Beyond the God Particle (12 page)

     “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!”
     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 for a muon? Well, the muons are decaying into electrons and neutrinos with a half-life of 2 microseconds. That is, half of the muons have given their all in 2 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 “front” of the muon as opposed to the “back,” 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. on 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 directions, were rotated in a magnetic field. So the electron's time of arrival now corresponded to their direction. By recording the time, 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 that registered 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.
     All of this started working, and by 5 a.m. we had “20 standard deviations” of scientific proof, i.e., proof positive, that the directions in which electrons are emitted changes with the angle, relative to the muon's spin. Our muons were all right-handed. The mirror image, a left-handed version, does not exist in our laboratory, and hence, by extension, it does not exist in any laboratory. The Conservation Law of Parity was
not valid
for this weak force process either—the radioactive decay of the muon! We had made a profound discovery in a few days of hard work, observing parity violation in the weak decays of both pions and muons. And a few hours of data accumulation. By about 9 a.m. the word had, somehow (?), spread and we began receiving calls from physicists around the nation and, soon thereafter, from around the world. The irascible Austrian Wolfgang Pauli was soon quoted, showing his shock and disbelief: “I cannot believe that God is a weak left-hander.” Yes, fame, fortune, and promotion followed in due course. And Marcel got his Ph.D.!

—Leon M. Lederman
¹⁴

Let's summarize. The result obtained by performing the experiment of negatively charged pion decay turns out to be shocking: the handedness of
the negatively charged muon produced in decay is always L, that is, we always see events as in
figure 3.5 (A)
, and we never see events as in
figure 3.5 (B)
!

FIGURE 3.5. Parity Violation in Pion Decay
. The spins of produced particles from (negatively charged) pion decays, in the weak interaction process
π
^–
→
μ
^–
+
⁰
. In (A) the muon spin is aligned with direction of motion (right-handed muon); in (B) the muon spin is counter-aligned with direction of motion (left-handed muon). We always observe (A) in the laboratory, and we never observe (B). If we did observe (B) we could tell that we were looking at the process through a mirror.

This indeed implies that if we ever “see” a film or a DVD of a negatively charged pion decay producing an L negatively charged muon, as in
figure 3.5 (B)
, then we can loudly proclaim: “We are seeing an image of the process reflected in a mirror! Such a process can happen only in Alice's looking-glass house. This never happens on our side of the mirror!”
¹⁵

The mirror world with left-handed, or L, negatively charged muons coming from negative pion decay doesn't exist. (Actually, the L muon is produced instantaneously in the pion decay, but its mass flips into an R muon that balances the spin of the anti-neutrino; we'll soon have much more to say about that.) The shocking implication of the experiment is that in our world, the laws of physics contain forces and interactions that are not symmetric under parity. This happens for the class of interactions called the “weak interactions” that are producing the decay of the pion and, subsequently, the decay of the muon. Indeed, this is an example of a “broken symmetry” that occurs throughout the weak interactions, which also produce numerous other effects. The very matter out of which we are composed, hence our very existence, depends upon these feeble forces in nature, and we now learn that these forces distinguish our world from its mirror image!

Historically, until the mid 1950s, physicists had believed that parity was an exact symmetry of physics. Thus, the looking-glass world would have been indistinguishable in any movie of any process that we might ever encounter. The question of parity (P) non-conservation in the weak interactions was first raised by two young theorists, T. D. Lee and C. N. Yang, in 1956.
¹⁶
Parity symmetry was practically considered to be a bread-and-butter established fact in nature and had been used for decades in compiling data on nuclear and atomic physics. The breakthrough of Lee and Yang was the idea that the reflection symmetry—parity—could be perfectly respected in most of the interactions that physicists encountered, such as the strong force that holds the atomic nucleus together, and the electromagnetic forces together with gravity. But Lee and Yang proposed that the weak force, with its particular form of beta-decay radioactivity, might not possess this mirror symmetry.

In 1957, parity violation was discovered experimentally, by Leon Lederman, Richard Garwin, and Marcel Weinrich, by using the charged pion decay and stopped muon decay techniques we have just described. Independently, the effect was seen by Chien-Shiung Wu, using another more complex technique. It was astounding news—the weak processes are not invariant under the parity. Parity was overthrown!

Madame Wu observed the radioactive disintegration of cobalt 60 (
⁶⁰
Co) at very low temperatures in a strong magnetic field.
¹⁷
This experiment
was a very challenging undertaking, requiring the heroic efforts of many groups with different expertise. The
⁶⁰
Co is a metal out of which ordinary electrons stream, coming from beta-decay processes within the material. Wu discovered that, in the strong magnetic field, the electrons were emitted in the direction of the magnetic field (this happens because the magnetic field, at low temperatures, aligns the spins of the nuclei in the cobalt, and the decay pattern is determined by the spin of the nucleus). However, her observation was enough to conclude that there was a violation of parity symmetry. The alignment of outgoing electron velocity with the magnetic field, it turns out, is the same as a handedness, and it would be reversed in a mirror. If we saw a movie or DVD showing the electrons coming out of
⁶⁰
Co decay counter-aligned to the magnetic field, then, again, we could announce: “This is a mirror image of the real process and does not occur in our world.”

Parity is violated. Parity is not a symmetry. The mirror world of Alice through the looking glass is different in a fundamental way from ours. The lowly muon has led us to this. Perhaps that's why someone at the Chinese lunch table “ordered the muon” after all. There is a difference between left and right in our world.

And this is where the story of the Higgs boson begins.

It's 2 a.m., the early morning hours of July 4, 2012. We have congregated in our largest seminar room, One West at Fermilab, where there is standing room only. We are here to audit two talks, one each from the gigantic experimental collaborations at CERN, known as ATLAS and CMS, talks that are being beamed in live at 9 a.m. from Geneva, Switzerland.