The "downfall of parity," as the events of January 1957 were later described, is a quintessential example of how physicists think, how they adapt to shock, how theory and mathematics bend to the winds of measurement and observation. What is far from typical about this story is the speed and relative simplicity of the discovery.
THE SHANGHAI CAFE
Friday, January 4, 12 noon. Friday was our traditional Chinese-lunch day, and the faculty of the Columbia University Physics Department gathered outside the office of Professor Tsung Dao Lee. Between ten and fifteen physicists trooped down the hill from the 120th Street Pupin Physics Building to the Shanghai Café on 125th and Broadway. The lunches started in 1953, when Lee arrived at Columbia from the University of Chicago with a fairly new Ph.D. and a towering reputation as a theoretical superstar.
What characterized the Friday lunches was uninhibited noisy conversations, sometimes three or four simultaneously, punctuated by the very satisfactory slurping of winter melon soup and the sharing out of the dragon meat phoenix, shrimp balls, sea cucumbers, and other spicy exotica of northern Chinese cuisine, not yet trendy in 1957. Already on the walk down, it was clear that this Friday the theme would be parity and the hot news from our Columbia colleague C. S. Wu, who was conducting an experiment at the Bureau of Standards in Washington.
Before entering into the serious business of lunch discussion, T. D. Lee carried out his weekly chore of composing the lunch menu on a small pad offered by the respectful waiter-manager. T. D. composes a Chinese menu in the grand manner. It is an art form. He glances at the menu, at his pad, fires a question in Mandarin at the waiter, frowns, poises his pencil over the pad, carefully calligraphs a few symbols. Another question, a change in one symbol, a glance at the embossed tin ceiling for divine guidance, and then a flurry of rapid writing. A final review: both hands are poised over the pad, one with fingers outstretched, conveying the blessings of the pope on the assembled throng, the other holding the stub of a pencil. Is it all there? The yin and the yang, the color, texture, and flavor in proper balance? Pad and pencil are handed to the waiter, and T. D. plunges into the conversation.
"Wu telephoned and said her preliminary data indicated a huge effect!" he said excitedly.
***
Let's return to the laboratory (the real world as She made it) with one wall a mirror. Our normal experience is that whatever we hold up to the mirror, whatever experiments we do in the lab—scattering, production of particles, gravity experiments like Galileo's—all the mirror-lab reflections will conform to the same laws of nature that govern in the lab. Let's see how a violation of parity would show up. The simplest objective test of handedness, one we could communicate to inhabitants of the planet Twilo, employs a right-handed machine screw. Facing the slotted end, turn the screw "clockwise." If the screw advances into a block of wood, it is defined as right-handed. Obviously the mirror view shows a left-handed screw because the mirror guy is turning it counterclockwise, but it still advances. Now suppose we live in a world so curious (some Star Trek universe) that it is impossible—against the laws of physics—to make a left-handed screw. Mirror symmetry would break down; the mirror image of a right-handed screw could not exist; and parity would be violated.
This is the lead-in to how Lee and his Princeton colleague Chen Ning Yang proposed to examine the validity of the law for weak-force processes. We need the equivalent of a right-handed (or left-handed) particle. Like the machine screw, we need to combine a rotation and a direction of motion. Consider a spinning particle—call it a muon. Picture it as a cylinder spinning around its axis. We have rotation. Since the ends of the cylinder-muon are identical, we cannot say whether it is spinning clockwise or counterclockwise. To see this, place the cylinder between you and your favorite antagonist. While you swear it is rotating to the right, clockwise, she insists that it is rotating to the left. And there is no way to resolve the dispute. This is a parity-conserving situation.
The genius of Lee and Yang was to bring in the weak force (which they wanted to examine) by watching the spinning particle decay. One decay product of the muon is an electron. Suppose nature dictates that the electron comes off only one end of the cylinder. This gives us a direction. And we can now determine the sense of rotation—clockwise or counterclockwise—because one end is defined (the electron comes off here). This end plays the role of the point of the machine screw. If the sense of spin rotation relative to the electron is right-handed, like the sense of the machine screw relative to its point, we have defined a right-handed muon. Now if these particles always decay in such a way as to define right-handedness, we have a particle process that violates mirror symmetry. This is seen if we align the spin axis of the muon parallel to our mirror. The mirror image is a left-handed muon— which doesn't exist.
***
The rumors about Wu had begun over the Christmas break, but the Friday after New Year was the first gathering of the Physics Department since the holidays. In 1957 Chien Shiung Wu, like me a professor of physics at Columbia, was quite a well-established experimental scientist. Her specialty was the radioactive decay of nuclei. She was tough on her students and postdocs, exceedingly energetic, careful in evaluating her results, and much appreciated for the high quality of the data she published. Her students (behind her back) called her Generalissimo Mme. Chiang Kai-shek.
When Lee and Yang challenged the validity of parity conservation in the summer of 1956, Wu went into action almost immediately. She selected as the object of her study the radioactive nucleus of cobalt-60, which is unstable. The cobalt-60 nucleus changes spontaneously into a nucleus of nickel, a neutrino, and a positive electron (a positron). What one "sees" is that the cobalt nucleus suddenly shoots off a positive electron. This form of radioactivity is known as beta decay, because the electrons, whether negative or positive, emitted during the process were originally called beta particles. Why does this happen? Physicists call it a weak interaction, and think of a force operating in nature that generates these reactions. Forces not only push and pull, attract and repel, but are also capable of generating changes of species, such as the process of cobalt changing to nickel and emitting leptons. Since the 1930s a large number of reactions have been attributed to the weak force. The great Italian-American Enrico Fermi was the first to put the weak force into a mathematical form, enabling him to predict many details of reactions such as that which occurs with cobalt-60.
Lee and Yang, in their 1956 paper called "The Question of Parity Conservation in the Weak Force," selected a number of reactions and examined the experimental implications of the possibility that parity—mirror symmetry—was not respected by the weak force. They were interested in the directions in which the emerging electron is ejected from a spinning nucleus. If the electron favored one direction over another, that would be like dressing the cobalt nuclei in buttoned shirts. One could tell which was the real experiment, which was a mirror image.
What is it that differentiates a great idea from a routine piece of scientific work? Analogous questions can be asked about a poem, a painting, a piece of music—in fact, gasp and choke, even a legal brief. In the case of the arts, it is the test of time that ultimately decides. In science, experiment determines whether an idea is "right." If it is brilliant, a new area of research is opened, a host of new questions are generated, and a large number of old questions are put to bed.
T. D. Lee's mind worked in subtle ways. In ordering a lunch or in commenting on some old Chinese pottery or on the abilities of a student, his remarks all had hard edges, like a cut precious stone. In Lee and Yang's parity paper (I didn't know Yang that well), this crystalline idea had many sharp sides. To question a well-established law of nature takes a lot of Chinese chutzpah. Lee and Yang realized that all of the vast amount of data that had led to the "well-established" parity law was irrelevant to that piece of nature that caused radioactive decay, the weak force. This was another brilliant, sharp edge: here, for the first time to my knowledge, the different forces of n
ature were permitted to have different conservation laws.
Lee and Yang rolled up their sleeves, poured perspiration on their inspiration, and examined a large number of radioactive decay reactions that represented likely candidates for a test of mirror symmetry. Their paper provided laboriously detailed analyses of likely reactions so dumb experimentalists could test the validity of mirror symmetry. Wu devised a version of one of these, using the cobalt reaction. The key to her approach was to make sure that the cobalt nuclei—or at least a very good fraction of them—were spinning in the same sense. This, Wu argued, could be ensured by running the cobalt-60 source at very low temperatures. Wu's experiment was extremely elaborate, requiring hard-to-find cryogenic apparatus. This led her to the Bureau of Standards, where the technique of spin alignment was well developed.
***
The next to last course that Friday was a large carp braised in black bean sauce with scallions and leeks. It was during this serving that Lee reiterated the key information: the effect Wu was observing was large, more than ten times larger than expected. The data were rumored, tentative, and therefore very preliminary but (T. D. served me the fish head, knowing I liked the cheeks) if the effect was that large, it was just what we would expect if neutrinos were two-component ... I lost the rest of his excitement because an idea had started growing in my own mind.
After lunch there was a seminar, some departmental meetings, a social tea, and a colloquium. In all of these activities I was distracted, bugged by the notion that Wu was seeing a "big effect." From Lee's talk at Brookhaven in August I remembered that the effects produced by the suggested violations of parity when pions and muons decayed were assumed to be minuscule.
Big effect? I had looked briefly in August at the "pi-mu" (pion-muon) chain of decays and had realized that to design a reasonable experiment one would need to have parity violation in two sequential reactions. I kept recalling the calculations we had done in August before deciding that the experiment was borderline or less in chances of success. However, if the effect was large...
By 6 P.M. I was in my car heading north to dinner at home in Dobbs Ferry and then to a quiet evening shift with my graduate student at the nearby Nevis Lab in Irvington-on-Hudson. The Nevis 400 MeV accelerator was a workhorse for producing and studying the properties of mesons, relatively new particles in the 1950s. In those happy days, there were very few mesons to worry about, and Nevis worried about pions as well as muons.
At Nevis we had intense beams of pions coming off a target bombarded by protons. The pions were unstable, and during their flight from the target, out of the accelerator, through the shielding wall, and out into the experimental hall, some 20 percent would undergo the weak decay into a muon and a neutrino.
π → μ + ν (in flight)
The muons generally traveled in the same direction as the parent pion. If the parity law was violated, there would be an excess of muons with spin axis aligned in the direction of the motion of the muon over muons with spin axes pointing, say, opposite to the flight. If the effect was large, nature could be providing us with a sample of particles all spinning in the same sense. This is the situation Wu had to organize by cooling cobalt-60 to extremely low temperatures in a magnetic field. The key was to watch those muons whose direction of spin axis was known to decay into an electron plus some neutrinos.
THE EXPERIMENT
The heavy traffic on the drive north on the Saw Mill River Parkway on Friday evening tends to obscure the lovely forested hills that line this road, which winds along the Hudson River, past Riverdale, Yonkers, and points north. It was somewhere on this road that the implications of the "big effect" possibility dawned upon me. In the case of a spinning object, if any direction of the spin axis is favored in the decay, that is the effect. A small effect might be 1,030 electrons emitted in one direction relative to the spin axis versus 970 in the other, and this would be very difficult to determine. But a big effect, say 1,500 versus 500, would be much easier to find, and the same fortunate bigness would help in organizing the spins of the muons. To do the experiment, we need a sample of muons all spinning in the same direction. Since they will be moving from the cyclotron to our apparatus, the direction of motion of the muons becomes a reference for the muon spin. We need most of the muons to be right-handed (or left, it doesn't matter), now using the direction of motion as a "thumb." Muons will arrive, pass through a few counters, and stop in a carbon block. Then we count how many electrons are emerging in the direction in which the muons were moving against how many are emerging in the opposite direction. A significant difference would be proof of parity violation. Fame and fortune!
Suddenly, my usual Friday night calm was destroyed by the thought that we could trivially do the experiment. My graduate student, Marcel Weinrich, had been working on an experiment involving muons. His setup, with simple modifications, could be used to look for a big effect. I reviewed the way muons were created in the Columbia accelerator. In this I was a sort of expert, having worked with John Tinlot on the design of external pion and muon beams some years ago when I was a brash graduate student and the machine was brand new.
In my mind I visualized the entire process: the accelerator, a 4,000-ton magnet with circular pole pieces about twenty feet in diameter sandwiches a large stainless steel evacuated box, the vacuum chamber. A stream of protons is injected via a tiny tube in the center of the magnet. The protons spiral outward as strong radio-frequency voltages kick them on each turn. Near the end of their spiral trip, the particles have an energy of 400 MeV. Near the edge of the chamber almost at the place where we would run out of magnet, a small rod carrying a piece of graphite waits to be bombarded by the energetic protons. Their 400 million volts is enough to create new particles—pions—as they collide with a carbon nucleus in the graphite target.
In my mind's eye I could see the pions spewing forward from the momentum of the proton's impact. Born between the poles of the powerful cyclotron magnet, they sweep in a gradual arc toward the outside of the accelerator and do their dance of disappearance; muons appear in their place, sharing the original motion of the pions. The rapidly vanishing magnetic field outside the pole pieces helps to sweep the muons through a channel in a ten-foot-thick concrete shield and into the experimental hall where we are waiting.
In the experiment Marcel had been setting up, muons would be slowed down in a three-inch-thick filter and then be brought to rest in one-inch-thick blocks of various elements. The muons would lose their energy via gentle collisions with the atoms in the material and, being negative, would finally be captured by the positive nuclei. Since we did not want anything to influence the muons' direction of spin, capture into orbits could be fatal, so we switched to positive muons. What would positively charged muons do? Probably just sit there in the block spinning quietly until their time came to decay. The material of the block would have to be chosen carefully, and carbon seemed appropriate.
Now comes the key thought of the driver heading north on a Friday in January. If all (or almost all) of the muons, born in the decay of pions, could somehow have their spins aligned in the same direction, it would mean that parity is violated in the pion-to-muon reaction and violated strongly. A big effect! Now suppose the axis of spin remained parallel to the direction of motion of the muon as it swept through its graceful arc to the outside of the machine, through the channel. (If g is close to 2, this is exactly what happens.) Suppose further that the innumerable gentle collisions with carbon atoms, which gradually slowed the muon, did not disturb this relationship of spin and direction. If all this were indeed to happen, mirabile dictu! I would have a sample of muons coming to rest in a block all spinning in the same direction!
The muon's lifetime of two microseconds was convenient. Our experiment was already set up to detect the electrons that emerge from the decaying muons. We could try to see if equal numbers of electrons emerged in the two directions defined by the spin axis. The mirror symmetry test. If the numbers are not equal, parity i
s dead! And I killed it! Arggghh!
It looked as if a confluence of miracles would be needed for a successful experiment. Indeed, it was just this sequence that had discouraged us in August when Lee and Yang read their paper, which implied small effects. One small effect can be overcome with patience, but two sequential small effects—say, one percent of one percent—would make the experiment hopeless. Why two sequential small effects? Remember, nature has to provide pions that decay into muons, mostly spinning in the same sense (miracle number one). And the muons have to decay into electrons with an observable asymmetry relative to the muon spin axis (miracle number two).
By the Yonkers toll booth (1957, toll five cents) I was quite excited. I felt pretty sure that if the parity violation was large, the muons would be polarized (spins all pointing in the same direction). I also knew that the magnetic properties of the muon's spin were such as to "clamp" the spin in the direction of the particle's motion under the influence of the magnetic field. I was less certain of what happens when the muon enters the energy-absorbing graphite. If I was wrong, the muon spin axis would be twisted in a wide assortment of directions. If that happened there would be no way to observe the emission of electrons relative to the spin axis.
Let's go over that again. The decay of pions generates muons that spin in the direction in which they are moving. This is part of the miracle. Now we have to stop the muons so we can observe the direction of the electrons they emit upon decay. Since we know the direction of motion just before they hit the block of carbon, if nothing screws them up we know the spin direction when they stop and when they decay. Now all we have to do is rotate our electron detection arm about the block where the muons are at rest to check for mirror symmetry.
The God Particle Page 34
