Newer unified field theories, however, predict that baryon number will not be strictly conserved. This prediction has stimulated impressive efforts to detect proton decay, so far without success. But it does illustrate the existence of approximate conservation laws. Parity was one example. Strangeness was devised to understand why a number of baryons lived much longer than they should, given all the possible final states into which they could decay. We learned later that strangeness in a particle—lambda or kaon, for example—means the presence of the's quark. But lambda and kaon do decay, and the's quark does change into a lighter d quark in the process. However this involves the weak force—the strong force will have no part of an s → d process; in other words, the strong force conserves strangeness. Since the weak force is weak, the decay of lambda, kaon, and its family members is slow, and the lifetime is long— 10−10 seconds instead of an allowed process that typically takes 10−23 seconds.
The many experimental handles on conservation laws are fortunate, because an important mathematical proof showed that conservation laws are related to symmetries that nature respects. (And symmetry, from Thales to Sheldon Glashow, is the name of the game.) This connection was discovered by Emmy Noether, a woman mathematician, about 1920.
But back to our story.
NIOBIUM BALLS
Despite the omega minus and other successes, no one had ever seen a quark. I'm speaking here in the physicist sense, not the skeptical-lady-in-the-audience sense. Zweig claimed from the beginning that aces/quarks were real entities. But when John Peoples, the current director of Fermilab, was a young experimenter in search of quarks, Gell-Mann told him not to worry about them, that quarks were merely "an accounting device."
Saying this to an experimenter is like throwing down a gaundet. Searches for quarks began everywhere. Of course, any time you put up a "Wanted" sign, false sightings appear. People looked in cosmic rays, in deep ocean sediment, in old, fine wine ('Shno quarks here, hic!) for a funny electric charge trapped in matter. All the accelerators were used in attempts to smash quarks out of their prisons. A charge of ⅓ or ⅔ would have been relatively easy to find, but still most searches came up empty. One Stanford University experimenter, using tiny, precisely engineered balls made of pure niobium, reported trapping a quark. The experiment languished when it couldn't be repeated, and disrespectful undergrads wore T-shirts inscribed "You have to have niobium balls if you want to trap quarks."
Quarks were spooky; the failure to find free quarks and the ambivalence of the original concept slowed the acceptance of the concept until the late sixties, when a different class of experiments demanded quarks, or at least quarklike things. Quarks were invented to explain the existence and classification of the huge number of hadrons. But if a proton had three quarks, why didn't they show up? Well, we gave it away earlier. They can be "seen." It's Rutherford all over again.
"RUTHERFORD" RETURNS
A series of scattering experiments was undertaken using new electron beams at SLAC in 1967. The objective was a more incisive study of the structure of the proton. The electron at high energy goes in, hits a proton in a hydrogen target, and an electron of much lower energy comes out, but at a large angle to its initial path. The pointlike structures inside the proton act in some sense as the nucleus did for Rutherford's alpha particles. The issue here, however, was more subtle.
The Stanford team, led by SLAC physicist Richard Taylor, a Canadian, and two MIT physicists, Jerome Friedman and Henry Kendall, were enormously aided by the theoretical kibitzing of Richard Feynman and James Bjorken. Feynman had been lending his energy and imagination to the strong interactions and in particular to "what's inside the proton?" He was a frequent visitor to Stanford from his base at Cal Tech in Pasadena. Bjorken (everyone calls him "Bj"), a Stanford theorist, was intensely interested in the experimental process and in the rules underlying seemingly inchoate data. These rules, Bjorken reasoned, would be indicators of the basic laws (inside the black box) controlling the structure of the hadrons.
Here we have to go back to our good friends Democritus and Boscovich, both of whom shed light on the subject. Democritus's test for an a-tom is that it must be indivisible. In the quark model the proton is actually a gooey agglomerate of three quickly moving quarks. But because those quarks are always inextricably tethered to one another, experimentally the proton appears indivisible. Boscovich added a second test. An elementary particle, or a-tom, must be pointlike. This test the proton fails decidedly. The MIT-SLAC team, with assists from Feynman and Bj, came to realize that the operative criterion in this instance was "points" rather than indivisibility. Translating their data into a model of pointlike constituents required much more subtlety than Rutherford's experiment did. That's why it was so convenient to have two of the world's best theorists on the team. The outcome was that the data did indeed indicate the presence of pointlike moving objects inside the proton. In 1990, Taylor, Friedman, and Kendall picked up their Nobel for establishing the reality of quarks. (They are the scientists referred to by Jay Leno at the beginning of the chapter.)
A good question: how can these guys see quarks when quarks are never free? Consider a sealed box with three steel balls inside. You shake the box, tilt it in various ways, listen, and conclude: three balls. The more subtle point is that quarks are always detected in proximity to other quarks, which may change their properties. This factor had to be dealt with but... piano, piano.
The quark theory made more converts, especially as theorists watching the data began imbuing the quarks with increasing reality, adding to their properties and converting the inability to see free quarks into a virtue. "Confinement" became the buzzword. Quarks are permanently confined because the energy required to separate quarks increases as the distance between quarks increases. Then, as one tries harder the energy becomes sufficient to create a quark-antiquark pair, and now we have four quarks, or two mesons. It's like trying to take home one end of a string. One snips it and, oops, two strings.
Reading quark structure out of electron-scattering experiments was very much a West Coast monopoly. I must note, however that very similar data were being collected at the same time by my group at Brookhaven. I've often joked that if Bjorken had been an East Coast theorist, I would have discovered quarks.
The two contrasting experiments at SLAC and Brookhaven demonstrate that there is more than one way to skin a quark. In both experiments the target particle was a proton. But Taylor Friedman, and Kendall were using electrons as probes, and we were using protons. At SLAC they sent electrons into the "black box of the collision region" and measured the electrons coming out. Lots of other things, such as protons and pions, also came out, but these were ignored. At Brookhaven we were colliding protons on a piece of uranium (going after the protons therein) and concentrating on pairs of muons coming out, which we measured carefully. (For those of you who haven't been paying attention, electrons and muons are both leptons with identical properties except that the muon is two hundred times heavier.)
I said earlier that the SLAC experiment was similar to Rutherford's scattering experiment that revealed the nucleus. But Rutherford simply bounced alpha particles off the nucleus and measured the angles. At SLAC the process was more complicated. In the language of the theorist and in the mental image evoked by the mathematics, the incoming electron in the SLAC machine sends a messenger photon into the black box. If the photon has the right properties, it can be absorbed by one of the quarks. When the electron tosses a successful messenger photon (one that gets eaten), the electron alters its energy and motion. It then leaves the black box area and goes out and gets itself measured. In other words, the energy of the outgoing electron tells us something about the messenger photon it threw, and, more important, what ate it. The pattern of messenger photons could be interpreted only as being absorbed by a pointlike substructure in the proton.
In the dimuon experiment (so called because it produces two muons) at Brookhaven, we send high-energy protons into the black b
ox region. The energy from the proton stimulates a messenger photon to be radiated from the black box. This photon, before leaving the box, converts into a muon and its antimuon, and these particles leave the box and get measured. This tells us something about the properties of the messenger photon, just as the SLAC experiment did. However, the muon-pair experiment was not theoretically understood until 1972 and, indeed, required many other subtle proofs before its unique interpretation was given.
This interpretation was first done by Sidney Drell and his student Tung Mo Yan at Stanford, not surprisingly, where quarks ran in the blood. Their conclusion: the photon that generates our muon pair is generated when a quark in the incoming proton collides with and annihilates an antiquark in the target (or the other way around). This is widely known as the Drell-Yan experiment even though we invented it and Drell "merely" found the right model.
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 poi
nts, 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 this 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.
The God Particle: If the Universe Is the Answer, What Is the Question? Page 39
