362 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 63 



*- o i/ 



(muon coupled) 



pi meson — »»-mu meson 

 + anti-neutrino 



"0 o 

 (electron coupled) 



e • ^ o 



(muon coupled; 



mu meson - 

 neutrino + 



-^- electron + 

 anti-neutrino 



Figure 2. — Decays of pion and muon both involve neutrinos (or antineutrinos). It is 

 now known that two kinds of neutrino are involved, as indicated in the diagram. 



be the result of a particle with a mass equal to several hundred, elec- 

 tron masses. A search for the pi meson in cosmic rays led to the 

 discovery of a particle with a mass not too different from the predicted 

 one. However, detailed experiments showed that this new particle 

 did not participate in the strong interactions and hence could not be 

 the pion. It was subsequently called the mu meson (or muon). Its 

 origin was a complete mystery until sometime later when the pion 

 was finally found and observed to decay spontaneously into the muon. 

 But now the picture was again not quite complete. During this 

 decay of a pion into a muon there was also an apparent failure to 

 conserve energy and momentum. Another particle had to be formed 

 as well. Careful measurement indicated that the neutrino fitted per- 

 fectly, and so for many years the same neutrino which participated 

 in beta decay was presumed to participate in pion decay (fig. 2) . Fur- 

 thermore, a study of the muon itself showed that it apparently decayed 

 in several millionths of a second into an electron, a neutrino, and an 

 antineutrino. 



PARITY VIOLATED 



Historically, this brings us to the mid-fifties and the beginning of 

 a new era in the understanding of the weak interaction. One of the 

 fundamental "principles" in the development of quantum mechanics 

 until that time was the law of parity conservation. It states that 

 the laws of physics which one would deduce from observing nature 



