The Higgs Boson: Searching for the God Particle
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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).
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ANALYSIS OF EXPERIMENTAL DATA from CERN shows that if squarks and gluinos do exist, their mass is probably greater than about 70 billion electron volts (white region).
Illustration by Gabor Kiss
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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?
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CHARACTERISTICS OF THE STANDARD MODEL are the quarks and leptons, the photon (which mediates the electromagnetic force), the W+, W- and Z0 particles (transmitting the weak force) and gluons (mediating the strong force). Each quark has a different flavor, but quarks and leptons in the same column belong to the same family. The numbers to the right indicate the electric charge of all particles in the same row. For every quark and lepton there is an antiquark or antilepton with the opposite charge. Quarks have another quantum number, called color, that has not been indicated. There are a total of eight gluons, each with a different combination of color quantum numbers.
Credit: Ian Worpole
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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. Altern
atively, a quark can interact with itself by emitting and reabsorbing a Z0, which gives rise to a weak neutral current, or WNC.
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DECAY OF A Z0 PARTICLE is captured by the Aleph detector at CERN. The Z0, which was firt seen in 1983, transmits the weak force between other particles such as quarks, giving rise to a weak neutral current. Here it breaks up into a quark and an antiquark, which further splay into more stable particles such as mesons.
Illustration by CERN
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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 Z0, 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 Z0 is exchanged. Among other interactions, researchers assumed that the Z0 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.
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UPPER LIMIT to the fraction of kaons decaying into a pion (by emitting a neutrino and an antineutrino) has gone down steadily over 30 years. Fewer than one kaon in a billion decays this way. The absence of this flavor-changing decay, involving the transformation of a strange quark into a down quark, led to the discovery of the charm quark and has restricted several extensions of the Standard Model. The most recent search is being conducted at Brookhaven National Laboratory.
Credit: Ian Worpole
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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.
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 1016 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 1016 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.
&nb
sp; 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.