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

by Leon Lederman

  Rubbia's third front was to keep the CERN authorities enthusiastic, roil the world community, and set the stage for the great W experiment. All of Europe was rooting for this, for it meant the coming of age of European science. One journalist claimed that a failure would crush "popes and prime ministers."

  The experiment got under way in 1981. Everything was in place—UA-1, UA-2, the AA ring—tested and ready. The first runs, designed as checkout trials of everything in the complex system of collider plus detector, were reasonably fruitful. There were leaks, mistakes, accidents, but eventually, data! And all at a new level of complexity. The 1982 Rochester Conference was to be in Paris, and the CERN lab went all out to get results.

  Ironically, UA-2, the afterthought detector, made the first splash by observing jets, the narrow bundles of hadrons that are the signatures of quarks. UA-1, still learning, missed this discovery. Whenever David beats Goliath, everyone except Goliath feels warm. In this case Rubbia, who hates to lose, recognized that the observation of jets was a real triumph for CERN—that all of the effort in machines, detectors, and software had paid off in a strong indicator. It all worked! If jets were seen, W's were soon.

  A RIDE ON NO. 29

  Perhaps a fantastic voyage can best illustrate the way detectors work. Here I will switch over to the CDF detector at Fermilab because it is more modern than UA-1, although the general idea of all the "four pi" detectors is the same. (Four pi—4π—means that the detector completely surrounds the point of collision.) Remember that when a proton and an antiproton collide, a spray of particles comes off in all directions. On the average, one third are neutral, the rest charged. The task is to find out exactly where each particle goes and what it does. As with any physical observation, one is only partially successful.

  Let's ride on one particle. Say it's track No. 29. It zips out at some angle to the line of the collision, encounters the thin metal wall of the vacuum vessel (the beam tube), zips through this, no sweat, and for the next twenty or so inches passes through a gas containing an immense number of very thin gold wires. Although there is no sign, this is Charpak territory. The particle may pass close to forty or fifty of these wires before reaching the end of the tracking chamber. If the particle is charged, each nearby wire records its passage, together with an estimate of how close it came. The accumulated information from the wires defines the particle's path. Since the wire chamber is in a strong magnetic field, the charged particle's path is curved, and a measurement of this curve, calculated by the on-board computer gives the physicist the momentum of particle No. 29.

  Next the particle passes through the cylindrical wall defining the magnetic wire chamber and passes into a "calorimeter sector" which measures particle energy. Now the particle's subsequent behavior depends on what it is. If it is an electron, it fragments on a series of closely spaced thin lead plates, giving up its entire energy to sensitive detectors that provide the meat for the lead sandwiches. The computer notes that the progress of No. 29 ceases after three or four inches of lead-scintilla tor calorimeter and concludes: electron! If, however, No. 29 is a hadron, it penetrates ten to twenty inches of calorimeter material before exhausting all of its energy. In both cases the energy is measured and cross-checked against the momentum measurement, determined by the particle's curvature in the magnet. But the computer graciously leaves it up to the physicist to draw a conclusion.

  If No. 29 is a neutral particle, the tracking chamber doesn't record it at all. When it turns up in the calorimeter, its behavior is essentially the same as that of a charged particle. In both cases the particle produces nuclear collisions with calorimeter materials, and the debris produces further collisions undl all the original energy is exhausted. So we can record and measure neutrals, but we can't chart the momentum, and we lose precision in the direction of motion since no track is left in the wire chamber. One neutral particle, the photon, can be easily identified by its relatively quick absorption by the lead, like the electron. Another neutral, the neutrino, leaves the detector entirely, carrying away its energy and its momentum, leaving behind not even a hint of its fragrance. Finally, the muon moves through the calorimeter leaving a small amount of energy (it has no strong nuclear collision). When it emerges, it finds some thirty to sixty inches of iron, through which it passes only to find a muon detector—wire chambers or scintillation counters. This is how muons are tagged.

  One does all this for forty-seven particles, or whatever the number is, in this one particular event. The system stores the data, close to one million bits of information—equivalent to the' amount of information in a hundred-page book—for each event. The data collection system must quickly decide whether this event is interesting or not; it must discard or record the event or pass the data into a "buffer" memory and clear all registers in order to be ready for the next event. This arrives on the average of a millionth of a second later if the machine is working very well. In the most recent full run at the Tevatron (1990–91), the total amount of information stored on magnetic tape of the CDF detector was equivalent to the text of one million novels or five thousand sets of Encyclopaedia Britannica.

  Among the outgoing particles are some with very short lifetimes. These may move only a few tenths of an inch away from the collision point in the beam tube before spontaneously disintegrating. W's and Z's are so short lived that their flight distance is unmeasurable, and one must identify their existence from measurements on the particles to which they give rise. These are often hidden among the debris that typically flies out of each collision. Since the W is massive, the decay products have higher than average energy, which helps locate them. Such exotics as a top quark or a Higgs particle will have a set of expected decay modes that must be extracted from the mess of emerging particles.

  The process of converting enormous numbers of electronic data bits to conclusions about the nature of the collisions takes impressive efforts. Tens of thousands of signals have to be checked and calibrated; tens of thousands of lines of code must be inspected and verified by looking at events that have to "make sense." Small wonder that it takes a battalion of highly skilled and motivated professionals (even though they may officially be classified as graduate students or postdocs) armed with powerful work stations and well-honed analysis codes two or three years to do justice to the data collected in a Tevatron collider run.

  TRIUMPH!

  At CERN, where collider physics was pioneered, it all worked, validating the design. In January 1983, Rubbia announced W's. The signal was five clear events that could be interpreted only as the production and subsequent disintegration of a W object.

  A day or so later UA-2 announced that it had four additional events. In both cases, the experimenters had to sort through about one million collisions that produced all manner of nuclear debris. How does one convince oneself as well as the multitude of skeptics? The particular W decay most conducive to discovery is W+ → e+ + neutrino, or W− → e− + antineutrino. In a detailed analysis of this kind of event one has to verify (1) that the single observed track is indeed an electron and not anything else, and (2) that the electron energy adds up to about half the mass of the W. The "missing momentum," which the invisible neutrino carries off, can be deduced by adding up all the momentum seen in the event and comparing it to "zero," which is the momentum of the initial state of colliding particles. The discovery was greatly facilitated by the lucky accident that W's are made almost at rest under the CERN collider parameters. To discover a particle, lots of constraints must be satisfied. An important condition is that all the candidate events yield the same value (within allowable measurement errors) for the W mass.

  Rubbia was given the honor of presenting his results to the CERN community, and, uncharacteristically, he was nervous; eight years of work had been invested. His talk was spectacular. He had all the goods and the showmanship to display them with passionate logic(!). Even the Rubbia-haters cheered. Europe had its Nobel Prize, duly given to Rubbia and Van der Meer in 1985.

/>   Some six months after the W success, the first evidence appeared for the existence of the neutral partner the Z zero. With zero electric charge, it decays into, among many possibilities, an e+ and an e− (or a pair of muons, μ+ and μ−). Why? For those who fell asleep during the previous chapter since the Z is neutral, the charges of its decay products must cancel each other out, so particles of opposite signs are logical decay products. Because both electron and muon pairs can be precisely measured, the Z0 is an easier particle to recognize than the W. The trouble is that the Z0 is heavier than the W, and fewer are made. Still, by late 1983, the Z0 was established by both UA-1 and UA-2. With the discovery of the W's and the Z0 and a determination that their masses are just what was predicted, the electroweak theory—which unified electromagnetism and the weak force—was solidly confirmed.

  TOPPING OFF THE STANDARD MODEL

  By 1992, tens of thousands of W's had been collected by UA-1 and UA-2, and the new kid, CDF, at the Fermilab Tevatron. The mass of the W is now known to be about 79.31 GeV. Some two million Z0's were collected by CERN's Z0 factory," LEP (Large Electron-Positron Storage Ring), a seventeen-mile-around electron accelerator. The Z0 mass is measured to be 91.175 GeV.

  Some accelerators became particle factories. The first factories—in Los Alamos, Vancouver, and Zurich—produced pions. Canada is now designing a kaon factory. Spain wants a tau-charm factory. There are three or four proposals for beauty or bottom factories, and the CERN Z0 factory is, in 1992, in full production. At SLAC a smaller Z0 project might more properly be called a loft, or perhaps a boutique.

  Why factories? The production process can be studied in great detail and, especially for the more massive particles, there are many decay modes. One wants samples of many thousands of events in each mode. In the case of the massive Z0, there are a huge number of modes, from which one learns much about the weak and electroweak forces. One also learns from what isn't there. For example, if the mass of the top quark is less than half that of the Z0, then we have (compulsory) Z0 → top + antitop. That is, a Z zero can decay, albeit rarely, into a meson, composed of a top quark lashed to an antitop quark. The Z0 is much more likely to decay into electron pairs or muon pairs or bottom-quark pairs, as mentioned. The success of the theory in accounting for these pairs encourages us to believe that the decay of Z0 into top/antitop is predictable. We say it is compulsory because of the totalitarian rule of physics. If we make enough Zs, according to the probabilities of quantum theory, we should see evidence of the top quark. Yet in the millions of Z0's produced at CERN, Fermilab, and elsewhere, we have never seen this particular decay. This tells us something important about the top quark. It must be heavier than half of the Z0 mass. That's why the Z0 can't produce it.

  WHAT ARE WE TALKING ABOUT?

  A very broad spectrum of hypothetical particles has been proposed by theorists following one trail or another toward unification. Usually the properties of these particles, except for the mass, are well specified by the model. Not seeing these "exotics" provides a lower limit for their mass, following the rule that the larger the mass the harder it is to produce.

  Some theory is involved here. Theorist Lee says: a p/p-bar collision will produce a hypothetical particle—call it the Lee-on—if there is enough energy in the collision. However, the probability or relative frequency of producing the Lee-on depends on its mass. The heavier it is, the less frequently it is produced. The theorist hastens to supply a graph relating the number of Lee-ons produced per day to the particle's mass. For example: mass = 20 GeV, 1,000 Lee-ons (mind-numbing); 30 GeV, 2 Lee-ons; 50 GeV, one thousandth of a Lee-on. In the last case one would have to run the equipment for 1,000 days to get one event, and experimenters usually insist on at least ten events since they have additional problems with efficiency and background. So after a given run, say of 150 days (a year's run), in which no events are found, one looks at the curve, follows it down to where, say, ten events should have been produced—corresponding to a mass of, say, 40 GeV for the Lee-on. A conservative estimate is that some five events could have been missed. So the curve tells us that if the mass were 40 GeV, we would have seen a weak signal of a few events. But we saw nothing. Conclusion: the mass is heavier than 40 GeV.

  What next? If the Lee-on or the top quark or the Higgs is worth the game, one has a choice of three strategies. First, run longer, but this is a tough way to improve. Second, get more collisions per second; that is, raise the luminosity. Right on! That is exactly what Fermilab is doing in the 1990s, with the goal of improving the collision rate by about a hundredfold. As long as there is plenty of energy in the collision (1.8 TeV is plenty), raising the luminosity helps. The third strategy is to raise the energy of the machine, which increases the probability of producing all heavy particles. That's the Super Collider route.

  With the discovery of the W and Z, we have identified six quarks, six leptons, and twelve gauge bosons (messenger particles). There is a bit more to the standard model that we have not yet fully confronted, but before we approach that mystery, we should beat on the model a bit. Writing it as three generations at least gives it a pattern. We note some other patterns, too. The higher generations are successively heavier, which means a lot in our cold world today but wouldn't have been very significant when the world was young and very hot. All the particles in the very young universe had enormous energies—billions and billions of TeV, so a little difference in rest mass between a bottom quark and an up quark wouldn't mean much. All quarks, leptons, and so on were once upon a time on an equal footing. For some reason She needed and loved them all. So we have to take them all seriously.

  The Z0 data at CERN suggest another conclusion: it is very unlikely that we have a fourth or fifth generation of particles. How is that for a conclusion? How could these scientists working in Switzerland, lured by the snow-capped mountains, deep, icy lakes, and magnificent restaurants, come to such a limiting conclusion?

  It's a neat argument. The Z0 has plenty of decay modes, and each mode, each possibility for decay, shortens its life a bit. If there are a lot of diseases, enemies, and hazards, human life is also shortened. But that is a sick analogy. Each opportunity to decay opens a channel or a route for the Z0 to shake this mortal coil. The sum total of all routes determines the lifetime. Let's note that not all Z0's have the same mass. Quantum theory tells us that if a particle is unstable—doesn't live forever—its mass must be somewhat indeterminate. The Heisenberg relations tell us how the lifetime affects the mass distribution: long lifetime, narrow width; short lifetime, broad width. In other words, the shorter the lifetime, the less determinate the mass and the broader the range of masses. The theorists can happily supply us a formula for the connection. The distribution width is easy to measure if you have a lot of Z0's and a hundred million Swiss francs to build a detector.

  The number of produced Z0's is zero if the sum of the e+ and the e− energies at the collision is substantially less than the average Z0 mass of 91.175 GeV. The operator raises the energy of the machine until a low yield of Z0's is recorded by each of the detectors. Increase the machine energy, and the yield increases. It is a repeat of the J/psi experiment at SLAC, but here the width is about 2.5 GeV; that is, one finds a peak yield at 91.175, which decreases to about half on either side, at 89.9 GeV and 92.4 GeV. (If you'll recall, the J/psi width was much narrower: about 0.05 MeV.) The bell-shaped curve gives us a width, which is in effect a lifetime. Every possible Z0 decay mode decreases its lifetime and increases the width by about 0.20 GeV.

  What has this to do with a fourth generation? We note that each of the three generations has a low-mass (or zero-mass) neutrino. If there is a fourth generation with a low-mass neutrino, then the Z0 must include, as one of its decay modes, the neutrino vx and its antiparticle, of this new generation. This possibility would add 0.17 GeV to the width. So the width of the Z0 mass distribution was carefully studied. And it turned out to be exactly what the three-generation standard model had predicted. The data on the width
of the Z0 excludes the existence of a low-mass fourth-generation neutrino. All four LEP experiments chimed in to agree that their data allowed only three neutrino pairs. A fourth generation with the same structure as the other three, including a low- or zero-mass neutrino, is excluded by the Z0 production data.

  Incidentally, the same remarkable conclusion had been claimed by cosmologists years earlier. They based their conclusions on the way neutrons and protons combined to form the chemical elements during an early phase of the expansion and cooling of the universe after that humongous bang. The amount of hydrogen compared to the amount of helium depends (I won't explain) on how many neutrino species there are, and the data on abundances strongly suggested three species. So the LEP research is relevant to our understanding of the evolution of the universe.

  Well, here we are with an almost complete standard model. Only the top quark is missing. The tau neutrino is too, but that is not nearly so serious, as we have seen. Gravity must be postponed until the theorists understand it better and, of course, the Higgs is missing, the God Particle.

  SEARCH FOR TOP

  A NOVA TV program called "Race for the Top" was shown in 1990 when CERN's p-bar/p collider and Fermilab's CDF were both running. CDF had the advantage of three times higher energy, 1.8 TeV against CERN's 620 GeV. CERN, by cooling their copper coils a bit better had succeeded in raising their beam energies from 270 GeV to 310 GeV, squeezing every bit of energy they could in order to be competitive. Still, a factor of three hurts. CERN's advantage was nine years of experience, software development, and know-how in data analysis. Also they had redone the antiproton source, using some of Fermilab's ideas, and their collision rate was slightly better than ours. In 1989–90, the UA-1 detector was retired. Rubbia was now director general of CERN with an eye to the future of his laboratory, so UA-2 was given the task of finding top. An ancillary goal was to measure the mass of the W more precisely, for this was a crucial parameter of the standard model.