The God Particle

by Leon Lederman

  When Richard Feynman called my dimuon experiment the "Drell-Yan experiment" in a book—surely he was joking—I phoned Drell and told him to call all the people who bought the book and ask them to cross out Drell and Yan on [>] and write in Lederman. I didn't dare bug Feynman. Drell cheerfully agreed, and justice triumphed.

  Since those days, Drell-Yan-Lederman experiments have been carried out in all the labs and have given complementary and confirmatory evidence of the detailed way in which quarks make protons and mesons. Still, the SLAC/Drell-Yan-Lederman studies did not convert all physicists into quark believers. Some skepticism remained. At Brookhaven there was a clue right in front of our eyes that would have answered the skeptics had we known what it meant.

  In our 1968 experiment, the first of its kind, we were examining the smooth decrease in the yield of muon pairs as the mass of the messenger photons increased. A messenger photon can have a transitory mass of any value, but the higher the mass, the shorter the time it lives and the harder it is to generate. Heisenberg again. Remember the higher the mass, the smaller the region of space that is being explored, so we should see fewer and fewer events (numbers of pairs of muons) as the energy increases. We chart this on a graph. Along the bottom of the graph, the x-axis, we show increasing masses. On the vertical y-axis we show numbers of muon pairs. So what we should get is a graph that looks like this:

  We should see a smooth descending line indicating ever-decreasing muon pairs as the energy of the photons coming out of the black box increases. But instead we got something that looked like this:

  At about the 3 GeV mass level this smooth decrease was interrupted by a "shoulder" now called the Lederman Shoulder. A shoulder or a bump in the graph indicates an unexpected event, something that can't be explained by the messenger photons alone, something sitting on top of the Drell-Yan events. We did not report this shoulder as a new particle. It was the first clear miss of a discovery that would finally establish the reality of the quark hypothesis.

  Incidentally, our chagrin at missing the discovery of pointlike structures in the proton, a discovery that by Swedish decree went to Friedman, Kendall, and Taylor, is mock chagrin. Even Bjorken might not have seen through the subtleties of relating the Brookhaven dimuons to quarks in 1968. The dimuon experiment, in retrospect, is my favorite. The concept was original and imaginative. Technically it was childishly simply—so simple that I missed the discovery of the decade. The data had three components—Drell-Yan proof of pointlike structures, proof of the concept of "color" in its absolute rates (discussed later), and the J/Psi discovery (directly ahead)—each of which was of Nobel quality. The Royal Swedish Academy could have saved at least two prizes had we done it right!

  THE NOVEMBER REVOLUTION

  Two experiments began in 1972 and 1973 that would change physics. One took place at Brookhaven, an old army camp amid the scrub pines and sand, a mere ten minutes from some of the most beautiful beaches in the world, on the south shore of Long Island, host to the Atlantic rollers coming straight from Paris. The other site was SLAC, in- the brown hills above the Spanish-style campus of Stanford University. Both experiments were fishing expeditions. Neither was sharply motivated but both would come together in November of 1974 with a crash heard round the world. The events of late 1974 go down in physics history as the November Revolution. It is told around fireplaces wherever physicists gather to talk of old times and great heroes and to sip Perrier. The prehistory is the almost religious idea of theorists that nature must be pretty, symmetrical.

  We should first mention that the quark hypothesis did not threaten the electron's status as an elementary particle, as an a-tom. Now there were two classes of pointlike a-toms—the quarks and the leptons. The electron, along with the muon and the neutrino, is a lepton. That would have been fine, except that Schwartz, Steinberger, and Lederman had fouled up the symmetry with the two-neutrino experiment. Now we had four leptons (electron, electron neutrino, muon, and muon neutrino) but three quarks (up, down, and strange). A chart in 1972 might have looked like this in physics shorthand:

  quarks: u d s

  leptons: e μ

  νe νμ

  Ugh. Well, you wouldn't have made such a chart because it didn't make much sense. The leptons are in a nice two-by-two pattern, but the quark sector was relatively ugly in a threesome, when theorists were already disillusioned with the number 3.

  Theorists Sheldon Glashow and Bjorken had more or less noted (in 1964) that it would be simply charming if there were a fourth quark. This would restore the symmetry between quarks and leptons, which had been destroyed by our discovery of the muon neutrino, the fourth lepton. In 1970 a more cogent theoretical reason for suspecting the fourth quark appeared in a complicated but lovely argument made by Glashow and his collaborators. It converted Glashow into a passionate quark advocate. Shelly, as he is known to his admirers and his enemies, has written a number of books that establish just how passionate he can get. A major architect of our standard model, Shelly is also much appreciated for his stories, his cigars, and his critical commentaries on theoretical trends.

  Glashow became an active marketer of the theoretical invention of a fourth quark, which of course he called charm. He traveled from seminar to workshop to conference, insisting that experimenters look for a charmed quark. His idea was that this new quark and a new symmetry in which quarks also come in matched pairs—up/down and charm/strange—would cure many pathologies (Doctor, here is where it hurts) in the theory of the weak force. It would for example serve to cancel certain reactions that had not been seen but had been predicted. Slowly he won adherents, at least among theorists. In the summer of 1974, a seminal review paper, "The Search for Charm," was written by theorists Mary Gaillard (one of the tragically few women in physics and one of the top theorists of any sex), Ben Lee, and Jon Rosner. The paper was especially instructive for experimenters because it pointed out that such a quark, call it c, and its antiparticle c, or c-bar, could be made in the black collision box and emerge as a neutral meson in which c and c were bound together. They even proposed that the old Brookhaven data my group had taken of muon pairs may have been evidence of a cc decaying into two muons, and that this could be the interpretation of the Lederman Shoulder near 3 GeV. That is, 3 GeV was presumably the mass of the thing.

  BUMP HUNTING

  Still, these were only theorists talking. Other published accounts of the November Revolution have implied that the experimenters involved were somehow working their tails off to verify the ideas of the theorists. Dream on. They were fishing. In the case of the Brookhaven physicists, they were "bump hunting," looking for blips in the data that might indicate some new physics—something that would upset the apple cart, not steady it.

  At the time that Glashow, Gaillard, and others were talking charm, experimental physics was having its own problems. By then, the competition between electron-positron (e− e+) colliders and proton accelerators was clearly recognized. The "lepton people" and the "hadron people" had a spirited debate going. Electrons hadn't done much. But you should have heard the propaganda! Because electrons are thought to be structureless points, they offer a clean initial state: an e− (electron) and an e+ (positron, the electron's antiparticle) heading toward each other in the black-box collision domain. Clean, simple. The initial step here, the model insisted, is that the particle-antiparticle collision generates a messenger photon of energy equal to the sum of the two particles.

  Now, the messenger photon has a brief existence, then materializes into pairs of particles of appropriate mass, energy, spin, and other quantum numbers imposed by the laws of conservation. These come out of the black box and what we commonly see are (1) another e+ e− pair (2) a muon-antimuon pair, or (3) hadrons in a wide variety of combinations but constrained by the initiating condition—the energy and quantum properties of the messenger photon. The variety of possible final states, all derived from a simple initial state, speaks to the power of the technique.

  Contrast thi
s with the collision of two protons. Each proton has three quarks, which are exerting strong forces on one another. This means that they are rapidly exchanging gluons, the messenger particles of the strong force (we'll meet gluons later in the chapter). To add to the complexity of our unlovely proton, a gluon, on its way from, say, an up quark to a down quark, can momentarily forget its mission and materialize (like the messenger photons) into any quark and its antiquark, say s and (s-bar). The appearance is very fleeting, since the gluon has to get back together again in time to be absorbed, but in the meantime it makes for a complicated object.

  Physicists who were stuck with using electron accelerators sneeringly called protons "garbage cans" and portrayed a proton-proton or proton-antiproton collision, not without some justice, as a collision of two garbage cans, out of which flew eggshells, banana peels, coffee grounds, and torn parimutuel tickets.

  In 1973–74, the Stanford electron-positron (e− e+) collider called SPEAR, began taking data, and ran into an inexplicable result. It appeared that the fraction of collisions yielding hadrons was higher than theoretical estimates. The story is complicated and not too interesting until October of 1974. The SLAC physicists, led by Burton Richter, who, in the hallowed tradition of group leaders, was away at the time, began to close in on some curious effects that appeared when the sum of the energies of the two colliding particles was near 3.0 GeV, a suggestive mass, as you may recall.

  What added salsa to the affair was that three thousand miles east at Brookhaven, a group from MIT was repeating our 1967 dimuon experiment. Samuel C. C. Ting was in charge. Ting, who is rumored to have been the leader of all the Boy Scouts in Taiwan, got his Ph.D. at Michigan, did a postdoc term at CERN, and in the early sixties joined my group as assistant professor at Columbia, where his rough edges were sharpened.

  A meticulous, driven, precise, organized experimenter Ting worked with me at Columbia for a few years, had several good years at the DESY lab near Hamburg, Germany, and then went to MIT as a professor. He quickly became a force (the fifth? sixth?) to be reckoned with in particle physics. My letter of recommendation deliberately played up some of his weak points—a standard ploy in getting someone hired—but I did it in order to conclude: "Ting—a hot and sour Chinese physicist." In truth, I had a hang-up about Ting, which dates back to the fact that my father operated a small laundry, and as a child I listened to many stories about the Chinese competition across the street. Since then, any Chinese physicist has made me nervous.

  When Ting worked with the electron machine in the DESY lab, he became an expert in analyzing e+ e− pairs from electron collisions, so he decided that detecting electron pairs is the better way to do the Drell-Yan, oops, I mean the Ting dilepton experiment. So here he was in 1974 at Brookhaven, and, unlike his counterparts at SLAC who were colliding electrons and positrons, Ting was using high-energy protons, directing them into a stationary target, and looking at the e+ e− pairs that came out of the black box with the latest word in instrumentation—a vastly more precise detector than the crude instrument we had put together seven years earlier. Using Charpak wire chambers, he was able to determine precisely the mass of the messenger photon or whatever else would give rise to the observed electron-positron pair. Since muons and electrons are both leptons, which pair you chose to detect is a matter of taste. Ting was bump hunting, fishing for some new phenomenon rather than trying to verify some new hypothesis. "I am happy to eat Chinese dinners with theorists," Ting once reportedly said, "but to spend your life doing what they tell you is a waste of time." How appropriate that such a personality would be responsible for finding a quark named charm.

  The Brookhaven and SLAC experiments were destined to make the same discovery, but until November 10, 1974, neither group knew much about the other's progress. Why are the two experiments connected? The SLAC experiment collides an electron against a positron, creating a virtual photon as the first step. The Brookhaven experiment has an unholy complicated mishmash initial state, but it looks at virtual photons only if and when they emerge and dissolve into an e+ e− pair. Both deal then with the messenger photon, which can have any transitory mass/energy; it depends on the force of the collision. The well-tested model of what goes on in the SLAC collision says a messenger photon is created that can dissolve into hadrons—three pions, say, or a pion and two kaons, or a proton, antiproton, and two pions, or a pair of muons or electrons, and so on. There are many possibilities, consistent with the input energy, momentum, spin, and other factors.

  So if something new exists whose mass is less than the sum of the two colliding beam energies, it also can be made in the collision. Indeed, if the new "thing" has the same popular quantum numbers as the photon, it can dominate the reaction when the sum of the two energies is precisely equal to the new thing's mass. I've been told that just the right pitch and force in a tenor's voice can shatter a glass. New particles come into being in a similar fashion.

  In the Brookhaven version the accelerator sends protons into a fixed target, in this case a small piece of beryllium. When the relatively large protons hit the relatively large beryllium nuclei, all kinds of things can and do happen. A quark hits a quark. A quark hits an antiquark. A quark hits a gluon. A gluon hits a gluon. No matter what the energy of the accelerator collisions of much lower energies occur, because the quark constituents share the total energy of the proton. Thus, the lepton pairs that Ting measured in order to interpret his experiment came out of the machine more or less randomly. The advantage of such a complex initial state is that you have some probability of producing everything that can be reached at that energy. So much is going on when two garbage cans collide. The disadvantage is that you have to find the new "thing" among a big pile of debris. To prove the existence of a new particle, you need many runs to get it to show up consistently. And you need a good detector. Fortunately, Ting had a beauty.

  SLAC's SPEAR machine was the opposite. It collided electrons with positrons. Simple. Pointlike particles, matter and antimatter colliding, annihilating one another. The matter turns into pure light, a messenger photon. This packet of energy in turn coalesces back into matter. If each beam is, say, 1.5525 GeV, you get double that, a 3.105 GeV collision, every time. And if a particle exists at that mass, you can produce this new particle instead of a photon. You're almost forced to make the discovery; that's all the machine can do. The collisions it produces have a predetermined energy. To switch to another energy, the scientists have to reset the magnets and make other adjustments. The Stanford physicists could fine-time the machine energy to a precision far beyond what had been designed into it, a remarkable technological accomplishment. Frankly, I didn't think it could be done. The disadvantage of a SPEAR-type machine is that you must scan the energy domain, very slowly, in extremely small steps. On the other hand, when you hit the right energy—or if you're tipped off somehow, and this was to become an issue—you can discover a new particle in a day or less.

  Let's return for a moment to Brookhaven. In 1967–68, when we observed the curious dimuon shoulder our data went from 1 GeV to 6 GeV, and the number of muon pairs at 6 GeV was only one millionth of what it was at 1 GeV. At 3 GeV there was an abrupt leveling of the yield of muon pairs, and above approximately 3.5 GeV the plunge resumed. In other words, there was this plateau, this shoulder from 3 to 3.5 GeV. In 1969, when we were getting ready to publish our data, we seven authors argued about how to describe the shoulder. Was it a new particle whose effect was smeared out by the highly distorting detector? Was it a new process that produced messenger photons with a different yield? No one knew, in 1969, how the muon pairs were produced. I decided that the data were not good enough to claim a discovery.

  Well, in a dramatic confrontation on November 11, 1974, it turned out that the SLAC and Brookhaven groups each had clear data on an enhancement at 3.105 GeV. At SLAC, when the machine was tuned to that energy (no mean feat!), the counters recording collisions went mad, increasing by a hundredfold and dropping back to the base value when
the accelerator was tuned to 3.100 or 3.120. The sharpness of the resonance was the reason it had taken so long to find; the group had gone over that territory before and had missed the enhancement. In Ting's Brookhaven data, the outgoing pairs of leptons, precisely measured, showed a sharp bump centered near 3.10 GeV. He, too, concluded that the bump could mean only one thing: he had discovered a new state of matter.

  The problem of scientific priority in the Brookhaven/SLAC discovery was a very thorny controversy. Who did it first? Accusations and rumors flew. One charge was that the SLAC scientists, aware of Ting's preliminary results, knew where to look. The countercharge was that Ting's initial bump was inconclusive and was massaged in the hours between SLAC's discovery and Ting's announcement. The SLAC people named the new object ψ (psi). Ting named it J. Today it is generally called the J/ψ or J/psi. Love and harmony have been restored in the community. More or less.

  WHY THE FUSS? (AND SOME SOUR GRAPES)

  All very interesting, but why the tremendous fuss? Word of the November 11 joint announcement spread instantly around the world. One CERN scientist recalled: "It was indescribable. Everybody in the corridors was talking about it." The Sunday New York Times put the discovery on its front page: NEW AND SURPRISING TYPE OF ATOMIC PARTICLE FOUND. Science: TWO NEW PARTICLES DELIGHT AND PUZZLE PHYSICISTS. And the dean of science writers, Walter Sullivan, wrote later in the New York Times: "Hardly, if ever has physics been in such an uproar ... and the end is not in sight." A brief two years later Ting and Richter shared the 1976 Nobel Prize for the J/psi.