Beyond the God Particle by Leon M. Lederman, Christopher T. Hill Page B
Authors: Leon M. Lederman, Christopher T. Hill
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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
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
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