360 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 63 



• e* 



proton — ^ neutron + neutron — ^ proton + 



positron + neutrino electron + anti neutrino 



Figure 1. — Two examples of beta-decay, that' on the left occurring only within the nu 

 cleus. It was to carry away excess energy in beta-decay that the neutrino was first 

 postulated in the 1930's. Decay is governed by the weak interaction. 



This maximum value corresponded to the energy that might have 

 been expected for all of the electrons. The lack of uniqueness of 

 the electron energy appeared to be prima-facie evidence of a lack 

 of energy conservation in beta decay. 



Now, with the level of sophistication which prevails in today's 

 physics, the answer would have been obvious. As it was, it took the 

 enormous insight of Wolfgang Pauli to see it. To solve the dilemma, 

 he suggested that another particle had to be emitted — a particle carry- 

 ing no charge and having a mass less than that of the electron. This 

 particle would serve to carry away the energy that was clearly 

 missing. The only known particle with mass less than the electron 

 was the photon — the quantum of light. That was easily ruled out 

 in the case of beta decay and so the particle which was sought was 

 one whose presence was completely unknown until then, 



A very short time later, Enrico Fermi struck the crucial blow in 

 favor of the Pauli hypothesis. He developed a theory which ex- 

 plained, in large measure, the detailed shape of the electron spectrum 

 from the beta decay. That is to say, he was able to predict, with 

 accuracy, the probability of observing particular electron energies. 

 The key ingredient of his theory was a new particle which he labeled 

 the neutrino — "the little neutral one." To agree with experiment, the 

 mass of the neutrino had to be very small compared to that of the 

 electron. At present, it is assumed to be zero. 



PARTICLES AND ANTIPARTICLES 



Before proceeding, let us make a slight digression based on work 

 which has taken place since that time. For every particle which 

 can exist in nature there is an antiparticle whose existence is also 

 allowed. In the case of a particle which carries an electrical charge, 

 the antiparticle carries an equal and opposite charge. The positron, 

 for example, is the antiparticle of the electron. In the case of an 

 electrically neutral particle, the antiparticle is, of course, also neutral. 

 Now in the latter case the antiparticle and particle may be completely 



