Let's backtrack quickly to Becquerel and Rutherford. Recall that Becquerel had serendipitously discovered radioactivity in 1896 when he stored some uranium in a drawer where he kept his photographic paper. When the photographic paper came out black, he eventually traced the cause to invisible rays shooting out of the uranium. After the discovery of radioactivity and the elucidation by Rutherford of alpha, beta, and gamma radiation, many physicists the world over concentrated on the beta particles, which were soon identified as electrons.
Where did the electrons come from? Physicists very quickly figured out that the electron was emitted from the nucleus when it underwent a spontaneous change of state. In the 1930s researchers determined that nuclei consist of protons and neutrons, and traced the radioactivity of nuclei to the instability of their constituent protons and neutrons. Obviously, not all nuclei are radioactive. The conservation of energy and the weak force play important roles in whether and how readily a proton or a neutron decays in a nucleus.
In the late 1920s careful before-and-after measurements of radioactive nuclei were made. One measures the mass of the initial nucleus, the mass of the final nucleus, and the energy and mass of the emitted electron (remembering that E = mc2). And here an important discovery was made: it didn't add up. Energy was missing. The input was bigger than the output. Wolfgang Pauli made his (then) daring suggestion that a small neutral object was carrying the energy away.
In 1933 Enrico Fermi put it all together. The electrons were coming from the nucleus, but not directly. What happens is that the neutron in the nucleus decays into a proton, an electron, and the small neutral object that Pauli had invented. Fermi named it the neutrino, meaning "little neutral one." A force is responsible for this reaction in the nucleus, said Fermi, and he called it the weak force. It is enormously feeble compared to the strong nuclear force and electromagnetism. For example, at low energy the weak force is about one thousandth the strength of electromagnetism.
The neutrino, having no charge and almost no mass, could not be directly detected in the 1930s; it can be detected today only with great effort. Though the neutrino's existence was not proven experimentally until the 1950s, most physicists accepted it as a fact because it had to exist to make the bookkeeping come out right. In today's more exotic reactions in accelerators, involving quarks and other weird things, we still assume that any missing energy flies out of the collision in the form of undetectable neutrinos. This artful little dodger seems to leave its invisible signature all over the universe.
But back to the weak force. The decay that Fermi described—neutron gives way to proton, electron, and neutrino (actually, an antineutrino)—occurs routinely with free neutrons. When the neutron is imprisoned in the nucleus, however, it can happen only under special circumstances. Conversely, the proton as a free particle cannot decay (as far as we know). Inside the crowded nucleus, however the bound proton can give rise to a neutron, a positron, and a neutrino. The reason that the free neutron can undergo weak decay is simple energy conservation. The neutron is heavier than the proton, and when a free neutron changes into a proton there is enough additional rest mass energy to make the electron and the antineutrino and send each of them off with a little energy. A free proton has too little mass to do this. However inside the nucleus the presence of all the other guys in effect alters the mass of a bound particle. If the protons and neutrons inside can, by decaying, increase the stability and lower the mass of the nucleus in which they are stuck, they do it. However if the nucleus is already in its lowest mass-energy state, it is stable and nothing happens. It turns out that all the hadrons—the protons, neutrons, and their hundreds of cousins—are induced to decay via the weak force, with the free proton being the only apparent exception.
The theory of the weak force was gradually generalized and, in constant confrontation with new data, evolved to a quantum field theory of the weak force. A new breed of theorists emerging mostly in American universities helped to mold the theory: Feynman, Gell-Mann, Lee, Yang, Schwinger Robert Marshak, and many others. (I keep having this nightmare in which all the theorists I've failed to cite meet in a suburb of Teheran and offer a reward of prompt admission to Theory Heaven for anyone who instantly and totally renormalizes Lederman.)
SLIGHTLY BROKEN SYMMETRY, OR WHY WE ARE ALL HERE
A crucial property of the weak force is parity violation. All the other forces respect this symmetry; that one force can violate it was a shock. Another deep symmetry, one that compares the world to the anti-world, had been demonstrated to fail by the same experiments that showed P (parity) violation. This second symmetry was called C, for charge conjugation. The failure of C symmetry also occurred only with the weak force. Before C violation was demonstrated, it was thought that a world in which all objects are made of antimatter would obey the same laws of physics as the regular old matter world. No, said the data. The weak force doesn't respect that symmetry.
What were the theorists to do? They quickly retreated to a new symmetry: CP symmetry. This says that two physical systems are essentially identical if one is related to the other by simultaneously reflecting all objects in a mirror (P) and also changing all particles to antiparticles (C). CP symmetry, the theorists said, is a much deeper symmetry. Even though nature does not respect C and P separately, simultaneous CP symmetry must endure. It did until 1964 when Val Fitch and James Cronin, two Princeton experimenters studying neutral kaons (a particle my group discovered in Brookhaven experiments in 1956–1958), came upon clear and compelling data that CP symmetry was, in fact, not perfect.
Not perfect? The theorists sulked, but the artist in all of us rejoiced. Artists and architects love to tweak us with canvases or architectural structures that are almost, but not exactly, symmetrical. The asymmetric towers in the otherwise symmetric cathedral at Chartres is a good example. The CP violation effect was small—a few events out of a thousand—but clear, and theorists were back to square one.
I mention CP violation for three reasons. First, it is a good example of what became recognized, in the other forces, as "slightly broken symmetry." If we believe in the intrinsic symmetry of nature, something, some physical agency, must enter to break that symmetry. A closely related agency doesn't actually destroy the symmetry, it just hides it so that nature appears to be asymmetrical. The God Particle is such a disguiser of symmetry. We will return to it in Chapter 8. The second reason for mentioning CP violation is that in the 1990s understanding this concept is one of the most pressing needs for clearing up the problems in our standard model.
The final reason, and the element that brought the Fitch-Cronin experiment to the respectful attention of the Royal Swedish Academy of Science, is that when applied to cosmological models of the evolution of the universe, CP violation explained a puzzle that had plagued astrophysicists for fifty years. Before 1957 a large number of experiments indicated perfect symmetry between matter and antimatter. If matter and antimatter are so symmetric, why is our planet, our solar system, our galaxy, and, evidence indicates, all other galaxies devoid of antimatter? And how could an experiment carried out on Long Island in 1965 explain it all?
Models indicated that as the universe cooled after the Big Bang, all the matter and antimatter annihilated, leaving essentially pure radiation, ultimately too cool—too low in energy—to create matter. But matter, that's us! Why are we here? The Fitch-Cronin experiment shows the way out. The symmetry isn't perfect. A slight excess of matter over antimatter (for every 100 million quark-antiquark pairs there is one extra quark) is a result of the slightly broken CP symmetry, and this tiny excess accounts for all the matter in the presently observed universe, including us. Thanks Fitch, thanks Cronin. Splendid fellows.
TRAPPING THE LITTLE NEUTRAL ONE
Much of the detailed information on the weak force was provided by neutrino beams, and herein lies another story. Pauli's 1930 hypothesis—that a small, neutral particle exists that feels only the weak force—was tested in many ways from 1930 to 1960. Precise measurements of an
increasingly large number of weakly decaying nuclei and particles tended to confirm the hypothesis that a little neutral thing was escaping from the reaction carrying away energy and momentum. This was a convenient way to understand decay reactions, but could we actually detect neutrinos?
This was no easy task. Neutrinos float through vast thicknesses of matter unscathed because they obey only the weak force, whose short range reduces the probability of a collision enormously. It was estimated that to ensure a collision of a neutrino with matter would require a target of lead one light-year thick! Quite an expensive experiment. However; if we use a very large number of neutrinos, the required thickness to see a collision every once in a while is correspondingly reduced. In the mid-1950s, nuclear reactors were used as intense sources of neutrinos (so much radioactivity!), to which a huge vat of cadmium dichloride (cheaper than a light-year's worth of lead) was exposed. With so many neutrinos (actually, antineutrinos, which is mostly what you get from reactors), it was inevitable that some of them would strike protons, causing inverse beta decay; that is, a positron and a neutron were released. The positron, in its wandering, would eventually find an electron and annihilate into two oppositely moving photons. These fly outward into dry cleaning fluid, which flashes when struck by the photons. The detection of a neutron and a pair of photons represented the first experimental evidence of the neutrino, about thirty-five years after Pauli thought up the critter.
By 1959 another crisis, two in fact, arose to tweak the physicist's mind. The center of the storm was at Columbia University, but the crisis was liberally shared and appreciated around the world. All of the data on the weak force to that time were kindly provided by particles during natural decay. Greater love hath no particle than to give its all for the edification of physicists. To study the weak force we simply watched particles, such as the neutron or the pion, decay into other particles. The energies involved were provided by the rest masses of the decaying particles—typically from a few MeV to around 100 MeV or so. Even the free neutrinos shooting out of reactors and undergoing weak-force collisions involved only a few MeV. After we had modified the weak-force theory with the experimental results of parity violation, we had one zinger of an elegant theory that fit all the available data provided by zillions of nuclear decays as well as the decays of pions, muons, lambdas, and probably, though difficult to prove, Western civilization.
THE EXPLODING EQUATION
Crisis No. 1 had to do with the mathematics of the weak force. In the equations, the energy at which the force is measured appears. Depending on the data, you stick in the rest mass energy of the decaying particle—1.65 MeV or 37.2 MeV or whatever—and out comes the right answer. You manipulate the terms, bump and grind, and, sooner or later out come predictions as to the lifetimes, decays, spectra of electrons—things that can be compared to experiment—and they are right. But if one puts in, say, 100 GeV (billion electron volts), the theory goes haywire. The equation explodes in your face. In the jargon of physics, this is called "the unitarity crisis."
Here's the dilemma. The equation was okay, but it had a pathology at high energy. Little numbers worked; big ones didn't. We didn't have the ultimate truth, only a truth valid for the low-energy domain. There had to be some new physics that modified the equations at high energy.
Crisis No. 2 was the mystery of the unobserved reaction. One could calculate how often a muon decayed into an electron and a photon. Our theory of the weak processes said that this should happen. Looking for this reaction was a favorite Nevis experiment, and several new Ph.D.'s spent godknowshowmany beam hours searching with no success. Murray Gell-Mann, the pundit on all matters arcane, is often quoted as the source of something called the Totalitarian Rule of Physics: "Anything that isn't forbidden is compulsory." If our laws do not rule out an event, it not only can happen! it must happen! Since a muon decaying into an electron and a photon was not forbidden, why weren't we seeing it? What forbade this mu-e-gamma decay? (For "gamma" read "photon.")
Both crises were exciting. Both offered up the possibility of new physics. Theoretical speculations abounded, but experimental blood boiled. What to do? We experimenters must measure, hammer, saw, file, stack lead bricks—do something. So we did.
MURDER INC. AND THE TWO-NEUTRINO EXPERIMENT
Melvin Schwartz, an assistant professor at Columbia, after listening to a detailed review of the troubles by Columbia theorist T. D. Lee in November 1959 came up with his GREAT IDEA. Why not create a beam of neutrinos by letting a high-energy pion beam drift through enough space that some fraction, say 10 percent, of the pions decayed into a muon and a neutrino. Pions, in flight, would disappear; muons and neutrinos, sharing the pion's original energy, would appear. So here, flying through space, we have muons and neutrinos from the 10 percent of pions that decayed, plus the 90 percent of pions that didn't decay, plus a bunch of nuclear debris originating from the target that produced the pions. Now, said Schwartz, let's aim it all into a big thick wall of steel, forty feet thick, as it turned out. The wall would stop everything but the neutrinos, which would have no trouble passing through forty million miles of steel. We'd have a pure beam of neutrinos on the other side of the wall, and since the neutrino obeys only the weak force, we'd have a handy way of studying both the neutrino and the weak force via neutrino collisions.
The scheme addressed both Crisis No. 1 and Crisis No. 2. Mel's idea was that this neutrino beam would allow us to study the weak force at energies of billions rather than millions of electron volts. It would give us a view of the behavior of the weak force at high energy. It might also provide some ideas on why we don't see muons decay into electrons plus photons, based on the notion that neutrinos are somehow involved.
As happens so often in science, an almost equivalent idea was published almost simultaneously by a Soviet physicist, Bruno Pontecorvo. If the name seems more Italian than Russian, it is because Bruno is an Italian who defected to Moscow in the 1950s on ideological grounds. His physics, ideas, and imagination were nevertheless outstanding. Bruno's tragedy was in trying to carry out his imaginative ideas within a system of stultifying bureaucracy. International conferences are venues for displaying the traditional warm friendship of scientists. At one such conference in Moscow, I asked a friend, "Yevgeny, tell me, which one of you Russian physicists is really a communist?" He looked around the hall and pointed to Pontecorvo. But that was in 1960.
When I returned to Columbia from a pleasant sabbatical at CERN in late 1959, I listened to the discussions about crises in the weak force, including Schwartz's idea. Schwartz had somehow concluded that no existing accelerator was powerful enough to make a sufficiently intense neutrino beam, but I disagreed. The 30 GeV AGS (for Alternating Gradient Synchrotron) was nearing completion at Brookhaven, and I did the numbers and convinced myself and then Schwartz that the experiment was, in fact, doable. We designed what was, for 1960, a huge experiment. Jack Steinberger, a colleague at Columbia, joined us and with students and postdocs we formed a group of seven. Jack, Mel, and I were well known for our gentle and kindly demeanor. Once as we were walking across the Brookhaven accelerator floor I overheard a physicist in a group exclaim, "There goes Murder; Incorporated!"
To block all the particles except the neutrinos, we made a thick wall around a massive detector, using thousands of tons of steel from outdated naval vessels. I once made the mistake of telling a reporter that we took apart the battleship Missouri to make the wall. I must have gotten the name wrong, because the Missouri is apparently still out there someplace. But we certainly had a battleship cut up for scrap. I also made the mistake of joking that if there was a war we'd have to paste the ship back together, and that story got embellished and pretty soon there was a rumor that the navy had confiscated our experiment to fight some war (what war this could have been—it was 1960—remains a puzzle).
What is also somewhat fabricated is my story about the cannon. We got a twelve-inch naval cannon with a suitable bore and thick walls—it made a beauti
ful collimator, a device for focusing and aiming a beam of particles. We wanted to fill it up with beryllium as a filter but the bore had these deep rifling grooves. So I sent a skinny graduate student inside to stuff steel wool into the grooves. He spent about an hour in there and crawled out all hot, sweaty, and irritated and said, "I quit!" "You can't quit," I cried. "Where will I find another student of your caliber?"
Once our preparations were finished, steel from obsolete ships surrounded a detector made from ten tons of aluminum tastefully arranged so that if neutrinos collided with an aluminum nucleus, the products of the collision would be observed. The detector idea we eventually used, called a spark chamber, had been invented by a Japanese physicist, Shuji Fukui. We learned a lot by talking to Jim Cronin of Princeton who had mastered the new technique. Schwartz won the ensuing contest as to the best design that could be scaled up from a few pounds to ten tons. In this spark chamber, nicely machined one-inch-thick plates of aluminum were spaced about a half inch apart and a huge voltage difference applied between adjacent plates. If a charged particle passed through the gap, a spark would follow the trail of the particle and could be photographed. How easily this is said! The technique was not without its technical problems. But the results! Zap—and the path of a subnuclear particle was rendered visible in the red-yellow light of glowing neon gas. It was a lovely device.
We built models of spark chambers and put them in beams of electrons and pions to learn their characteristics. Most chambers of that day were about a foot square and had ten to twenty plates. The design we set about had one hundred plates, each four feet square. Each plate was one inch thick, pleading with the neutrinos to collide. Seven of us worked day and night as well as other times to assemble the apparatus and the electronics, inventing all sorts of devices—hemispherical spark gaps, automated gluing facilities, circuitry. We had help from engineers and several technicians.
The God Particle Page 37