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Particles and parity
THEORETICAL PHYSICS
Particles and parity PHYSICISTS were shaken but excited a few months ago when two American theoreticians, Professors T. D. Lee and C. N. Yang, suggested that in certain cases parity in elementary particle interactions was not conserved. Experimental confirmation soon followed and a number of investigations were started to provide a theoretical basis for the new results. The first showing some successful conclusion is by Lee and Yang, the original iconoclasts (Physical Review, vol 105, p 1671). Independently, L. Landan in Moscow and A. Salam in England have arrived at the same explanation. Parity is a somewhat subtle mathematical concept concerned with the positions and motions of groups of elementary particles such as protons, neutrons, electrons, neutrinos, mesons and others, including the recently discovered “anti” particles. Each of these particles possesses “spin”, something like the Earth spinning about its axis. Parity is a measure of the appearance of a group of these particles if their positions and motions only are seen reflected in a mirror, leaving their directions of spin unchanged. If the reflected group is indistinguishable in appearance from the original, parity is said to be even but if there is a distinction between the two the parity is said to be odd. Reflection of velocity without change of spin is well known to billiard players, who cue a ball with “side” and observe that the spin of the ball is unchanged when the ball bounces off the cushion. However, the elementary particles spin about an axis which lies along their directions of motion. For these particles the invariance of spin even when the motion is reversed is like the rotation of the cooling fan of a motor car, which always revolves in the same direction whether the car is driven backwards or forwards. And just as a car can be built with its fan rotating either clockwise or anticlockwise, so an electron, for example, can spin in either direction about its line of motion. www.newscientist.com
061118_Arch_1950s.indd 5
ALAN RICHARDS/INSTITUTE FOR ADVANCED STUDY
The neutrino seems to have an exclusive position in calculations concerning the looking-glass world of basic particles
An electron with a clockwise spin would travel forwards like a righthanded screw. Evidently, such a single electron has odd parity, since its reflection without change of spin would give it a left-handed screw motion. But two electrons, one with clockwise and the other with anticlockwise spin, would together have even parity, since both the reality and the reflection would contain one left-handed and one right-handed screw motion. The law of conservation of parity states that, if an isolated group of elementary particles, whatever the size of the group, is fragmented, then the parity of all the fragments together is the same as that of the original group. Before it was overthrown, this law was of enormous help in distinguishing between possible mechanisms of radioactive decay, nuclear transformation and so on. The new idea put forward by Lee and Yang is that, unlike electrons and other elementary particles, neutrinos can only spin in one direction about their line of motion. Thus neutrinos can only exhibit one sort of motion which, for the sake of argument, can be
Soon after receiving the Nobel prize for their idea, Lee (left) and Yang fell out over who had proposed it
supposed to be that of a right-handed screw. By this account, “a neutrino with a left-handed screw motion” would be as fanciful a juxtaposition of words as “a circular square”. Hence the complicated reflection which turned the electron with the right-handed screw motion into one with a lefthanded screw motion would turn any neutrino into a non-existing object. Lee and Yang suggest, therefore, not so much that the law of conservation of parity is broken by neutrinos, as that it does not even begin to apply to them. The law will not necessarily be obeyed in those physical phenomena in which neutrinos play a part. So far there is an accord between experiments and the theory put forward by Lee and Yang. But many more experiments need to be carried out before the theory can be finally accepted. 9 MAY 1957
HINDSIGHT Chen Ning Yang and Tsung-Dao Lee were awarded the Nobel Prize for Physics later in 1957 for their “penetrating investigation” of parity laws.
Fifty years of New Scientist | 5
3/11/06 2:17:58 pm
Particles and parity PHYSICISTS were shaken but excited a few months ago when two American theoreticians, Professors T. D. Lee and C. N. Yang, suggested that in certain cases parity in elementary particle interactions was not conserved. Experimental confirmation soon followed and a number of investigations were started to provide a theoretical basis for the new results. The first showing some successful conclusion is by Lee and Yang, the original iconoclasts (Physical Review, vol 105, p 1671). Independently, L. Landan in Moscow and A. Salam in England have arrived at the same explanation. Parity is a somewhat subtle mathematical concept concerned with the positions and motions of groups of elementary particles such as protons, neutrons, electrons, neutrinos, mesons and others, including the recently discovered “anti” particles. Each of these particles possesses “spin”, something like the Earth spinning about its axis. Parity is a measure of the appearance of a group of these particles if their positions and motions only are seen reflected in a mirror, leaving their directions of spin unchanged. If the reflected group is indistinguishable in appearance from the original, parity is said to be even but if there is a distinction between the two the parity is said to be odd. Reflection of velocity without change of spin is well known to billiard players, who cue a ball with “side” and observe that the spin of the ball is unchanged when the ball bounces off the cushion. However, the elementary particles spin about an axis which lies along their directions of motion. For these particles the invariance of spin even when the motion is reversed is like the rotation of the cooling fan of a motor car, which always revolves in the same direction whether the car is driven backwards or forwards. And just as a car can be built with its fan rotating either clockwise or anticlockwise, so an electron, for example, can spin in either direction about its line of motion. www.newscientist.com
061118_Arch_1950s.indd 5
ALAN RICHARDS/INSTITUTE FOR ADVANCED STUDY
The neutrino seems to have an exclusive position in calculations concerning the looking-glass world of basic particles
An electron with a clockwise spin would travel forwards like a righthanded screw. Evidently, such a single electron has odd parity, since its reflection without change of spin would give it a left-handed screw motion. But two electrons, one with clockwise and the other with anticlockwise spin, would together have even parity, since both the reality and the reflection would contain one left-handed and one right-handed screw motion. The law of conservation of parity states that, if an isolated group of elementary particles, whatever the size of the group, is fragmented, then the parity of all the fragments together is the same as that of the original group. Before it was overthrown, this law was of enormous help in distinguishing between possible mechanisms of radioactive decay, nuclear transformation and so on. The new idea put forward by Lee and Yang is that, unlike electrons and other elementary particles, neutrinos can only spin in one direction about their line of motion. Thus neutrinos can only exhibit one sort of motion which, for the sake of argument, can be
Soon after receiving the Nobel prize for their idea, Lee (left) and Yang fell out over who had proposed it
supposed to be that of a right-handed screw. By this account, “a neutrino with a left-handed screw motion” would be as fanciful a juxtaposition of words as “a circular square”. Hence the complicated reflection which turned the electron with the right-handed screw motion into one with a lefthanded screw motion would turn any neutrino into a non-existing object. Lee and Yang suggest, therefore, not so much that the law of conservation of parity is broken by neutrinos, as that it does not even begin to apply to them. The law will not necessarily be obeyed in those physical phenomena in which neutrinos play a part. So far there is an accord between experiments and the theory put forward by Lee and Yang. But many more experiments need to be carried out before the theory can be finally accepted. 9 MAY 1957
HINDSIGHT Chen Ning Yang and Tsung-Dao Lee were awarded the Nobel Prize for Physics later in 1957 for their “penetrating investigation” of parity laws.
Fifty years of New Scientist | 5
3/11/06 2:17:58 pm