My palms started to sweat as I reviewed what we had to do. The counters all existed. The electronics that signaled the arrival of the high-energy muon and the entrance into the graphite block of the now slowed muon were already in place and well tested. A "telescope" of four counters for detecting the electron that emerged after muon decay also existed. All we had to do was mount these on a board of some sort that we could pivot around the center of the stopping block. One or two hours' work. Wow! I decided that it would be a long night.
When I stopped at home for a quick dinner and some bantering with the kids, a telephone call came from Richard Garwin, a physicist with IBM. Garwin was doing research in atomic processes at the IBM research labs, which were then just off the Columbia campus. Dick hung around the Physics Department a lot, but he had missed the Chinese lunch and wanted to know the latest on Wu's experiment.
"Hey, Dick, I've got a great idea on how we can test for parity violation in the simplest way you can imagine." I explained hastily and said, "Why don't you drive over to the lab and give us a hand?" Dick lived nearby in Scarsdale. By 8 P.M. we were disassembling the apparatus of one very confused and upset graduate student. Marcel saw his Ph.D. thesis experiment being taken apart! Dick was assigned the job of thinking through the problem of rotating the electron telescope so we could determine the distribution of electrons around the assumed spin axis. This wasn't a trivial problem, since wrestling the telescope around could change the distance to the muons and thus alter the yield of detected electrons.
It was then that the second key idea was invented, by Dick Garwin. Look, he said, instead of moving this heavy platform of counters around, let's leave it in place and turn the muons in a magnet. I gasped as the simplicity and elegance of the idea penetrated. Of course! A spinning charged particle is a tiny magnet and will turn like a compass needle in a magnetic field, except that the mechanical forces acting on the muon-magnet make it rotate continuously. The idea was so simple it was profound.
It was a piece of cake to calculate the value of the magnetic field needed to turn the muons through 360 degrees in a reasonable time. What is a reasonable time to a muon? Well, the muons are decaying into electrons and neutrinos with a half-life of 1.5 microseconds. That is, half of the muons have given their all in 1.5 microseconds. If we turned the muons too slowly, say 1 degree per microsecond, most of the muons would have disappeared after being rotated through a few degrees and we wouldn't be able to compare the zero-degree and 180-degree yield—that is, the number of electrons emitted from the "top" of the muon as opposed to the "bottom," the whole point of our experiment. If we increased the turning rate to, say, 1,000 degrees per microsecond by applying a strong magnetic field, the distribution would whiz past the detector so fast we would have a blurred-out result. We decided that the ideal rate of turning would be about 45 degrees per microsecond.
We were able to obtain the required magnetic field by winding a few hundred turns of copper wire on a cylinder and running a current of a few amperes through the wire. We found a Lucite tube, sent Marcel to the stockroom for wire, cut the graphite stopping block down so it could be wedged inside the cylinder, and hooked the wires to a power supply that could be controlled remotely (there was one on the shelf). In a blur of late-night activity, we had everything ready by midnight. We were in a hurry because the accelerator was always turned off at 8 A.M. Saturday for maintenance and repairs.
By 1 A.M. the counters were recording data; accumulation registers recorded the number of electrons emitted at various directions. But remember with Garwin's scheme, we didn't measure these angles directly. The electron telescope remained stationary while the muons or, rather, their spin axis vectors, were rotated in a magnetic field. So the electrons' time of arrival now corresponded to their direction. By recording the rime, we were recording the direction. Of course, we had lots of problems. We badgered the accelerator operators to give us as many protons hitting the target as possible. All the counters registering the muons coming in and stopping had to be adjusted. The control of the small magnetic field applied to the muons had to be checked.
After a few hours of data taking, we saw a remarkable difference in the counts of electrons emitted at zero degrees and those emitted at 180 degrees relative to the spin. The data were very crude, and we mixed excited optimism with skepticism. When we examined the data at eight the next morning, our skepticism was confirmed. The data now were much less convincing, not really inconsistent with the hypothesis that all directions of emission were equivalent—a predictor of mirror symmetry. We had pleaded with the accelerator operators to give us an additional four hours, but to no avail. Schedules are schedules. Discouraged, we walked down to the accelerator room, where the apparatus was set up. There we were greeted by a small catastrophe. The Lucite cylinder on which we had wound the wire had become warped due to the heat produced by the current in the wires. This warping had permitted the stopping block to fall. Obviously, the muons were no longer in the magnetic field we had designed for them. After some recriminations (blame the graduate student!) we cheered up. Our original impression might still be correct!
We made a plan for the weekend. Design a proper magnetic field. Think about increasing the data rate by increasing the number of muons stopping and the fraction of the decay electrons counted. Think about what happens to the positively charged muons in their collisions on the way down to rest and in the microseconds in which they sit in the lattice of carbon atoms. After all, if a positive muon managed to capture one of the many electrons that are free to move about in graphite, the electron could easily depolarize (mess up the spin of) the muon so that they would not all be doing the same thing in lockstep.
The three of us went home to sleep for a few hours before reassembling at 2 P.M. We worked through the weekend, each at an assigned task. I managed to recalculate the motion of the muon from birth as it is kicked forward by its decaying pion parent, through its sweep toward the channel and through the concrete wall into our apparatus. I kept track of spin and direction. I assumed maximum violation of mirror symmetry so that all the muons would be spinning precisely along in the direction of their motion. Everything indicated that if the violation was large, even half of maximum, we should see an oscillating curve. This not only would prove parity violation but would give us a numerical result as to how much parity was violated, from 100 percent down to (no! no!) zero. Anyone who tells you that scientists are dispassionate and coldly objective is crazy. We desperately yearned to see parity violated. Parity was not a young lady, and we weren't teenagers, but we lusted to make a discovery. The test of scientific objectivity is not to let the passion influence the methodology and the self-criticism.
Eschewing the Lucite cylinder Garwin wound a coil directly on a new piece of graphite and tested the system at currents twice as high as we would need. Marcel rearranged the counters, improved the alignment, moved the electron telescope closer to the stopping block, tested, and improved the efficiency of all counters, all the while praying that something publishable would come out of this frantic activity.
The work went slowly. By Monday morning, some news of our intense activity had leaked out to the operator crew and to some of our colleagues. The accelerator maintenance gang found some serious problems in the machine, so Monday was out—no beam until Tuesday, 8 A.M. at best. Okay, more time to fume, fuss, check. Colleagues from the Columbia campus arrived at Nevis, curious as to what we were up to. One clever young man who had been at the Chinese lunch asked a few questions and, by my disingenuous answers, deduced that we were trying the parity experiment.
"It'll never work," he assured me. "The muons will depolarize as they lose energy in the graphite filter." I was easily depressed but not discouraged. I remembered my mentor, the great Columbia savant 1.1. Rabi, telling us: spin is a very slippery thing.
About 6 P.M. on Monday, ahead of schedule, the machine began to show signs of life. We hastened our preparations, checking all the devices and arrangements. I n
oticed that the target with its elegant copper wire wrapping, positioned on a four-inch slab, looked a bit low. Some squinting through a surveying scope convinced me, and I looked for something that would raise it an inch or so. Over in the corner I saw a Maxwell House coffee can partially filled with wood screws, and I substituted it for the four-inch slab. Perfect! (When the Smithsonian Institution later wanted the coffee can in order to replicate the experiment, we couldn't find it.)
The loudspeaker announced that the machine was about to be turned on and that all experimenters must leave the accelerator room (or get fried). We scrambled up the steep iron staircase and across the parking lot to the lab building, where the cables from the detectors were connected to electronic racks containing circuitry, scalers, oscilloscopes. Garwin had gone home hours ago, and I sent Marcel to get some dinner while I started a checkout procedure on the electronic signals arriving from the detectors. A large, thick lab notebook was used to note all relevant information. It was gaily embellished with graffiti—"Oh shit!" "Who the hell forgot to turn off the coffee pot?" "Your wife called"—as well as the necessary record of things to do, things done, conditions of the circuits. ("Watch scaler No. 3. It tends to spark and miss counts.")
By 7:15 P.M. the proton intensity was up to standard and the pion-producing target was moved remotely into position. Instantly, the scalers began registering arriving particles. I looked at the crucial row of scalers that would register the number of electrons emitted at various intervals after the muons had stopped. The numbers were still very small: 6, 13, 8...
Garwin arrived at about 9:30 P.M. I decided to get some sleep and relieve him at 6 the next morning. I drove home very slowly. I had been up for about twenty hours and was too tired to eat. It seemed as if i had just hit the pillow when the phone rang. The clock said 3 A.M. It was Garwin. "You'd better come in. We've done it!"
At 3:25 I parked at the lab and dashed in. Garwin had pasted paper strips of the scaler read-outs in the book. The numbers were devastatingly clear. More than twice as many electrons were emitted at zero degrees as at 180 degrees. Nature could tell the difference between a right-handed spin and a left-handed spin. By now the machine had come up to its best intensity, and the scaler registers were changing rapidly. The scaler corresponding to zero degrees was reading 2,560, the scaler corresponding to 180 degrees was reading 1,222. On a purely statistical basis this was overwhelming. The in-between scalers seemed satisfactorily in between. The implications of parity violation on this level were so vast ... I looked at Dick. My breathing was becoming difficult, my palms were wet, my heartbeat accelerated, I felt lightheaded—many (not all!) of the symptoms of sexual arousal. This was big stuff. I began to make a checklist: what elements could fail in such a way as to simulate the result we were seeing? There were so many possibilities. We spent an hour, for example, checking the circuits used to count the electrons. No problem. How else could we test our conclusions?
Tuesday, 4:30 A.M. We asked the operator to shut down the beam. We ran down and physically rotated the electron telescope through 90 degrees. If we knew what we were doing, the pattern should shift by a time interval corresponding to 90 degrees. Bingo! The pattern shifted as we had predicted!
6 A.M. I picked up the telephone and called T. D. Lee. He answered after one ring. "T. D., we've been looking at the pi-mu-e chain and we now have a twenty-standard-deviation signal. The law of parity is dead." T. D.'s reaction squirted through the telephone. He asked rapid-fire questions: "What energy electrons? How did the asymmetry vary with electron energy? Was the muon spinning parallel to the direction of arrival?" To some questions we had answers. Others came later in the day. Garwin began drawing graphs and entering the scaler readings. I made another list of things we had to do. At seven we started getting calls from Columbia colleagues who had heard. Garwin faded by eight. Marcel (temporarily forgotten!) arrived. By nine the room was crowded with colleagues, technicians, secretaries trying to find out what was going on.
It was hard to keep the experiment going. My breathing and sweating symptoms returned. We were the repository of new and profound information about the world. Physics was changed. And the violation of parity had given us a powerful new tool: polarized muons that were responsive to magnetic fields and whose spins could be tracked through the electron decay.
The phone calls from Chicago, California, and Europe came over the next three or four hours. People with particle accelerators in Chicago, Berkeley, Liverpool, Geneva, and Moscow swarmed to their machines like pilots rushing to their wartime battle stations. We continued the experiment and continued the process of checking our assumptions for a solid week, but we were desperately anxious to publish. We took data, in one form or another, twenty-four hours a day, six days a week, for the next six months. Data poured out. Other labs soon confirmed our results.
C. S. Wu was of course less than delighted by our clean, unequivocal result. We wanted to publish with her but, to her everlasting credit, she insisted she still needed a week to check her results.
It is difficult to express just how startling the results of this experiment were to the physics community. We had challenged—in fact, destroyed—a cherished belief, that nature exhibits mirror symmetry. In later years, as we shall see, other symmetries were also disproved. Even so, the experiment shook up many theorists, including Wolfgang Pauli, who made the famous statement "I cannot believe God is a weak left-hander." He didn't mean that God should be right-handed, but that She should be ambidextrous.
The annual meeting of the American Physical Society drew 2,000 physicists to the ballroom of the Hotel Paramount in New York on February 6,1957. People hung from rafters. Front-page articles in all the major newspapers heralded the result. The New York Times published our press release verbatim, with pictures of particles and mirrors. But none of this matched the 3 A.M. feeling of mystical euphoria when two physicists came to know a new and profound truth.
7. A-TOM!
Yesterday three scientists won the Nobel Prize for finding the smallest object in the universe. It turns out that it's the steak at Denny's.
—Jay Leno
THE 1950S AND '60S were great years for science in America. Compared to the much tougher 1990s, in the '50s anyone with a good idea and a lot of determination, it seemed, could get his idea funded. Perhaps this is as good a criterion for healthy science as any. The nation is still benefiting from the science that got done in these decades.
The flood of subnuclear structures opened up by the particle accelerator was as surprising as the heavenly objects revealed by Galileo's telescope. As in the Galilean revolution, mankind acquired new, previously unsuspected knowledge about the world. That this knowledge concerned inner rather than outer space made it no less profound. Pasteur's discovery of microbes and the invisible biological universe of microorganisms is an analogous event. The bizarre guess of our hero Democritus ("Guess?!" I hear him screeching. "Guess?!?!") was no longer even remarked upon. That there was a particle so small that it eluded the human eye was not a matter for further debate. Clearly, the search for the smallest particle called for extensions of the human eye: glasses, microscopes, now particle accelerators zooming down in quest of the true a-tom. And what we saw were hadrons, lots of hadrons, those Greek-letter particles created in the strong collisions induced by accelerator beams.
This is not to say that the proliferation of hadrons was an unalloyed pleasure. It did make for full employment, spreading the wealth so that the discoverers of new particles now made up a nonexclusive club. Want to find a brand-new hadron? Just wait for the next accelerator run. At a conference on the history of physics at Fermilab in 1986, Paul Dirac recounted how difficult it was for him to accept the consequences of his equation—the existence of a new particle, the positron, which Carl Anderson discovered a few years later. In 1927 it was counter to the ethos of physics to think so radically. When Victor Weisskopf remarked from the audience that in 1922 Einstein had speculated about the existence of a positive ele
ctron, Dirac waved his hand dismissively: "He was lucky." In 1930 Wolfgang Pauli had agonized before predicting the existence of the neutrino. He finally embraced the particle with great reluctance and only to favor a lesser evil, since nothing less was at stake than the principle of conservation of energy. Either the neutrino had to exist, or the conservation of energy had to go. This conservatism toward the introduction of new particles didn't last. As Professor Bob Dylan commented, the times they were a-changin'. Pioneer of the change in philosophy was theorist Hideki Yukawa, who began the process of freely postulating new particles to explain new phenomena.
In the 1950s and early '60s theorists were busy classifying the hundreds of hadrons, seeking patterns and meaning in this new layer of matter and hounding their experimental colleagues for more data. These hundreds of hadrons were exciting, but they were a headache as well. Where was the simplicity we had been seeking since the days of Thales, Empedocles, and Democritus? There was an unmanageable zoo of these entities, and we were beginning to fear that their legions were infinite.
In this chapter, we shall see how the dream of Democritus, Boscovich, and others was finally realized. We will chronicle the construction of the standard model, which contains all the elementary particles needed to make all the matter in the universe, past or present, plus the forces that act upon these particles. In some ways it is more complex than Democritus's model, in which each form of matter had its own indivisible a-tom, and the a-toms joined together because of their complementary shapes. In the standard model, the matter particles bind to each other via three different forces carried by yet more particles. All of these particles interact with each other in an intricate kind of dance, which can be described mathematically but cannot be visualized. Yet in some ways the standard model is simpler than Democritus ever imagined. We don't need a separate a-tom for feta cheese, one for kneecaps, another for broccoli. There are only a small number of a-toms. Combine them in various ways, and you can make anything. We've already met three of these elementary particles, the electron, the muon, and the neutrino. Soon we'll meet the others and see how they all fit together.
The God Particle Page 35