THE SCREAM OF THE QUARK
The pattern of hadrons, arranged by assorted quantum numbers, also screamed for substructure. It isn't easy, however, to hear the screams of subnuclear entities. Two keen-eared physicists did, and wrote about it. Gell-Mann proposed the existence of what he referred to as mathematical structures. In 1964 he postulated that the patterns of organized hadrons could be explained if three "logical constructs" existed. He called these constructs "quarks." It is generally assumed that he lifted the word from James Joyce's diabolical novel Finnegans Wake ("Three quarks for Muster Mark!"). George Zweig, a colleague of Gell-Mann's, had an identical idea while working at CERN; he named his three things "aces."
We will probably never know precisely how this seminal idea came about. I know one version because I was there—at Columbia University in 1963. Gell-Mann was giving a seminar on his Eightfold Way symmetry of hadrons when a Columbia theorist, Robert Serber, pointed out that one basis for the "eight" organization would involve three subunits. Gell-Mann agreed, but if these subunits were particles they would have the unheard-of property of having third-integral electric charges—⅓, ⅔, −⅓, and so on.
In the particle world, all electric charges are measured in terms of the charge on the electron. All electrons have exactly 1.602193 × 10−19 coulombs. Never mind what coulombs are. Just know that we use the previous complicated figure as a unit of charge and call it 1 because it's the charge on the electron. Conveniently, the proton's charge is also 1.0000, as is that of the charged pion, the muon (here the precision is much higher), and so on. In nature, charges come in integers—0, 1, 2 ... All the integers are understood to be multiples of the number of coulombs given above. Charges also come in two styles: plus and minus. We don't know why. That's the way it is. One might imagine a world in which the electron could, in a bruising collision or in a poker game, lose 12 percent of its electric charge. Not in this world. The electron, proton, pi plus, et al. always have charges of 1.0000.
So when Serber brought up the idea of particles with third-integral charges—forget it. Such things had never been seen, and the rather curious fact that all observed charges were equal to an integral multiple of a unique, unchanging standard charge became, over time, incorporated into the intuition of physicists. This "quantization" of electric charge was in fact used to seek some deeper symmetry that would account for it. However, Gell-Mann reconsidered and proposed the quark hypothesis, simultaneously blurring the issue, or so it seemed to some of us, by suggesting that quarks aren't real but are convenient mathematical constructs.
The three quarks born in 1964 are today called "up," "down," and "strange," or u, d, and's. There are, of course, three antiquarks: and . The properties of the quarks had to be delicately chosen so that they could be used to build all of the known hadrons. The u quark is given a charge of +⅔ the d quark is -⅓ as is the's quark. The antiquarks have equal but opposite charges. Other quantum numbers are also selected so that they add up correctly. For example, the proton is made of three quarks—uud—with charges +⅔, +⅔, and -⅓, the sum being +1.0, which jibes with what we know about the proton. The neutron is a udd combination, with charges +⅔, -⅓, -⅓, for a sum of 0.0, which makes sense because the neutron is neutral, zero charge.
All hadrons consist of quarks, sometimes three and sometimes two, according to the quark model. There are two classes of hadrons: baryons and mesons. Baryons, which are relatives of protons and neutrons, are three-quark jobs. Mesons, which include pions and kaons, consist of two quarks—but they must be a quark combined with an and quark. An example is the positive pion (π+), which is ud. The charge is +⅔ +⅓, which is equal to 1. (Note that the d-bar, the antidown quark, has a charge of +⅓.)
In fashioning this early hypothesis, the quantum numbers of the quarks, and properties such as spin, charge, isospin, and so on, were fixed in order to account for just a few of the baryons (proton, neutron, lambda, and so on) and mesons. Then these numbers and other relevant combinations were found to fit all the hundreds of known hadrons. It all worked! And all the properties of a composite—for example, a proton—are subsumed by the properties of the constituent quarks, moderated by the fact that they are in intimate interaction with one another. At least, that is the idea and the task for generations of theorists and generations of computers, given, of course, that they are handed the data.
Quark combinations raise an interesting question. It is a human trait to modify one's behavior in company. However; as we shall see, quarks are never alone, so their true unmodified properties can only be deduced from the variety of conditions under which we can observe them. In any case, here are some typical quark combinations and the hadrons they produce:
BARYONS MESONS
uud proton positive pion
udd neutron dū negative pion
uds lambda neutral pion
uus sigma plus positive kaon
dds sigma minus sū negative kaon
uds sigma zero neutral kaon
dss xi minus neutral antikaon
uss xi zero
Physicists gloried in the spectacular success of reducing hundreds of seemingly basic objects to composites of just three varieties of quarks. (The term "aces" faded—no one can compete with Gell-Mann when it comes to naming.) The test of a good theory is whether it can predict, and the quark hypothesis, guarded or not, was a brilliant success. For example, the combination of three strange quarks, sss, was not among the record of discovered particles, but that didn't stop us from giving it a name: omega minus (Ω−). Because particles containing the strange quark had established properties, the properties of a hadron with three strange quarks, sss, would also be predictable. The omega minus was a very strange particle with a spectacular signature. In 1964 it was discovered in a Brookhaven bubble chamber and was exactly what Dr. Gell-Mann had ordered.
Not that all issues were settled—not by a long shot. Lots of questions: for starters, how do quarks stick together? This strong force would be the subject of thousands of theoretical and experimental papers over the next three decades. The jawbreaking title "quantum chromodynamics" would propose a new breed of messenger particles, gluons, to cement(!!) quarks together. All in due course.
CONSERVATION LAWS
In classical physics there are three great conservation laws: energy, linear momentum, and angular momentum. They have been shown to be deeply related to concepts of space and time, as we will see in Chapter 8. Quantum theory introduced a great number of additional quantities that are conserved; that is, they do not change during a variety of subnuclear, nuclear and atomic processes. Examples are electric charge, parity, and a host of new properties like isospin, strangeness, baryon number, and lepton number. We have already learned that the forces of nature differ in their respect for different conservation laws; for example, parity is respected by the strong and electromagnetic forces but not by the weak force.
To test a conservation law, one examines a huge number of reactions in which a particular property, say the electric charge, can be ascertained before and after the reaction. We recall that energy conservation and momentum conservation were so solidly established that when certain weak processes appeared to violate them, the neutrino was postulated as a saving mechanism, and it was right. Other clues to the existence of a conservation law have to do with the refusal of certain reactions to take place. For example, an electron does not decay with two neutrinos because that would violate charge conservation. Another example is proton decay. Recall that it doesn't. Protons are assigned a baryon number that is ultimately derived from its three-quark structure. So protons, neutrons, lambdas, sigmas, and so on—all three-quark fellows—have baryon number +1. The corresponding antiparticles have baryon number -1. All mesons, force carriers, and leptons have baryon number 0. If baryon number is strictly conserved, then the lightest baryon, the proton, can never decay, since all the lighter decay-product candidates have baryon number 0. Of course, a proton-antiproton collision has total baryon number
0 and can give rise to anything. So baryon number "explains" why the proton is stable. The neutron, decaying into a proton, an electron, and an antineutrino, and the proton inside the nucleus, which is able to decay into a neutron, a positron, and a neutrino, conserve baryon number.
Pity the guy who lives forever. The proton can't decay into pions because it would violate baryon number conservation. It can't decay into a neutron and a positron and a neutrino because of energy conservation. It can't decay into neutrinos or photons because of charge conservation. There are more conservation laws, and we feel that the conservation laws shape the world. As should be obvious, if the proton could decay it would threaten our existence. Of course, that does depend on the proton's lifetime. Since the universe is fifteen or so billion years old, a lifetime much longer than this would not influence the fate of the Republic too much.
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 boun
ced 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 box 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.
The God Particle Page 39