The Higgs Boson: Searching for the God Particle (25 page)

In the example, three jets of hadrons
(one for each of the two quarks and one for the antiquark) and two photinos would emerge into the region around the interaction. When the gluino decays, its energy is shared among the quark, the antiquark and the photino.
The energy need not be shared equally, however; sometimes one or another of the three decay products gets most of the available energy. Although the energy imbalance and other related effects make comparison of theory and experiment more difficult at proton colliders than at electron colliders,
the comparison can be made.

As an example, in 1982 Jacques P.
Leveille, then at the University of Michigan, and one of us (Kane) proposed that evidence for supersymmetry would be obtained if certain observations
(such as one or more jets ofparticles in the presence of a large amount of missing momentum) were seen at a high-energy hadron collider.
Much excitement was generated the following year when events of this type were reportedly seen at the UA1 (for Underground Area 1) and UA2 detectors at CERN. A number of groups of theorists have examined the possibility that supersymmetric particles were being produced. We have done detailed analyses in collaboration with R. Michael Barnett of the Lawrence Berkeley Laboratory. Other analyses have been made by John Ellis of CERN and Henry Kowalski of DESY (the electron accelerator in Hamburg), by Yernon D. Barger of the University of Wisconsin at Madison and his collaborators,
and by Ewald Reya of the University of Dortmund in West Germany and D. P. Roy of the Tata Institute of Fundamental Research in India.

The number of missing energy events observed in the original, albeit small, data sample initially suggested the possibility of new particles.
Events predicted by the standard model,
however, can appear in a configuration similar to the configurations observed at CERN. Statistical analyses are therefore needed to prove that inherently new phenomena are being seen.
Now that more data have been accumulated the situation remains unclear,
although the possibility that the discovery of supersymmetric particles has occurred is less likely than it seemed initially. More data will be collected at the CERN collider this year and may make it clearer whether any of the few candidate events corresponds to the production of supersymmetric particles.

Together with Barnett we have done a comprehensive analysis of the data from CERN. First, we assumed the photino is the least massive supersymmetric particle. Working from the additional assumption that the missing energy events can be accounted for by the standard model, we concluded that gluinos and squarks must be heavier than about 75 proton masses, or 70 BeV. Although this is a large mass, it is still not as large as the
W-
boson mass (81 BeV).

Yet if it turns out that some of the events do correspond to the production of supersymmetric particles, our analysis implies the photino is not the least massive supersymmetric particle. This conclusion is remarkable, given that the masses of the supersymmetric particles are not known. We were able to arrive at it because the supersymmetric theory is strongly constrained,
thereby yielding detailed predictions that can easily be tested by experiment.

In collaboration with Mariano Quiros of the Institute for the Structure of Matter of the Council for Scientific Research (CSIC) in Spain we have argued that the least massive supersymmetric particle might be the higgsino
(the supersymmetric partner of the Higgs boson). If this turns out to be the case, the photino would be unstable and would decay into a photon and a higgsino. In this scenario the limits that can be placed on squark and gluino masses are somewhat weaker.

As machines with larger energies and intensities become available in the future, they could produce and detect super symmetric particles of greater mass. The electron-positron colliders that will start up in the next few years
(TRISTAN in Japan in 1986, SLC at the Stanford Linear Accelerator Center in 1987 and LEP at CERN in 1989) will be able to detect sleptons up to about 50 BeV in mass. The proton-antiproton collider at Fermilab, which should begin to yield data at the end of this year,
will be able to detect squarks and gluinos of mass between 100 and 150 BeV, depending on its intensity. Before 1990, then, sleptons of 50 BeV and squarks of about 150 BeV will have been either found or excluded.

To go beyond those masses will require machines that are being planned but have not yet been approved. The U.S. particle-physics community has committed itself to proposing a proton-
proton collider called the Superconducting Supercollider (ssc), having energies of 20,000 BeV per beam and an intensity about 1,000 times as great as that of the CERN or Fermilab proton-
antiproton colliders. At the ssc squarks and gluinos of masses up to more than 20 times as great as the
W-
boson mass could be found. When such a machine is in operation, if not before, physicists expect to find experimental clues pointing toward a theory that goes beyond the standard model. In particular,
data from the ssc could definitively decide whether nature is supersymmetric on the scale of the electroweak force and could thus help in understanding the laws of nature on that scale. The alternative is that supersymmetry could at best be a mathematical property of quantum field theories, relevant to energies far greater than those that investigators can ever hope to probe directly

-Originally published: Scientific American 252(6), 52-60 (June 1986)

Low-Energy Ways to Observe High-Energy Phenomena

by David B. Cline

In the fall of 1993 Congress canceled the Superconducting Super Collider, or SSC. The SSC was designed to search for particles beyond the energy range of current accelerators. The Large Hadron Collider at CERN, the European laboratory for particle physics near Geneva, will probably be built in the first few years of the 21st century. But its energy is only about half of that which the SSC might have achieved. So how can physicists seek the massive particles that give logic and symmetry to theories of the fundamental elements of matter?

Fortunately, nature has provided a loophole through which scientists can look more deeply into its puzzles. Within the Standard Model of particle physics, some types of interactions are conceivable but in practice never seen. For example, a strange quark is not observed to decay into a down. Different means by which the interaction might occur manage to cancel one another out. Interactions that are not found to occur are said to be forbidden.

But it is entirely possible that particles not yet known to us might be able to mediate such an interaction by passing from one (known) particle to another. If researchers test ever more precisely, they may ultimately succeed in finding a faint signal for the process. Indeed, the detection will be made possible by the fact that the result one expects from the Standard Model is zero. Although it is difficult to discern a minute deviation from a large (and usually ill-defined) quantity, it is relatively easy to measure a deviation from zero. Once scientists have observed this so-called forbidden interaction, they will have evidence of the presence of a new particle. They can then add the particle to the Standard Model, thereby extending it.

One class of such interactions goes by the name of flavor-changing neutral currents, or FCNCs. Although these interactions had never been observed (until recently), new and exotic particles would almost inevitably create FCNCs that could be detectable in extremely sensitive experiments. Already this window may have revealed the first signs of particles that lie beyond the Standard Model.

Traditionally physicists have sought additional characters of the Standard Model by smashing together beams of known particles in accelerators. The mass-energy contained in these particles is oftentimes channeled into creating unknown ones. But the heaviest particles, which require large inputs of energy, are inaccessible to accelerators. In this realm, too, FCNCs have an advantage. As a rule, the heavier an exotic particle, the more likely it is to interact with a known one. Thus, although heavy particles are hard to generate in accelerators, they are easier to detect through their effects at low energies.

Known particles belong to the low-energy world that human beings normally live in. One class of particles comprises
the leptons—electrons, muons and taus—and the elusive ultralight particles they decay into, the three neutrinos. Then there are the quarks.

Quarks seem to come in six types, or “flavors"—up, down, strange, charm, bottom and, now, top. Each quark is heavier than the preceding one in the list; the conservation of mass-energy allows a heavier quark to decay into one that is lighter, but not vice versa.

Up and down, strange and charm, and bottom and top are closely related to each other and are paired into “families.” Up and down, for instance, are the two lightest quarks and belong to the first family. In each family one quark has an electric charge of 2/3 (up, charm and top), and the other has an electric charge of 1/3 (down, strange and bottom). (The charge is measured in units of a proton's charge.) For every quark or lepton there is an antiquark or antilepton, which is identical except for having the opposite charge.

Quarks are able to change into one another by giving off or absorbing heavy particles. Three particles that transmit the weak nuclear force between quarks are the
Z°,
the
W
⁺
and the
W
^-
.
(The superscripts indicate electric charges of 0, +1 and -1, respectively.) For instance, a down quark can change into an up quark by a weak process, with the
W
^-
particle carrying away the extra charge. Because the decay involves the passage of a charged particle (the
W
^-
it is said to be mediated by a charged current. Alternatively, a quark can interact with itself by emitting and reabsorbing a
Z
⁰
, which gives rise to a weak neutral current, or WNC.

But never do experimenters see, as mentioned, a strange quark changing into a down, a process involving a flavor change. Because both these quarks have the same charge, such an interaction would have to proceed by a flavor-changing neutral current, or FCNC.

The absence of FCNCs in (almost) all experiments conducted to date has already led to the prediction—and discovery—of the charm and the top quarks. When physicists first became aware, in the late 1960s, that FCNCs did not seem to occur, they were at a loss to understand their absence. The theory of elec-troweak interactions had just been invented by Steven Weinberg, now at the University of Texas at Austin, and Ab-dus Salam of the International Centre for Theoretical Physics in Trieste, Italy.
Previously Sheldon L. Glashow of Harvard University had described the same theory. They had fit the weak and electromagnetic interactions into the same framework and predicted the existence of the
Z
⁰
,
W
⁺
and
W
^-
particles. These particles became analogues of the photon, which transmits electromagnetic forces.

But the electroweak theory, brilliantly confirmed over the next decades, required the existence of neutral currents, in which a
Z
⁰
is exchanged. Among other interactions, researchers assumed that the
Z
⁰
might mediate the decay of the strange quark to the down. An experiment mounted at Lawrence Berkeley Laboratory in 1963, which I helped to initiate, did not find any such decays. What we did not realize at the time was that we were looking for a special, forbidden process: an FCNC. We simply concluded, on the basis of our experiments, that no neutral currents existed.

The only quarks known then were the up, down and the strange. In 1970 Glashow, John Iliopoulos of the Ecole Normale Superieure to Paris and Luciano Maiani of the University of Rome noticed that if a fourth quark existed, it could cancel the interaction of the strange quark with the down. Thus, the absence of FCNCs would be accounted for. Also, weak neutral currents that do not change flavor would exist. Because it would solve a long-standing dilemma, the theorists called their hypothetical fourth quark the “charm."

Meanwhile scientists at CERN and at Fermi National Accelerator Laboratory (Fermilab) to Batavia, Ill., had been looking for WNCs to processes involving neutrinos. Neutrinos interact with other particles only by weak interactions and with other neutrinos only by WNCs. For some time, different and confusing signals for WNCs from one of the major experiments led the physics community to claim, tongue to cheek, that “alternating neutral currents” had been discovered.

In 1973 both the experiments at CERN and Fermilab found WNCs. In 1974, also at Fermilab, a charm quark made a fleeting appearance. Furthermore, large numbers of charm particles were produced to 1976 at the Stanford linear Accelerator Center, thus confirming the theorists' scenario. Their formula for getting rid of FCNCs, called the GIM mechanism, has since turned out to have much broader validity than earlier envisaged. Within each family, one quark
prevents the other from decaying via anFCNC.

Like the charm, the top quark was predicted to exist—because the bottom was not seen to decay to a strange or a down. Because each quark has a familial pair, FCNCs cannot easily occur within the Standard Model. Only on rare occasions can the heavy quarks violate the GIM mechanism, which works best for the light quarks.

The rare FCNC that might be mediated by known particles—and, in fact, all particle interactions—is best illustrated by a kind of diagram invented by the late Richard P. Feynman of the California Institute of Technology
.
In a Feynman diagram the particles are drawn as leaving traces, rather like a jet plane leaving a vapor trail. Thus, when two particles interact, their traces join at a vertex; when a particle decays, its trace breaks up.

An FCNC can occur if a top quark mediates the interaction in a way described by a complicated Feynman diagram known as a penguin. (The name has an unusual source. John Ellis of CERN once lost a game of darts with Melissa Franklin, now at Harvard. The penalty was that he had to put the word “penguin” into his next published paper—in which this diagram first appeared.) This decay, however, takes place infrequently, if at all. The penguin diagram has many variations; in most of them, exotic particles serve to mediate the decay.

Such particles are invariably postulated in theories that address the deficiencies of the Standard Model. One such problem is the question of why the fundamental particles have such diverse masses. The top quark, for example, is some 30,000 times heavier than the more common up quark, one of the principal constituents of ordinary matter.

Particles are believed to gain mass by interacting with the heavy Higgs particle, which is also predicted by the electroweak theory. Because each quark has a different mass, however, it must couple with the Higgs with a different strength. These coupling strengths, or, alternatively, the quark masses themselves, are among the 21 parameters of the Standard Model that do not emerge from its fundamental assumptions. The properties have instead to be determined by experiment. This large set of arbitrary numbers is less than appealing—at least to those scientists who believe that at the deepest level of structure, the universe must be simple.