THE NEUTRINOS — SCHWARTZ 363 



directly must be identical to the laws of physics one would deduce 

 from observing nature through a mirror. About that time, however, 

 physicists observed what appeared to be a violation of the parity law 

 in the decay of the K meson — which also took place by way of the 

 weak interactions. 



As is always the case, physicists tried to preserve the rule by in- 

 venting all sorts of other schemes. However, T. D. Lee and C. N. 

 Yang, surveying all of the experimental evidence existing until that 

 time, pointed out (in a now famous paper which won the Nobel 

 Prize for physics in 1957) that the only evidence for parity conserva- 

 tion existed in the realm of the strong and electromagnetic inter- 

 actions. They proposed a series of experiments to investigate the 

 validity of this rule in the realm of the weak interaction. The first 

 crucial experiment was performed by E. Ambler and C. S. Wu at the 

 U.S. National Bureau of Standards in 1956 and showed conclusively 

 that parity was not conserved in beta decay. They thus resolved 

 the problem at hand and opened the way for a large series of addi- 

 tional experiments on beta decay, pion decay, and muon decay. 



In each of these reactions, the violation of the parity rule became 

 apparent. Insofar as the neutrinos were concerned, the parity viola- 

 tion gave rise to a most fascinating aspect of their behavior. A 

 neutrino always travels as though it were a left-handed screw. An 

 antineutrino, on the other hand, travels like a right-handed screw. 

 The verification of these and other properties of the weak interaction 

 encompassed one of the most productive periods in modern physics. 

 Further progress was made shortly afterwards when R. Feynman 

 and M. Gell-Mann, in a brilliant paper, showed that all the features 

 of both beta decay and muon decay can be explained by one relatively 

 simple theory which seemed to be quite universal in its aspects. In- 

 deed, almost too universal, for it predicted that there was no difference 

 in the basic interaction of the electron and muon with other particles. 

 In a sense, this was quite puzzling because the two particles differ in 

 mass by a factor of 200, and physicists tend to think of mass as largely 

 the result of interaction properties. This puzzle is, as yet, unresolved. 

 And, as we will shortly see, it has become even sharper in recent 

 months. 



THE NEUTRINO IS DETECTED 



The mid-fifties also saw another great achievement in neutrino 

 physics — the first direct observation of neutrino-induced reactions. 

 C. Cowan and F. Reines, working at a large nuclear reactor, observed 

 antineutrinos which were emitted by beta deca3^s within the reactor. 

 On the average, these particles could spend a full year traveling in a 

 straight line through solid lead before being absorbed. It was only by 

 passing a phenomenal number of them through a detector that they 



