THE NEUTRINOS — SCHWARTZ 361 



indistinguishable, in which case they are assumed to be the same 

 particle. Experiments done in the last 10 years have shown that the 

 neutrino and the antineutrino are in fact distinguishable in some of 

 their physical properties. Insofar as beta decay is concerned, the 

 appearance of an electron seems to be accompanied by the production 

 of an antineutrino while the appearance of a positron seems to be 

 accompanied by the production of a neutrino (fig. 1). The decision 

 as to which shall be called neutrino and which antineutrino is made 

 by convention. 



THE FOUR INTERACTIONS 



To return to our story, beta decay is an example of a class of inter- 

 actions which have acquired the label "weak." In nature there appear 

 to be four quite distinct types of interactions, each with its charac- 

 teristic strength. Listed in order of decreasing strength they are: 

 Strong, electromagnetic, weak, and gravitational. The first three are 

 the only ones which concern us when we discuss nuclear phenomena. 

 Their respective strengths are roughly in the ratio of 10" to 10" to 

 1. The strong interactions are responsible for holding a nucleus 

 together against the repulsive electromag-netic interactions among 

 the various protons. The weak interactions are responsible for beta 

 decay. Among the above three types of interactions the neutrinos 

 participate only in the weak. Were it not for this class of inter- 

 actions, neutrinos would not exist at all (or, at best, they would be 

 completely undetectable and irrelevant to the rest of nature). It is 

 the weakness of its interaction with matter that makes the neutrino 

 so elusive. Just how difficult it has been to detect will shortly become 

 apparent. 



THE NEUTRINO IS NEEDED AGAIN 



As we have said, the neutrino was born out of the theoretical need 

 to preserve one of the fundamental laws of physics. Since it was 

 first proposed, more detailed experiments have shown that its presence 

 was also necessary to preserve other conservation laws. For example, 

 measurements of the direction in which the residual nucleus went 

 showed an apparent violation of momentum conservation. The same 

 neutrino also resolved the difficulty here. Furthermore, it was neces- 

 sary for the neutrino to carry away angular momentum — indeed, pre- 

 cise measurement showed that the neutrino carries the same intrinsic 

 angular momentum as the electron. All of these experiments served 

 to endow the neutrino with practically all of its properties before 

 it was ever observed directly. 



But now we turn back the clock again some 20 years for the be- 

 ginning of another major chapter in the neutrino story — the dis- 

 covery of the pi meson (or pion, as it is often called) . Hideki Yukawa 

 had calculated that the forces which bind a nucleus together should 



