The Higgs Boson: Searching for the God Particle

by Scientific American Editors

  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 1012 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.

  * * *

  PENGUIN DECAY of a B meson was observed in June 1993 at the Cornell Electron Storage Ring. The collider produced a pair of B mesons. One decayed conventionally into a positive kaon (green), a negative pion (purple) and a photon, seen as a dark patch (bottom right). The other decayed via a flavor-changing neutral current, the end products of which are a negative kaon (blue), two positive pions (red), a negative pion (pink) and a photon (patch at top left). The flavor-changing decay may signal an exotic particle not within the Standard Model.

  Illustration by Cornell University

  * * *

  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 109 to 1010 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 108 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 1012 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 1011 B mesons could be manufactured.

  Many teams from the U.S. are now planning to work at the LHC. A sub-panel of the High Energy Physics Advisory Panel, chaired by Sidney D. Drell of the Stanford Linear Accelerator, recently emphasized to the U.S. Department of Energy the need to support such participation. Fortunately for those of us at U.C.L.A., our early involvement in the Compact Muon Solenoid guarantees our place in the LHC.

  The discovery of the top quark gives physicists a more accurate tool in evaluating decays of the bottom quark. Now that the mass of the top is known, theorists can calculate the frequency of penguin processes involving top quarks. Knowing the top's contribution, they can more precisely gauge which FCNCs signal exotic particles.

  The top quark could also decay in exotic ways that signal unusual physics. For instance, it might decay to a charm and two neutrinos, a decay mediated by technicolor or multiple Higgs particles. The high mass of the top—174 GeV—might be part of a general pattern, indicating that exotic particles are even heavier than theorists had anticipated. They could range from hundreds of GeV to 1 TeV.

  The observations of flavor-changing decays at Cornell and the limits on exotic particles from UA1 have put scientists in a new era of searches for phenomena beyond the Standard Model. With the profuse sources of B mesons experimenters will have in the near future, and information about top quarks, they can consolidate the early sightings of flavor-changing processes—and tease out the implications.

  The story of flavor-changing neutral currents illustrates the role that “null” experiments—those that see nothing-have played in guiding the development of particle physics. We hope the 30 years of arduous searches will be rewarded in the not too distant future with more discoveries. Even before the Large Hadron Collider comes on line, physicists may be able to peel partially yet another layer from the elementary-particle onion.

  -Originally published: Scientific American 271(3), 40-47 (September 1994)

  SECTION 3

  The Search Is On

  Building the Next-Generation Collider

  by Barry Barish, Nicholas Walker and Hitoshi Yamamoto

  A new era in physics will open up when the Large Hadron Collider (LHC) extends the reach of subatomic particle investigations to unprecedented energy scales. But even before researchers initiate the first high-energy collisions in the LHC’s giant storage ring, located under the French-Swiss border, they are already contemplating and working toward the next great particle accelerator. And the consensus choice of the particle physics community is a proposed facility called the International Linear Collider (ILC), a machine more than 30 kilometers long that would smash electrons and positrons together at velocities very close to the speed of light. (The positron is the antimatter counterpart of the electron, identical in mass but opposite in charge.)

  Far more powerful than previous electronpositron colliders, the ILC would enable physicists to follow up any groundbreaking discoveries made by the LHC. The LHC is designed to investigate the collisions of protons, each of which is actually a bundle of three quarks bound together by gluons (the particles carrying the strong nuclear force). Because the quarks and gluons within a proton are constantly interacting, a proton-proton collision is an inherently messy affair. Researchers cannot be certain of the energy of each quark at the moment of the collision, and this uncertainty makes it difficult to determine the properties of novel particles produced by the impact. But the electron and positron are fundamental particles rather than composites, so physicists working with an electron- positron collider can know the energy of each collision to great accuracy. This capability would make the ILC an extremely useful tool for precisely measuring the masses and other characteristics of newly discovered particles.

  More than 1,600 scientists and engineers from nearly 300 laboratories and universities around the world are now working on the design of the ILC and the development of the detectors that would analyze its particle collisions. In February 2007 our design team released a cost estimate for the machine: $6.7 billion (not including the expense of the detectors). We have done studies comparing the costs of locating the ILC at three possible sites—CERN, the European laboratory for particle physics near Geneva, the Fermi National Accelerator Laboratory in Batavia, Ill., and the mountains of Japan—and we are developing schemes for the governance of a truly international laboratory. Although the ILC’s price tag may seem steep, it is roughly comparable to the costs of large science programs such as the LHC and the ITER nuclear fusion reactor. And if everything proceeds as hoped, the ILC could start illuminating the frontiers of particle physics sometime in the 2020s.

  Birth of a Collider

  In August 2005 about 600 physicists from around the world gathered in Snowmass, Colo., to start planning the development of the ILC. But the true beginnings of the project go back to the commissioning of CERN’s Large Electron- Positron (LEP) collider in 1989. The LEP accelerated electrons and positrons in a storage ring with a circumference of 27 kilometers, then smashed the particles together, producing impacts with energies as high as 180 billion electron volts (GeV). It was clear, though, that the LEP would be the largest collider of its kind, because accelerating electrons and positrons to energies in the trillion-electron-volt (TeV) scale—also known as the terascale—would require a ring several hundred kilometers in circumference and would be completely cost-prohibitive.

  The major obstacle to a storage ring solution is synchrotron radiation: relatively light particles such as electrons and positrons happily radiate their energy as they speed around the ring, their paths continuously bent by the ring’s many dipole magnets. Because these losses make it progressively harder to accelerate the particles, the cost of building such a collider is proportional to the square of the collision energy: a machine that doubled the LEP energies would cost four times as much. (The energy losses are not as severe for colliders that accelerate heavier particles such as protons; hence, the tunnel dug for the LEP ring is now being used by the LHC.)

  A more cost-effective solution is a linear collider, which avoids synchrotron radiation by accelerating particles in straight lines rather than in a ring. In the ILC design, two 11.3-kilometer-long linear accelerators, or linacs—one for electrons, one for positrons—are aimed at each other, with the collision point in the middle. The downside is that the electrons and positrons must be accelerated from rest up to the collision energy on each pulse of the machine instead of building up speed with each circuit of the storage ring. To obtain higher collision energies, one can simply build longer linear accelerators. The cost of the facility is directly proportional to the collision energy, giving linear colliders a clear advantage over the storage ring concept at the TeV scale.

  At the same time that the LEP was being constructed in Europe, the U.S. Department of Energy was building a competing machine at the Stanford Linear Accelerator Center (SLAC). SLAC’s device, which was considered a proof of principle of the linear collider concept, used a three-kilometer-long linac to accelerate bunches of electrons and positrons in tandem, boosting them to energies of about 50 GeV. The bunches were then magnetic
ally separated and bent around to bring them into a head-on collision. Although SLAC’s machine—which operated from 1989 to 1998—was not exactly a true linear collider, because it employed only one linac, the facility paved the way for the ILC.

  Planning for a TeV-scale linear collider began in earnest in the late 1980s and early 1990s when several competing technologies were proposed. As researchers developed these proposals over the next decade, they focused on the need to keep the linear collider affordable. Finally, in August 2004, a panel of 12 independent experts assessed the proposed technologies and recommended a design conceived by the TESLA group, a collaboration of scientists from more than 40 institutions, coordinated by the DESY research center in Hamburg, Germany. Under this proposal, the electrons and positrons would travel through a long series of vacuum chambers called cavities. Constructed from the metal niobium, these cavities can be superconducting— when cooled to very low temperatures, they can conduct electricity without resistance. This phenomenon would enable the efficient generation of a strong electric field inside the cavities that would oscillate at radio frequencies, about one billion times per second. This oscillating field would accelerate the particles toward the collision point.

  The basic element of this superconducting radio- frequency (SCRF) design is a one-meterlong niobium cavity consisting of nine cells that can be cooled to a temperature of two kelvins (–456 degrees Fahrenheit). Eight or nine cavities would be attached end to end in a string and immersed in ultracold liquid helium in a tank called a cryomodule. Each of the two main linacs in the ILC would require about 900 cryomodules, giving the collider about 16,000 cavities in all. Researchers at DESY have so far constructed 10 prototype cryomodules, fi ve of which are currently installed in FLASH, a laser at DESY that employs high-energy electrons. The SCRF technology will also be incorporated into DESY’s upcoming European X-Ray Free-Electron Laser (XFEL), which will string together 101 cryomodules to form a superconducting linac that can accelerate electrons to about 17.5 GeV.