The Neutrinos' 



By Melvin Schwartz 



Professor of Physics, Columbia University 



[With one plate] 



In recent months, the attention of physics has centered upon the 

 most elusive of all elementary particles — the neutrino. A recent ex- 

 periment at Brookhaven National Laboratory and Columbia Uni- 

 versity has shown that there exists in nature t^vo independent types 

 of neutrinos — one associated with electrons and the other associated 

 with mu mesons. This experiment has also opened a new chapter 

 in high energy physics — namely, the study of energetic neutrino 

 interactions. 



To understand the neutrino and the history of its discovery, we 

 must go back some 30 or 40 years. At that time, much less was known 

 about nuclear physics than is known today but, on the other hand, 

 the triumphs of quantum mechanics were fresh and exciting and 

 many of the conservation laws of physics were on a very firm foot- 

 ing. In particular, conservation of energy was a cornerstone of the 

 edifice which had been built up in the three centuries since Newton's 

 time. 



While investigating the behavior of nuclei, physicists had noted the 

 phenomenon called beta decay. They observed that occasionally a 

 nucleus would spontaneously emit an electron (or its antiparticle, a 

 positron) and change into another nucleus (fig. 1). Now if this were 

 all that were happening, we vrould expect the electron and the re- 

 sidual nucleus to travel off in opposite directions, with the electron 

 having a unique energy. We Avould expect that the total energy 

 of the electron and the residual nucleus should add up to the total 

 energy of the initial nucleus (including the energy equivalent of the 

 masses involved by means of the relation E =mc^) . 



Now in these early experiments it was not possible to observe the 

 residual nucleus, but measurements of the electron energy alone indi- 

 cated a difficulty. Its energy was not unique; indeed, it showed a 

 continuous spectrum of energies up to a certain maximum value. 



1 Reprinted by permission from Discovery (London), vol. 23, No. 11, November 1962. 



359 



