at the relatively low mass of 100 GeV. Although hard to detect in current accelerators—because they are not very reactive—the particles would almost certainly mediate flavor-changing decays. Such decays would be most pronounced for bottom, and possibly top, quarks.

Another theory, known by the name of technicolor, suggests that the Higgs particle is a composite of two higher mass particles. This postulate allows the Higgs mechanism—by which the
W
and Z particles get their mass—to have
a more natural structure. The technicolor particles have masses likely above a trillion electron volts. Technicolor particles also tend to generate rather large FCNCs, which are currently unapparent. Refined versions of the theory—called running technicolor or walking technicolor—manage to reduce, but not eliminate, flavor-changing currents.

Thus, theorists predict a plethora of particles beyond the Standard Model that could give rise to FCNCs. Experimenters have looked for such currents for some 30 years now, reaching ever increasing levels of sensitivity.

Preliminary searches for neutral currents began, as mentioned, in the early 1960s. We used a kaon beam at Lawrence Berkeley Laboratory for the first definitive search. A kaon has one strange quark coupled with an antiup or antidown quark. Alternatively, it may have an antistrange quark coupled with an up or down. Kaons belong to a class of composite particles, each made of a quark and an anti-quark, that are called mesons. Whereas quarks do not exist freely in nature, mesons do—although they are often unstable. Hence, experiments often begin with a meson beam.

If the strange quark in a kaon were to decay into a down, the kaon would break up into a pion—a meson that combines a down with an antiup (or up with an antidown) quark. The decaying kaon would emit as well a neutrino and
an antineutrino. A pion is all too common; it is made in many nuclear processes. But the two neutrinos that would come along with it are a distinctive signal of the flavor-changing process.

Observing the decay in an experiment is not so easy. The trace of a neutrino, for example, is never seen in a detector. Nowadays the extreme sensitivity of this search has placed severe constraints on extensions of the Standard Model.

The next quark, the charm—a heavy relative of the strange—was until recently thought to be not a sensitive gauge of exotic physics. This was because it decays relatively fast, by Standard Model processes. Now we think it is interesting, for a different reason. The charm is weakly coupled to the top quark; thus, the top could decay into the charm, emitting neutrinos of very high energy. Interactions of neutrinos with charm quarks could also signal FCNCs. The latter processes could possibly be tested in future Fermilab experiments involving neutrino beams.

The most likely particle to reveal flavor-changing neutral currents is the bottom quark. Being much heavier than the strange or the charm, the bottom quark couples better with the heavy particles that are predicted by extensions of the Standard Model. Furthermore, bottom quarks are found in
B
mesons, which have a relatively long lifetime of 10
¹²
second—100 times longer than expected. The stability of
B
mesons allows experimenters to produce them in large numbers and in beams of high energy.

The bottom quark can decay in several ways via FCNCs. Any one of these decays could signal novel physics beyond the Standard Model. Besides being able to make
B
meson beams, we can now also use some extremely sensitive detectors. The
B
meson travels only a tenth of a millimeter before it decays. The latest detectors contain silicon strips in which the mesons and other particles leave tracks of electron charge. Even the very short tracks are clearly visible.

In one process, the bottom quark could decay to a strange quark by emitting an unknown object, possibly a supersymmetric particle or an exotic Higgs. The latter decays further, into a lepton and anulepton pair.

The most sensitive search to date for this decay was carried out by our group, in the unimaginatively dubbed UA1 (Underground Area 1) detector, at the CERN proton-antiproton collider. (In 1983 the UA1 collaboration reported the first observation of
W
and Z particles.) We looked for a muon-antimuon pair with a combined energy of more than 4 GeV. We found that fewer than five decays in 100,000 were flavor changing. The result was used to restrict the masses of technicolor and Higgs particles. If the particles interact as strongly as theorists believe them to, their masses must be less than 400 GeV.

In a different decay process, the bottom breaks down again to a strange quark, but by emitting a photon. The decay proceeds via a penguin diagram. In practice, the decaying bottom quark is contained in a
B
meson; the latter decays to an excited state of a kaon and gives off a photon.

In late 1993 such a decay was seen at the Cornell electron-positron storage ring. Only a few such events have been detected so far. Calculating the likelihood of this process is quite difficult. In particular, its presence could be signaling an exotic particle or an interaction involving a top quark. We know for sure only that it signals a penguin process. Until the decays take place frequently enough to be studied systematically, physicists cannot decide exactly which particles are mediating the penguin. At present, the finding serves to whet the appetite.

Another interaction—free of many of the theoretical uncertainties that plague the former—is one in which the
B
meson decays to any particle containing a strange quark, giving off a photon. The process includes the earlier one as a small component but is easier to calculate. Currently experimental limits have been placed on this process from the Cornell experiment. Of every 10,000
B
meson decays, fewer than five change flavor.

There is another exciting possibility for the decay of a bottom quark. It involves a flavor-changing neutral current in which a
B
meson decays, not to another quark but to a pair of leptons. In particular, the
B
could decay to a tau and an antitau. Grand unification puts the tau lepton in the same family as bottom quarks. Thus, this decay involves only the third family. Besides, it requires a flavor-changing neutral current.
If the decay is relatively profuse, it would point to the existence of supersymmetric particles.

Detecting this decay is a major challenge to experimental particle physics. At a recent meeting in Snowmass, Colo., a few of us initiated a study of schemes for its observation. To this end, we are conducting a series of computer simulations at the University of California at Los Angeles.

One approach is to detect the muons into which the tau lepton decays. A key detector in this search is the just approved Compact Muon Solenoid. It is to be used at the Large Hadron Collider (LHC) at CERN. Our group is part of a collaboration that designed and, we hope, will participate in building the detector. The current head of this experiment is Michel Della Negra of CERN.

In addition to detection schemes researchers also require intense sources of
B
particles. One such source might be derived from the proton-antiproton beams at Fermilab. When the two beams collide, they generate a profusion of particles, including between 10
⁹
to 10
¹⁰
B
mesons. Two “
B
factories” are being planned as well, at the Stanford Linear Accelerator Center and at the National Laboratory for High Energy Physics (better known as KEK) in Japan. These projects should each produce about 10
⁸
B
mesons.

Colliders to be built in the future will also be important for such searches. The European Union is going ahead with the LHC. This collider will smash
together, head-on, two proton beams, each with energies of 7 TeV. If all goes as planned, the LHC will turn on before the year 2003. It will create some 10
¹²
B
mesons in colliding beams. Another possible means of detecting
B
decays at the LHC is the super fixed target experiment. If a part of the main beam is extracted and made to hit a stationary target, up to 10
¹¹
B
mesons could be manufactured.