The God Particle: If the Universe Is the Answer, What Is the Question?

by Leon Lederman

  A third generation was in the making. Since both the electron and the muon have neutrinos associated with them, it seemed natural to assume that a neutrino-sub-tau (ντ) existed.

  Meanwhile, Lederman's group at Fermilab finally learned how to carry out the dimuon experiment correctly, and a new, vastly more effective organization of apparatus exploded open the mass domain from the J/psi peak at 3.1 all the way to pretty nearly 25 GeV, the limit allowed by Fermilab's 400 GeV energy. (Remember, we're talking about stationary targets here, so the effective energy is a fraction of the beam energy.) And there, at 9.4, 10.0, and 10.4 GeV sat three new bumps, as clear as the Tetons viewed on a brilliant day from Grand Targhee ski resort. The huge mass of data multiplied the world's collection of dimuons by a factor of 100. Christened the upsilon (it was the last Greek letter available, we thought), the new particle repeated the story of the J/psi, and the new thing that was conserved was the beauty quark—or, as some less artistic physicists call it, the bottom quark. The interpretation of the upsilon was that it was an "atom" made of a new b quark bound to an anti-b quark. The higher mass states were simply excited states of this new "atom." The excitement over this discovery nowhere near matched that of J/psi, but a third generation was indeed news and raised an obvious question: how many more? Also, why does nature insist on Xerox copies, one generation replicating the previous one?

  Let me offer a brief description of the work that led to the upsilon. Our group of physicists from Columbia, Fermilab, and Stony Brook (Long Island) included some crackerjack young experimenters. We had constructed a state-of-the-art spectrometer with wire chambers, magnets, scintillation hodoscopes, more chambers, more magnets. Our data acquisition system was "dernier cri," based on electronics designed by genius engineer William Sippach. We had all worked in the same domain of Fermilab beams. We knew the problems. We knew one another.

  John Yoh, Steve Herb, Walter Innes, and Charles Brown were four of the best postdocs I have seen. The important software was reaching the state of sophistication required for work at the frontier. Our problem was that we had to be sensitive to reactions that happened as rarely as only once in every hundred trillion collisions. Since we needed to record many of these rare dimuon events, we needed to harden the apparatus to a huge rate of irrelevant particles. Our team had developed a unique understanding of how to work in a high-radiation environment and still have survivable detectors. We had learned how to build in redundancy so that we could ruthlessly suppress false information no matter how cleverly nature tried to fool us.

  Early in the learning process, we ran in the dielectron mode and obtained about twenty-five electron pairs above 4 GeV. Strangely, twelve of these were clustered around 6 GeV. A bump? We debated and decided to publish the possibility that there was a particle at 6 GeV. Six months later, after the data had increased to three hundred events, poof—no bump at 6 GeV. We had suggested the name "upsilon" for the fake bump, but when better data contradicted the earlier data, the incident became known as oops-leon.

  Then came our new setup, with all of our experience invested in a rearrangement of target, shielding, placement of magnets, and chambers. We began taking data in May of 1977. The era of month-long runs of twenty-seven events or three hundred events were over; thousands of events per week were now coming in, essentially free of background. It isn't often in physics that a new instrument permits one to survey what amounts to a new domain. The first microscope and the first telescope are historic examples of far greater significance, but the excitement and joy when they were first used cannot have been much more intense than ours. After one week, a wide bump appeared near 9.5 GeV, and soon this enhancement became statistically solid. John Yoh had, in fact, seen a clustering near 9.5 GeV in our three-hundred-event run, but having been burned at 6 GeV, he merely labeled a bottle of Mumm's champagne "9.5" and hid it in our refrigerator.

  In June we drank the champagne and broke the news (which had leaked anyway) to the laboratory. Steve Herb gave the talk to a packed and excited auditorium. This was Fermilab's first major discovery. Later that month we wrote up the discovery of a broad bump at 9.5 GeV with 770 events in the peak—statistically secure. Not that we didn't spend endless man-hours (unfortunately we had no women collaborators) looking for a malfunction of the detector that could simulate a bump. Dead regions of the detector? A software glitch? We ruthlessly tracked down dozens of possible errors. All of our built-in security measures—testing the validity of the data by asking questions to which we knew what the answers should be—checked out. By August, thanks to additional data and more sophisticated analysis, we had three narrow peaks, the upsilon family: upsilon, upsilon prime, and upsilon double prime. There was no way to account for these data on the basis of the known physics of 1977. Enter beauty (or bottom)!

  There was little resistance to our conclusion that we were seeing a bound state of a new quark—call it the b quark—and its antiparticle twin. The J/psi was a cc meson. Upsilon was a bb meson. Since the mass of the upsilon bump was near 10 GeV, the b quark must have a mass near 5 GeV. This was the heaviest quark yet recorded, the c quark being near 1.5 GeV. Such "atoms" as cc and bb have a lowest-energy ground state and a variety of excited states. Our three peaks represented the ground state and two excited states.

  One of the fun things about the upsilon was that we experimentalists could handle the equations of this curious atom, composed of a heavy quark circling a heavy antiquark. Good old Schrodinger's equation worked fine, and with only a brief look at our grad school notes, we raced the professional theorists to calculate the energy levels and other properties that we had measured. We had fun ... but they won.

  Discoveries are always quasi-sexual experiences, and when John Yoh's "bicycle-on-line" quick analysis first indicated the existence of the bump, I experienced the now (for me) familiar feeling of intense euphoria, but tinged with the anxiety that "it can't really be true." The most obvious impulse is to communicate, to tell people. Who? Wives, best friends, children, in this case Director Bob Wilson, whose lab badly needed a discovery. We telephoned our colleagues at the DORIS machine in Germany and asked them to see if they could reach the energy required to make upsilons with their e+ e− collider. DORIS was the only other accelerator that had a chance at this energy. In a tour de force of machine magic, they succeeded. More joy! (And more than a little relief.) Later you think about rewards. Will this do it?

  The discovery was made traumatic by a fire that interrupted data taking after a good week of running. In May 1977 a device that measures the current in our magnets, supplied no doubt by a low bidder, caught fire, and the fire spread to the wiring. An electrical fire creates chlorine gas, and when your friendly firemen charge in with hoses and spray water everywhere, they create an atmosphere of hydrochloric acid. The acid settles on all the transistor cards and slowly begins to eat them.

  Electronic salvage is an art form. Friends at CERN had told me about a similar fire there, so I called to get advice. I was given the name and telephone numbers of a Dutch salvage expert working for a German firm and living in central Spain. The fire occurred on Saturday, and it was now 3 A.M. on Sunday. From my room at Fermilab, I called Spain and reached my man. Yes, he'd come. He'd get to Chicago Tuesday, and a cargo plane from Germany filled with special chemicals would arrive Wednesday. But he needed a U.S. visa, which usually takes ten days. I called the U.S. embassy in Madrid and spouted, "Atomic energy, national security, millions of dollars at stake..." I was connected to an assistant to the ambassador who was not impressed until I identified myself as a Columbia professor. "Columbia! Why didn't you say so? I'm class of fifty-six," he shouted. "Tell your fellow to ask for me."

  On Tuesday, Mr. Jesse arrived and sniffed at 900 cards, each carrying about 50 transistors (1975 technology). On Wednesday the chemicals arrived. Customs gave us more heartburn, but the U.S. Department of Energy helped. By Thursday we had an assembly line: physicists, secretaries, wives, girlfriends, all dipping cards in secret solution A, t
hen B, then drying with clean nitrogen gas, then brushing with camel's-hair brushes, then stacking. I half expected that we'd be required to accompany the ritual with a low moan of Dutch incantation, but this was not necessary.

  Jesse, a horseman, lived in Spain to train with the Spanish cavalry. When he learned I had three horses, he ran off to ride with my wife and the Fermilab horse club. A real expert, he gave everybody pointers. Pretty soon the prairie riders were trading tips on flying changes, passages, lavade, corbette, and capriole maneuvers. We now have a trained Fermilab cavalry to defend the lab should the hostile forces from CERN or SLAC decide to attack on horseback.

  Friday we installed all the cards, testing each one carefully. By Saturday morning we were up and running, and a few days later a quick analysis showed that the bump was still there. Jesse stayed on for two weeks, riding horses, charming everyone, advising on fire prevention. We never got a bill from him, but we did pay for the chemicals. And that was how the world acquired a third generation of quarks and leptons.

  The very name "bottom" suggests that there must be a "top" quark. (Or if you prefer the name "beauty," then there is a "truth" quark.) The new periodic table now reads:

  First generation Second generation Third generation

  QUARKS

  up (u) charm (c) top? (t)

  down (d) strange (s) bottom (b)

  LEPTONS

  electron neutrino (νe) muon neutrino (νμ) tau neutrino (ντ)

  electron (e) muon (μ) tau (τ)

  At this writing, the top quark has yet to be found. The tau neutrino has also never been pinned down experimentally, but no one really doubts its existence. Various proposals for a "three-neutrino experiment," a souped-up version of our two-neutrino experiment, have been submitted over the years at Fermilab, but all have been rejected because such a project would be enormously expensive.

  Note that the lower left-hand grouping (νe-e-vμ-μ) in our table was established in the 1962 two-neutrino experiment. Then the bottom quark and the tau lepton put the (almost) finishing touches on the model in the late 1970s.

  The table, once the various forces are added to it, is a compact summary of all the data emerging from all of the accelerators since Galileo dropped spheres of unequal weights from the nearly vertical tower at Pisa. This table is called the standard model or, alternately, the standard picture or standard theory. (Memorize.)

  In 1993 this model is still the ruling dogma of particle physics. The machines of the 1990s, primarily Fermilab's Tevatron and CERN's electron-positron collider (called LEP), are concentrating the efforts of thousands of experimentalists on clues to what lies beyond the standard model. The smaller machines at DESY, Cornell, Brookhaven, SLAC, and KEK (Tsukuba, Japan) are also attempting to refine our knowledge of the many parameters of the standard model and trying to find clues to a deeper reality.

  There is much to do. One task is to explore the quarks. Remember in nature only two kinds of combinations exist: (1) quark plus antiquark ()—these are the mesons—and (2) three quarks (qqq)—the baryons. Now we can play and compose hadrons such as and ... Have fun! And uud, ccd, ttb ... Hundreds of combinations are possible (somebody knows how many). All are particles that either have been discovered and listed in the tables or are ready to be discovered. By measuring the mass and the lifetimes and the decay modes, one learns more and more about the strong quark force mediated by gluons and about weak-force properties. Much to do.

  Another experimental high point is called "neutral currents," and it is crucial to our story of the God Particle.

  The Weak Force Revisited

  By the 1970s lots of data had been collected on the decay of unstable hadrons. This decay is really the manifestation of the constituent quarks undergoing reactions—for example, an up quark changing to a down quark or vice versa. Even more informative were the results of several decades of neutrino-scattering experiments. Together, the data insisted that the weak force had to be carried by three massive messenger particles: a W+, a W−, and a Z0. These had to be massive because the weak force has a very small sphere of influence, reaching no farther than approximately 10−19 meters. Quantum theory enforces a rough rule that the range of a force varies inversely as the mass of the messenger particle. The electromagnetic force reaches out to infinity (although it gets weaker with distance), and its messenger particle is the zero-mass photon.

  But why three force carriers? Why three messenger particles—one positively charged, one negatively charged, and one neutral—to propagate the field that induces the changes of species? To explain, we're going to have to do some physics bookkeeping, making sure that things come out equal on both sides of the arrow (→). This includes the electric-charge signs. If a neutral particle decays into charged particles, for example, the positive charges have to offset the negatives.

  First, here's what happens when a neutron decays into a proton, a typical weak-force process. We write it like this:

  We have seen this before: a neutron decays into a proton, an electron, and an antineutrino. Note that the positive proton cancels the negative charge of the electron on the right side of the reaction, the antineutrino being neutral. Everything works out. But this is a superficial view of the reaction, like watching an egg hatch into a blue jay. You don't see what the fetus is doing inside. The neutron is really a conglomerate of three quarks—one up and two downs (udd); a proton is two ups and a down (uud). So when a neutron decays into a proton, a down quark changes into an up quark. Thus it's more instructive to look inside the neutron and describe what's happening to the quarks. And in quark language, the same reaction can be written:

  That is, a down quark in the neutron changes to an up quark, emitting an electron and an antineutrino. However, this too is a simplified version of what really happens. The electron and antineutrino don't come directly out of the down quark. There's an intermediate reaction involving a W−. The quantum theory of the weak force therefore writes the neutron decay process in two stages:

  1) d− ⅓ → W− + u+⅔

  and then

  Note that the down quark decays first into a W− and an up quark. The W in turn decays into the electron and antineutrino. The W is the mediator of the weak force and participates in the decay reaction. In the above reaction it must be a negative W to balance the change in electric charge when d goes to u. When you add the −1 charge of the W− to the +⅔ charge of the up quark, you get −⅓, the charge of the down quark that started the reaction. Everything works out.

  In nuclei, up quarks can also decay into down quarks, turning protons into neutrons. In quark language the process is described: u → W+ + d and then W+ → e+ + νe. Here we need a positive W to balance the change of charge. Thus the observed decays of quarks, via the changes of neutrons to protons and vice versa, require both a W+ and a W−. But that's not the whole story.

  Experiments carried out in the mid-1970s involving neutrino beams established the existence of "neutral currents," which in turn required a neutral heavy force carrier. These experiments were stimulated by theorists like Glashow who were working the unification-of-forces frontier and were frustrated by the fact that weak forces seemed to require only charged force carriers. The hunt was on for neutral currents.

  A current is basically anything that flows. A current of water flows in a river or a pipe. A current of electrons flows in a wire or through a solution. The W− and W+ mediate the flow of particles from one state to another and the need to keep track of the electric charge probably generated the "current" concept. The W+ mediates a positive current; the W− mediates the negative current. These currents are studied in spontaneous weak decays, such as those just described. But they can also be generated by neutrino collisions in accelerators, made possible by the development of neutrino beams in the Brookhaven two-neutrino experiment.

  Let's look at what happens when a muon neutrino, the kind we discovered at Brookhaven, collides with a proton—or more specifically, with an up quark in the proton. The
collision of a muon antineutrino with an up quark generates a down quark and a positive muon.

  Or, in English, muon antineutrino plus up quark → down quark plus positive muon. Effectively, when the neutrino and up quark collide, the up turns into a down and the neutrino converts to a muon. Again, what really happens in the weak-force theory is a two-reaction sequence:

  The antineutrino collides with the up quark and leaves the collision as a muon. The up turns into a down, the whole reaction mediated by the negative W. So we have a negative current. Now, even as early as 1955, theorists (notably Glashow's teacher Julian Schwinger) noted that it would be possible to have a neutral current, like so:

  νμ + u → u + νμ

  What's happening here? We have muon neutrinos and up quarks on both sides of the reaction. The neutrino bounces off the up quark but emerges as a neutrino, not a muon as in the previous reaction. The up quark gets nudged but remains an up quark. Since the up quark is part of a proton (or a neutron), the proton, albeit jostled, remains a proton. If we were to look at this reaction superficially, we would see a muon neutrino hitting a proton and bouncing off intact. But it's more subtle than that. In the previous reactions, either a negative or a positive W was required to help facilitate the metamorphosis of an up quark into a down or vice versa. Here, the neutrino must emit a messenger particle to kick the up quark (and be swallowed by it). When we try to write this reaction, it's clear that this messenger particle must be neutral.