The Parity Errors

When we look in a mirror, we think of it as being a true image, except for one thing: left and right are reversed. Despite this reversal, we expect the laws of physics to remain the same on the "other side of the mirror". On the level that we might see in the ordinary world, physics continues to function there. A ball still falls downward, even dropped from the wrong hand. Negative charges still attract positive, regardless of direction. Molecules still hold together, even in a reversed configuration. And yet, on the level of elementary particles, not all the laws of physics hold through such a reversal.

Now, one might point out that in a mirror image, left and right are not really reversed, but mapped directly just as up and down are. Front and back (toward the mirror, or away from it) are actually reversed. However, if in normal life we wanted to reverse front and back, we would usually rotate, preserving only up and down. Thus, because the mirror image does not have an expected reversal, it seems reversed to us.

Events that occur without distinction between left and right (which is to say, the same whether one reflects or rotates them) are said to have parity conserved. Interactions related to the strong, electromagnetic, and weak forces can be shown in the laboratory to conserve parity, and by the mid-twentieth century the "law of conservation of parity" was taken for granted to be true.

Then, in 1956, a question came up. Kaons were observed to decay to two pions some of the time, and three pions on other occasions. Parity can be added like even and odd numbers, and since pions are odd, this produced an even parity for some kaons and an odd parity for others. No other difference could be observed between these kaons, so why should they break down differently?

Perhaps parity need not be conserved in weak interactions, suggested Tsung-Dao Lee and Chen Ning Yang. Their proposed experiment, performed successfully the following year by Madame Chien-Shiung Wu, involved cobalt-60, cooled to near absolute zero. Cobalt-60 gives off electrons in a weak interaction; placed in a c-shaped electromagnet and cooled so that its north and south poles will not reverse, it develops an up and a down--or rather, a left and a right. When the setup shown below is repeated in mirror image, with the electric current flowing the other direction around the magnet the poles are thus reversed. The electrons emitted are not the same to the left and right, but rather preferentially distributed to the south pole in each case; and thus while the apparatus is reflected, the cobalt and its electrons are rotated, an example of non-conservation of parity. This would be as if one's mirror image raised his right hand when one raised his own right hand--but in front of the left side of the room.

With this proof that in not all cases was parity conserved, a new theorem was quickly formulated. If reflection also meant the reversal of charges, the new theory claimed, parity would be conserved. This, called PC symmetry (for parity and charge conservation) was a rather short-lived idea.

PC conservation did solve the problem of the kaon--on one condition. Neutral kaons, which were known to have a long lifetime anyway, must never decay. It did solve the problem of neutrinos, which separately violate conservation of parity and charge-reflection symmetry.

Neutrinos spin as they travel, and thus trace out corkscrew paths in space. A neutrino is a right-handed screw; an antineutrino is a left-handed screw. Parity conservation without charge conservation would insist that there be left-handed neutrinos (to go with the right-handed ones) and right-handed antineutrinos (to go with the left-handed ones). Parity conservation does not concern itself with particles and antiparticles. Similarly, charge conjugation alone would insist that there be right-handed antineutrinos (to go with the neutrinos) and left-handed neutrinos (to go with the antineutrinos). Charge conjugation does not affect orientation, only anti-ness. Despite the double prediction of four neutrinos, only two exist. This can be observed in the laboratory, and soon was.

The kaon condition, also, was observed to be violated later on--sometimes. The experiments continue to this day, because results vary and inaccuracy can be great. Neutral particles as well as charged ones must be observed, which is difficult. See the experiment at Fermilab Kaons at the Tevatron for more details on this current research.

Resources and further reading:

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