Beyond the God Particle

by Leon M. Lederman

  An extremely important experimental point, and the reason we can do this experiment at all, is that we can slow down and stop a speeding muon, and its spin is not affected—it's a perfect gyroscope, remember? Since the slowing down and stopping of the muon doesn't change its spin direction, we therefore know the exact direction of the muon spin at the instant that it was produced from the decay of the pion. We can even measure its spin, which tells us what it was at the moment it was created in the pion decay. Furthermore, the muon itself decays (in two millionths of a second) into an electron and invisible neutrinos. The direction of its electron decay product actually reveals the muon's spin, so we have a neat way to measure it.

  So, we can set up an experiment to look in detail at the decay events of the pion and the muon. For this we go into a laboratory and use pions that are produced by an accelerator. We look for events where the muon comes out with its spin aligned along the muon direction of motion (we call this “right-handed” or R). And we can also look for events in which the muon spin is counter-aligned to the muon direction of its motion (we call this “left-handed” or L). Remember, since R and L are a form of handedness, like our right or left hands, the handedness of a particle is always reversed when viewed in a mirror.

  In the mid-1950s, Leon Lederman and his colleagues measured the handedness of the outgoing (negatively charged) muon, produced in (negatively charged) pion decay. Let's try to guess what the answer should be for the outcome of this experiment.

  If parity is a symmetry of the laws of physics, then both R and L muons should have occurred with equal probability from pion decays (quantum mechanics gives us only the probability of something happening, and it can't tell us exactly what will happen in any given event). That is, for many decay events we should get (approximately, since there are always statistical fluctuations) 50 percent L and 50 percent R muons coming out. Any given decay of the pion would have to produce a definite handedness for the muon, either L or R, and the mirror image of any particular event would have the opposite value handedness, either R or L. So any particular decay of the pion is different than its mirror image. But parity symmetry would require that things balance out over many, many decays. If parity is a symmetry, then we shouldn't be able to infer that we live on one or the other side of the mirror from pion decays. For example, if negatively charged pions produced 60 percent L muons and 40 percent R muons, over many, many decays, then there would be something wrong with parity symmetry, because in the mirror world we'd get the reversed situation, 40 percent L and 60 percent R decays. Pion decays would therefore be different in the mirror world than in our world. If that were the case, parity would not be a symmetry of the laws of physics (you might want to reread this paragraph a few times). In any case, this is how old Democritus might have reasoned it out.

  So what happened when Leon and his colleagues did the experiment? Let's hear it from the master himself.

  A PERSONAL RECOLLECTION OF THE DISCOVERY OF PARITY VIOLATION

  I was driving north from a long day spent in the Department of Physics. My thoughts were occupied with the discussions that started with the traditional Chinese lunch, a Columbia University physics department Friday tradition. Friday was also the day of the weekly physics colloquium and the invited speaker was usually from some distant and, of course, lesser institution, e.g. Harvard or Yale. The Columbia lunch was designed to generate a lively discussion, usually on the topic that the visitor was going to address at his colloquium. The hidden agenda was to so saturate the hapless visitor with superb Northern Chinese cuisine that his physics defenses would be weakened and his gracious hosts could more easily destroy him.

  Broadway was littered with the bones of destroyed Harvard professors. The drive from Columbia University's campus in Manhattan to the Nevis Laboratory in Irvington-on-Hudson usually takes about 40 minutes. Nevis, an old DuPont estate, was willed to the University much to the chagrin of the neighboring estate owners bordering the Hudson River.

  The Westchester affluence begins after Yonkers with villages like Hastings-on-Hudson, Dobbs Ferry, Tarrytown. On a particular Friday afternoon, January 5, 1957, my mind was occupied with the lunchtime conversation led by Columbia's leading theoretical physicist and gourmet, Professor Tsung Dao Lee. The issue was an experiment suggested some months earlier by Lee and his Princeton colleague, Frank Yang. The experiment was designed to test a centuries-old idea in a new domain of physics. The old idea was a belief in the symmetry between the real world and the mirror world. It had long been believed that the mirror reflections of real world processes were also real world processes. Look in the mirror. A man's jacket has its buttons on the right side of the coat. In the mirror, the buttons appear to be on the left side. But there is no law against securing the buttons on the left side.

  Which side gets the buttons is a matter of convention. Similarly, turning a screwdriver clockwise (as judged by the guy turning the screwdriver) advances the screw further into the block of wood. This is a “right-handed” screw, of course. By convention, screws are right-handed. The mirror image of the process makes the screw look left-handed. But again, there is no law that prevents you from going to the manufacturer and ordering left-handed screws. They may charge you too much, but the law of mirror symmetry says it can be done and experience confirms it.

  Mirror symmetry has vast implications in science. It is the same as the bilateral symmetry of the human body and of so many animals. Draw a vertical line down the center of a face and the two halves are mirror images. Molecules in which the atoms have a specific three-dimensional structure often have mirror images, which have different chemical behaviors; chemists and biologists are intensely interested in these mirror relations, but in all cases, the basic symmetry, i.e. the mirror world, obeys the same laws of physics, chemistry, and biology as the real world. Until January 6, 1957.

  And until Lee and Yang had published a paper questioning whether the mirror image of radioactive processes obey mirror symmetry. What makes radioactive processes different from buttons, screws, and biological molecules, is that radioactivity is a signature of the “weak force.” Certain puzzling reactions seen in the study of weak force driven decays would make sense if the weak force did not respect the symmetry. Incidentally, the validity of weak force symmetry (in the spirit of Emmy Noether's theorem) gives rise to a conservation law: the Law of Conservation of Parity. Parity is a measure of the “handedness” of a system.

  Earlier in the summer of 1950, Lee and Yang had suggested a number of processes that could be studied in order to verify mirror symmetry, or to disprove the symmetry. C. S. Wu, a Columbia colleague and a skilled experimenter and expert on radioactive decays, had decided then to attempt one of the experiments which involved the radioactive decay of Cobalt 60.

  The talk at our weekly Chinese lunch was that Wu had been recently seeing some interesting data, that the effect she was observing indicated a failure of mirror symmetry and that the effect could be large.

  It was this information that kept circulating in my head: “A large effect?” Back in August 1956 physicists collected at the Brookhaven Laboratory, conveniently situated near some of the finest ocean beaches bordering Long Island's Atlantic shore—summers at Brookhaven were a favorite for family fun and for more serious physics discussions. Hearing Lee and Yang's proposal that perhaps mirror symmetry may fail if we concentrate on only those processes controlled by the weak forces—those that produce radioactive decays—was a revelation.

  The idea sparkled for two reasons. The first was that this could explain some data that seemed totally contradictory—where 99 and 7/8ths of all reactions rigorously obeyed mirror symmetry (i.e. conserved parity) and a few reactions made no sense at all. The second deep idea was the possibility that rules and overriding laws of physics may in fact depend on the nature of the forces that are involved. As a crazy example, suppose the law of conservation of energy was valid for the electromagnetic forces, the strong forces, and the gravity force, but not valid
for the weak force. The stock market value of radioactivity would go through the stratosphere! Not to worry—it isn't so.

  Back to the trip north on Friday evening, following the graceful weaving of the Saw Mill River Parkway, paralleling the Hudson River. Suddenly the musings on the Wu report exploded into an idea. Suddenly, I visualized an experiment one could do with the Nevis cyclotron, an experiment so simple that one could carry it out in a few hours. (In those days, experiments, even with three or four collaborators, took months to carry out—now they take decades.)

  My graduate student, Marcel Weinrich, had been working on an experiment involving muons. Muons are produced in the radioactive disintegrations of certain particles (pions) produced in the Nevis cyclotron. They behave, in all measurements, exactly like electrons—except that they are two hundred times as heavy. The issue of why nature created a heavy twin made muons a favorite object for study. Little did we dream of the treasure the muons would reveal! Marcel's set up, 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 Nevis accelerator 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, adding energy on each turn. Near the end of their spiral trip, the particles have an energy of 400 MeV (1 MeV = 1 million electron volts, as though the protons had been kicked by a 400 million volt battery). 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 energy 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 would be waiting.

  In the experiment that 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, carrying a negative electric charge, would finally be captured by the positive nuclei. Since we did not want anything to influence the muon's direction of spin, its 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 came my key thought while 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 through the channel to the outside of the machine. Suppose further that the innumerable gentle collisions with carbon atoms, which gradually slowed down the muon, did not disturb this relationship between the muon's spin and its direction of motion. 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!

  Now, dear reader, listen carefully. A spinning object can be considered right-handed (say, clockwise) when viewed from the “back end,” or it can be considered left-handed when viewed from the front end. Its mirror image is the identical object turned upside down. Symmetry!

  However, if this object is a muon and it disintegrates, and out of one end, there appears an electron (with lots of muons, we'd get lots of electrons), then, like the classical screw, it is uniquely right-handed. If the fingers of your right hand curl in the, say clockwise, direction of the spin, the thumb gives the preferred direction of the emitted electron. Man! That is a right-handed process. The mirror image is a left-handed object.

  But if the laws of physics dictate that a positive muon is right-handed, the left-handed muon in the mirror doesn't exist—the symmetry is destroyed. The question then is: “Do electrons prefer to be emitted along the direction of your thumb or can they, with equal probability, be emitted in either direction?” In the latter case, the parity symmetry is valid. Thus, the experimental issue is extraordinarily simple—measure the direction of emission of the electrons from a collection of muons, all spinning in the same direction. If the electrons are emitted equally forward and backward (relative to the axis of spin), then parity is a symmetry and you get no promotion, no fame, and no fortune. Try again. But if there is a preference, then you have established a violation of parity, or mirror symmetry. (Okay, read it again!)

  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. Hence the mirror symmetry test. If the numbers were not equal, parity would be dead! And I would have killed it! Argggghh!

  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 would have to provide pions that decay into muons, mostly spinning with the same handedness (miracle number one). And the muons would 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 the direction of emission and therefore, to check if parity is a symmetry.

  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 hour's work. Wow! I decided that i
t 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 on atomic processes at the IBM research labs, which were then located 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 muon 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 of the experiment 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. 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. I seized the opportunity to comfort our now blubbering grad student: “Don't worry, Marcel, this experiment will make you famous!”