The God Particle

by Leon Lederman

  Only one question remained. Why would anyone believe any of this mathematical gobbledegook? Well, Tini Veltman (far from tiny) and Gerard 't Hooft had worked the same ground, perhaps more thoroughly, and had shown that if you did the (still mysterious) Higgs trick to break the symmetry, all the infinities that had characteristically lacerated the theory vanished, and the theory was squeaky clean. Renormalized.

  Mathematically, a whole set of terms appeared in the equations with signs such as to cancel terms that were traditionally infinite. But there were so many such terms! To do this systematically, 't Hooft wrote a computer program and, on a day in July 1971, watched the output as complicated integrals were subtracted from other complicated integrals. Each of these, if evaluated separately, would give an infinite result. As the readout emerged, term by term the computer printed "0." The infinities were all gone. This was't Hooft's thesis, and it must go down with de Broglie's as a Ph.D. thesis that made history.

  FIND THE ZEE ZERO

  Enough for theory. Admittedly, it's complicated stuff. But we'll return to it later, and a firm pedagogical principle acquired from forty or so years of facing students—freshmen to postdocs—says that even if the first pass is 97 percent incomprehensible, the next time you see it, it will be, somehow, hauntingly familiar.

  What implications did all this theory have for the real world? The grand implications will have to wait for Chapter 8. The immediate implication in 1970 for experimenters was that a Z0 had to exist to make everything work. And if the Z0 was a particle, we should find it. The Z0 was neutral, like its stepsister the photon. But unlike the massless photon, Z0 was supposed to be very heavy like its brothers, the twin W's. So our task was clear: look for something that resembles a heavy photon.

  W's had been searched for in many experiments, including several of mine. We looked in neutrino collisions, didn't see any, and asserted that failure to find the W could be understood only if the mass of the W was greater than 2 GeV. Had it been lighter, it would have shown up in our second series of neutrino experiments at Brookhaven. We looked in proton collisions. No W. So now its mass had to be greater than 5 GeV. Theorists also had opinions about the W properties and kept raising the mass until, by the late seventies, it was predicted to be about 70 GeV. Way too high for the machines of that era.

  But back to Z0. A neutrino scatters from a nucleus. If it sends out a W+ (an antineutrino will send out a W−), it changes to a muon. But if it can send out a Z0, then it remains a neutrino. As mentioned, since there is no change of electric charge as we follow the leptons, we call it a neutral current.

  A real experiment to detect neutral currents isn't easy. The signature is an invisible neutrino coming in, an equally invisible neutrino going out, along with a cluster of hadrons resulting from the struck nucleon. Seeing only a cluster of hadrons in your detector isn't very impressive. It's just what a background neutron would do. At CERN a giant bubble chamber called Gargamelle began operating in a neutrino beam in 1971. The accelerator was the PS, a 30 GeV machine that produced neutrinos of about 1 GeV. By 1972 the CERN group was hot on the trail of muonless events. Simultaneously the new Fermilab machine was sending 50 GeV neutrinos toward a massive electronic neutrino detector managed by David Cline (University of Wisconsin), Alfred Mann (University of Pennsylvania), and Carlo Rubbia (Harvard, CERN, northern Italy, Alitalia...).

  We can't do full justice to the story of this discovery. It's full of sturm und drang, human interest, and the sociopolitics of science. We'll skip all that and simply say that by 1973 the Gargamelle group announced, somewhat tentatively, the observation of neutral currents. At Fermilab the Cline-Mann-Rubbia team also had so-so data. Obfuscating backgrounds were serious, and the signal was not one than knocked you on your rear. They decided they had found neutral currents. Then they withdrew. Then decided again. A wag dubbed their efforts "alternating neutral currents."

  By the 1974 Rochester Conference (a biennial international meeting) in London, it was all clear: CERN had discovered neutral currents, and the Fermilab group had convincing confirmation of this signal. The evidence indicated that "something like a Z0" had to exist. But if we go strictly by the book, although neutral currents were established in 1974, it took another nine years to prove directly the existence of Z0. CERN got the credit, in 1983. The mass? Z0 was indeed heavy: 91 GeV.

  By mid-1992, incidentally, the LEP machine at CERN had registered more than 2 million Z0's, collected by its four huge detectors. Studying the production and the subsequent decay of Z0's is providing a bonanza in data and keeps some 1,400 physicists busy. Recall that when Ernest Rutherford discovered alpha particles, he then explained them and went on to use them as a tool to discover the nucleus. We did the same thing with neutrinos; and neutrino beams, as we've just seen, have become an industry also, useful for finding messenger particles, studying quarks, and a number of other things. Yesterday's fantasy is today's discovery is tomorrow's device.

  The Strong Force Revisited: Gluons

  We needed one more discovery in the 1970s to complete the standard model. We had the quarks, but they bind together so strongly that there's no such thing as a free quark. What is the binding mechanism? We called on quantum field theory, but the results were once again frustrating. Bjorken had elucidated the early experimental results at Stanford in which electrons were bounced off the quarks in the proton. Whatever the force was, the electron scattering indicated that it was surprisingly weak when the quarks were close together.

  This was an exciting result because one wanted to apply gauge symmetry here, too. Gauge theories could predict the counterintuitive idea that the strong force gets very weak at close approach and stronger as the quarks move apart. The process, discovered by some kids, David Politzer at Harvard and David Gross and Frank Wilczek at Princeton, carried a name that would be the envy of any politician: asymptotic freedom. Asymptotic roughly means "getting closer and closer but never touching." Quarks have asymptotic freedom. The strong force gets weaker and weaker as one quark approaches a second quark. What this means, paradoxically, is that when quarks are close together they behave almost as if they are free. But when they are farther apart, the forces get effectively stronger. Short distances imply high energies, so the strong force gets weaker at high energies. This is just the opposite of the electrical force. (Things do get curiouser, said Alice.) More important, the strong force needed a messenger particle like the other forces. Somewhere the messenger acquired the name gluon. But to name it is not to know it.

  Another idea, rattling around in the theoretical literature, is relevant now. Gell-Mann named this one. It's called color—or colour in Europe—and it has nothing to do with color as you and I recognize it. Color explains certain experimental results and predicts others. For example, it explained how a proton could have two up quarks and a down quark, when the Pauli principle specifically excluded two identical objects in the same state. If one of the up quarks is blue, and the other is green, we satisfy Pauli's rule. Color gives the strong force the equivalent of electric charge.

  Color must come in three types, said Gell-Mann and others who had worked in this garden. Remember that Faraday and Ben Franklin had determined that electric charge comes in two styles, designated plus and minus. Quarks need three. So now all quarks come in three colors. Perhaps the color idea was stolen from the palette because there are three primary colors. A better analogy might be that electric charge is one-dimensional, with plus and minus directions, and color is three-dimensional (three axes: red, blue, and green). Color explained why quark combinations are, uniquely, either quark plus antiquark (mesons) or three quarks (baryons). These combinations show no color; the quarkness vanishes when we stare at a meson or a baryon. A red quark combines with an antired antiquark to produce a colorless meson. The red and antired cancel. Likewise, the red, blue, and green quarks in a proton mix to make white (try this by spinning a color wheel). Again colorless.

  Even though these are nice reasons for using the word "co
lor," it has no literal meaning. We are describing another abstract property that the theorists gave to quarks to account for the increasing amount of data. We could have used Tom, Dick, and Harry or A, B, and C, but color was a more appropriate (colorful?) metaphor. So color along with quarks and gluons, seemed to be forever a part of the black box, abstract entities that won't make a Geiger counter click, will never leave a track in a bubble chamber, will never tickle wires in an electronic detector.

  Nevertheless, the concept that the strong force gets weaker as quarks approach one another was exciting from the point of view of further unification. As the distance between particles decreases, their relative energy increases (small distance implies high energy). This asymptotic freedom implies that the strong force gets weaker at high energy. The unification seekers were then given the hope that at sufficiently high energy, the strength of the strong force may approach that of the electroweak force.

  And what about the messenger particles? How do we describe the color-force-carrying particles? What emerged was that gluons carry two colors—a color and another anticolor—and, in their emission or absorption by quarks, they change the quark color. For example, a red-antiblue gluon changes a red quark to an antiblue quark. This exchange is the origin of the strong force, and Murray the Great Namer dubbed the theory quantum chromodynamics (QCD) in resonance with quantum electrodynamics (QED). The color-changing task means that we need enough gluons to make all possible changes. It turns out that eight gluons will do it. If you ask a theorist, "Why eight?" he'll wisely say, "Why, eight is nine minus one."

  Our uneasiness with the fact that quarks were never seen outside of hadrons was only moderately tempered by a physical picture of why quarks are permanently confined. At close distances, quarks exert relatively weak forces on one another. This is the glory domain for theorists, where they can calculate properties of the quark state and the quark's influence on collision experiments. As the quarks separate, however, the force becomes stronger, and the energy required to add distance between them rises rapidly until, long before we have actually separated the quarks, the energy input results in the creation of a new quark-antiquark pair. This curious property is a result of the fact that gluons are not simple, dumb messenger particles. They actually exert forces on each other. This is where QED differs from QCD, since photons ignore each other.

  Still, QED and QCD had many close analogies, especially in the high-energy domain. QCD's successes were slow in coming, but steady. Because of the fuzzy long-distance part of the force, calculations were never very precise, and many experiments would conclude with the rather nebulous statement that "our results are consistent with the predictions of QCD."

  So what kind of a theory do we have if we can never, ever see a free quark? We can do experiments that sense the presence of electrons and measure them, this way and that, even when they are all bound up in atoms. Can we do the same with quarks and gluons? Bjorken and Feynman had suggested that in very hard collisions of particles, the energized quarks would initially head out and, just before leaving the influence of their quark partners, would mask themselves into a narrow bundle of hadrons—three or four or eight pions, for example, or add some kaons and nucleons. These would be narrowly directed along the path of the parent quark. They were given the name "jets," and the search was on.

  With the machines of the 1970s, these jets were not easily distinguished because all we could produce were slower quarks that gave rise to broad jets of a small number of hadrons. We wanted dense, narrow jets. The first success belonged to a young woman experimentalist named Gail Hanson, a Ph.D. from MIT working at SLAC. Her careful statistical analysis revealed that a correlation of hadrons did appear in the debris of a 3 GeV e+ e− collision at SPEAR. She was helped by the fact that what went in were the electrons and what came out were a quark and an antiquark, back to back to conserve momentum. These correlated jets showed up, barely but decisively, in the analysis. When Democritus and I were sitting in the CDF control room, needlelike bundles of ten or so hadrons, two jets 180 degrees apart, were flashed on the large screen every few minutes. There is no reason why there should be such a structure unless the jet is the offspring of a very high energy, very high momentum quark, which dresses itself before going out.

  But the major discovery of the 1970s along these lines was made at the PETRA e+ e− machine in Hamburg, Germany. This machine, colliding at the total energy of 30 GeV, also showed, without need for analysis, the two-jet structure. Here one could almost see the quarks in the data. But something else was also seen.

  One of the four detectors on-line at PETRA had its own acronym: TASSO, for Two-Armed Solenoidal Spectrometer. The TASSO group was looking for events in which three jets would appear. A consequence of QCD theory is that when e+ and e− annihilate to produce a quark and an antiquark, there is a reasonable probability that one of the outgoing quarks will radiate a messenger particle, a gluon. There is enough energy here to convert "virtual" gluon to real gluon. The gluons share the quarks' shyness and, like quarks, dress themselves before leaving the black box of the encounter domain. Therefore three jets of hadrons. But this takes more energy.

  In 1978, runs of total energy of 13 and 17 GeV came out empty, but at 27 GeV, something happened. The analysis was pushed by another woman physicist, Sau Lan Wu, a professor at the University of Wisconsin. Wu's program soon uncovered more than forty events in which there were three jets of hadrons, each jet having three to ten tracks (hadrons). The array looked like the hood ornament of a Mercedes.

  The other PETRA groups soon got on the bandwagon. Looking through their data, they also found the three-jet events. A year later, thousands had been collected. The gluon had thus been "seen." The pattern of tracks was calculated by theorist John Ellis at CERN using QCD, and one must credit his intervention in motivating the search. The announcement of the gluon's detection was made at a conference at Fermilab in the summer of 1979, and it was my job to go on the Phil Donahue television show in Chicago to explain the discovery. I put more energy into explaining that the Fermilab buffalo were not roaming the lab as early warning devices for dangerous radiation. But in physics, the real news was the gluons—the bosons, not the bisons.

  So now we have all the messenger particles, or gauge bosons as they are more eruditely called. ("Gauge" came from gauge symmetry, and boson is derived from the Indian physicist S. N. Bose, who described the class of particles with integer values of spin.) Whereas the matter particles all have spin of ½ and are called fermions, the messenger particles all have spin 1 and are bosons. We've skipped over some details. The photon, for instance, was predicted by Einstein in 1905 and observed experimentally by Arthur Compton in 1923, using x-rays scattered from atomic electrons. Although neutral currents had been discovered in the mid-1970s, the Ws and Z's were not directly observed until 1983–84, when they were detected in the CERN hadron collider. As mentioned, the gluons were pinned down by 1979.

  In this long discussion of the strong force, we should note that we define it as the quark-quark force carried by gluons. But what about the "old" strong force between neutrons and protons? We now understand this as the residual effects of the gluons, sort of leaking out of the neutrons and protons that bind together in the nucleus. The old strong force that is well described by exchange of pions is now seen as a consequence of the complexities of quark-gluon processes.

  END OF THE ROAD?

  Entering the 1980s, we had figured out all the matter particles (quarks and leptons), and we had the messenger particles, or gauge bosons, of the three forces (excluding gravity) pretty much in hand. Adding the force particles to the matter particles, you have the complete standard model, or SM. Here, then, is the "secret of the universe":

  MATTER

  First generation Second generation Third generation

  QUARKS

  u c t?

  d s b

  LEPTONS

  νe νμ ντ

  e μ τ

  FORCES


  GAUGE BOSONS

  electromagnetism photon (γ)

  weak force W− W+ Z0

  strong force eight gluons

  Remember that the quarks come in three colors. So if one is nasty, one can count eighteen quarks, six leptons, and twelve gauge boson force carriers. There is also an and table in which all the matter particles appear as antiparticles. That would give you sixty particles total. But who's counting? Stick to the above table; it's all you need to know. At last we believe we have Democritus's a-toms. They are the quarks and leptons. The three forces and their messenger particles account for his "constant violent motion."

  It may seem arrogant to sum up our entire universe in a chart, albeit a messy one. Yet humans appear to be driven to construct such syntheses; "standard models" have been a recurrent theme in Western history. The current standard model wasn't given that name until the 1970s, and the term is peculiar to the recent modern history of physics. But certainly there have been other standard models through the centuries. The next page shows just a few of them.

  Why is our standard model incomplete? One obvious flaw is that the top quark hasn't yet been seen. Another is that one of the forces is missing: gravity. No one knows how to work this grand old force into the model. Another aesthetic flaw is that it's not simple enough—it should look more like Empedocles' earth, air, fire, and water plus love and strife. There are too many parameters in the standard model, too many knobs to twiddle.