This reaction is similar to the way we understand the electrical force, say between two protons; there is an exchange of a neutral messenger, the photon, and this produces the Coulomb law of force, which allows one proton to kick another. There is no change of species. The similarity is not fortuitous. The unification crowd (not the Reverend Moon but Glashow and his friends) needed such a process if they were to have a prayer of unifying the weak and electromagnetic forces.
So the experimental challenge was: can we do reactions in which neutrinos collide with nuclei and come out as neutrinos? A crucial ingredient is that we observe the impact on the struck nucleus. There was some ambiguous evidence of such reactions in our two-neutrino experiment at Brookhaven. Mel Schwartz called them "crappers." A neutral particle goes in; a neutral particle comes out. There's no change in electric charge. The struck nucleus breaks up, but very little energy appears in the relatively low-energy neutrino beam at Brookhaven—hence Schwartz's description. Neutral currents. For reasons I forget, the neutral weak messenger particle is called Z0 (zee zero, we say), rather than W0. But if you want to impress your friends, use the term "neutral currents," a fancy way of expressing the idea that a neutral messenger particle is required to kick off a weak-force reaction.
TIME TO BREATHE FASTER
Let's review a bit of what the theorists were thinking.
The weak force was first recognized by Fermi in the 1930s. When he wrote down his theory, Fermi modeled it in part on the quantum field theory of the electromagnetic force, quantum electrodynamics (QED). Fermi tried to see if this new force would follow the dynamics of the older force, electromagnetism (older, that is, in terms of our knowledge of it). In QED, remember, the field idea is carried by messenger particles, the photons. So the Fermi theory of the weak force should have messenger particles, too. But what would they be like?
The photon has zero mass, and that gives rise to the famous long-range inverse-square law of the electric force. The weak force was very short range, so in effect Fermi simply gave his force carriers infinite mass. Logical. Later versions of the Fermi theory, most notably by Schwinger; introduced the heavy W+ and W− as weak-force carriers. So did several other theorists. Let's see: Lee, Yang, Gell-Mann ... I hate to credit any theorists because 99 percent of them will be upset. If I occasionally neglect to cite a theorist, it's not because I've forgotten. It's probably because I hate him.
Now comes the tricky part. In program music, a recurring theme introduces an idea or person or animal—like the leitmotif in Peter and the Wolf that tells us Peter is about to come onstage. Perhaps more appropriate in this case is the ominous cello that signals the appearance of the great white shark in Jaws. I am about to slip in the first thematic notes of the denouement, the sign of the God Particle. But I don't want to reveal her too early. As in any tease show, slow is better.
In the late sixties and early seventies, several young theorists began to study quantum field theory in the hopes of extending the success of QED to the other forces. You may recall that these elegant solutions to action-at-a-distance were subject to mathematical troubles: quantities that should be small and measurable appear in the equation as infinite—and that's a lot. Feynman and friends invented the process of ›normalization to hide the infinities in the measured quantities, for example, e and m, the charge and mass of the electron. QED was said to be a renormalizable theory; that is, you can get rid of the stultifying infinities. However; when quantum field theory was applied to the other three forces—the weak force, the strong force, and gravity—it met with total frustration. It couldn't have happened to nicer guys. With these forces infinities ran wild, and things got so sick that the entire usefulness of quantum field theory was questioned. Some theorists reexamined QED to try to understand why that theory worked (for electromagnetism) and the other theories did not.
QED, the super-accurate theory that gives the g-value to eleven significant places, belongs to a class of theories known as gauge theories. The term gauge in this context means scale, as in HO-gauge model railroad tracks. Gauge theory expresses an abstract symmetry in nature that is very closely tied to experimental facts. A key paper by C. N. Yang and Robert Mills in 1954 stressed the power of gauge symmetry. Rather than proposing new particles to explain observed phenomena, one sought for symmetries that would predict these phenomena. When applied to QED, gauge symmetry actually generated the electromagnetic forces, guaranteed the conservation of charge, and provided, at no extra cost, a protection against the worst infinities. Theories exhibiting gauge symmetry are renormalizable. (Repeat this sentence until it rolls trippingly from the tongue, then try it out at lunch.) But the gauge theories implied the existence of gauge particles. They were none other than our messenger particles: photons for QED, and W+ and W− for weak. And for strong? Gluons, of course.
Some of the best and brightest theorists were motivated to work on the weak force for two, no three, reasons. The first is that the weak force was full of infinities, and it was not clear how to make it into a gauge theory. Second was the quest for unification, extolled by Einstein and very much on the minds of this group of young theorists. Their focus was on unifying the weak and electromagnetic forces, a daunting task since the weak force is vastly weaker than the electric force, has a much, much shorter range, and violates symmetries such as parity. Otherwise, the two forces are exactly alike!
The third reason was the fame and glory that would accrue to the guy who solved the puzzle. The leading contestants were Steven Weinberg, then at Princeton; Sheldon Glashow, a fellow science fiction club member with Weinberg; Abdus Salam, the Pakistani genius at Imperial College in England; Martinus Veltman at Utrecht, Netherlands; and his student Gerard't Hooft. The more elderly theorists (well into their thirties) had set the stage: Schwinger, Gell-Mann, Feynman. There were lots of others around; Jeffrey Goldstone and Peter Higgs were crucial piccolo players.
Eschewing a blow-by-blow account of the theoretical brouhaha from about 1960 to the mid-1970s, we find that a renormalizable theory of the weak force was finally achieved. At the same time it was found that a marriage with the electromagnetic force, QED, now seemed more natural. But to do all this, one had to assemble a common messenger family of particles for the combined "electroweak" force: W+, W−, Z0, and the photon. (It looks like one of those mixed families, with stepbrothers and stepsisters from previous marriages trying to live, at all odds, in harmony while sharing a common bathroom.) The new heavy particle, Z0, helped to satisfy the demands of gauge theory, and the foursome satisfied all the requirements of parity violation, as well as the apparent weakness of the weak force. Yet at this stage (before 1970) not only hadn't the W's and Z been seen, but neither had the reactions that Z0 might produce. And how can we talk about a unified electroweak force, when any child in the laboratory can demonstrate huge differences in behavior between the electromagnetic and weak forces?
One problem that the experts confronted, each in his own aloneness, at office or home or airplane seat, was that the weak force, being short range, needed heavy force carriers. But heavy messengers are not what gauge symmetry predicted, and the protest came in the form of infinities, sharp steel into the intellectual guts of the theorist. Also, how do three heavies, W+, W−, and Z0, coexist in a happy family with the massless photon?
Peter Higgs, of the University of Manchester (England), supplied a key—yet another particle, to be discussed soon—which was exploited by Steven Weinberg, then at Harvard, now at the University of Texas. Clearly, we plumbers in the lab see no weak-electromagnetic symmetry. The theorists know that, but they desperately want the symmetry in the basic equations. So we are faced with finding a way to install the symmetry, then break it when the equations get down to predicting the results of the experiment. The world is perfect in the abstract, see, but then it becomes imperfect when we get down to details, right? Wait! I didn't think up any of this.
But here's how it works.
Weinberg, via the work of Higgs, had dis
covered a mechanism by which a pristine set of zero-mass messenger particles, representing a unified electroweak force, acquired mass by feeding, in a very poetic manner of speaking, on the unwanted components of the theory. Okay? No? Using Higgs's idea to destroy the symmetry, lo!—the W's and Z's acquired mass, the photon remained the same, and in the ashes of the destroyed unified theory there appeared: the weak force and the electromagnetic force. Massive W's and Z's waddled around to create the radioactivity of particles and the reactions that occasionally interfered with neutrino transits of the universe, whereas the messenger photons gave rise to the electricity we all know, love, and pay for. There. Radioactivity (weak force) and light (electromagnetism) neatly(?) tied to one another. Actually, the Higgs idea didn't destroy the symmetry; it just hid it.
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 tw
o 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 "color," 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."
The God Particle: If the Universe Is the Answer, What Is the Question? Page 42
