364 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1963 



could be detected at all. Those few which did interact in the de- 

 tector initiated the reaction : 



anti-neutrino + proton — ^- 

 neutron + positron 



Figure 3 



It was the simultaneous appearance of the reaction products which 

 indicated to the experimenters that an antineutrino had been stopped. 

 The completion of this experiment yielded the final proof that the 

 neutrino really existed — anticlimatic in a sense but, nevertheless, 

 essential. 



As we have said before, the theoiy was, in some sense, in good 

 shape. It was possible to calculate low-energy neutrino phenomena 

 with substantial accuracy. However, in all of this there was a basic, 

 deep-rooted difficulty. The theory predicted that as the neutrino en- 

 ergy increased, its reaction rate should increase proportionately. 

 Above a certain energy this leads to serious difficulties which make 

 the theory untenable. Just how the theory would have to be modified 

 to avoid these difficulties is not at all apparent. The simplest pro- 

 posal — one which actually dates back to Yukawa — is that there exists 

 a particle which is responsible for the weak interactions. This par- 

 ticle, by introducing a certain level of structure to the weak inter- 

 actions, could serve to moderate the interaction rate at high energies 

 and, consequently, avoid the difficulties. The particle in question is 

 referred to by physicists as the "intermediate boson." 



One difficulty with the intermediate boson theory was pointed out by 

 G. Feinberg several years ago. He showed that if there were such a 

 particle involved in the weak interactions, then one should expect that 

 once in every 10,000 or so ordinary muon decays the muon should 

 decay into an electron and a gamma ray (rather than an electron, a 

 neutrino, and an antineutrino). Experimental tests showed that this 

 event happened less than once every 10^ normal muon decays. This 

 seemed to rule out the mtermediate boson and Lee and Yang also 

 pointed out that any mechanism for removing the weak interaction 

 difficulty at high energies would run into the same problem. The 

 only solution to this paradox and one which had been favored by 

 numerous theorists seemed to be that the neutrino coupled to the muon 



