THE NEUTRINOS — SCHWARTZ 365 



and the neutrino coupled to the electron (fig. 2) are not the same par- 

 ticle. If this were the case, the decay of a muon into an electron and 

 a gamma ray would be absolutely forbidden independent of the exist- 

 ence of an intermediate boson. 



DISCOVERY OF TWO NEUTRINOS 



It remained then to devise a proper test for this hypothesis. B. 

 Pontecorvo of the Soviet Union and the author independently pointed 

 out that it is feasible to do experiments with high energy neutrinos 

 in presently existing or planned accelerators. In these accelerators, 

 protons are brought up to energies in the multibillion-electron-volt 

 region and are then allowed to strike a target. Out of this target 

 come mainly pi mesons, most of which decay shortly into muons and 

 their neutrinos. If these neutrinos were identical to the electron- 

 coupled neutrinos, when they interacted with matter (a neutron in a 

 nucleus, for example) they would produce electrons as often as muons. 



Figure 4 



If they were different from the electron-coupled neutrinos, they 

 could produce only muons and no electrons at all. 



The proposed experiment has just been completed at Brookhaven 

 National Laboratory by a group including G. Danby, J. M. Gaillard, 

 K. Goulianos, L. Lederman, N. Mistry, J. Steinberger, and the author. 

 In this experiment (fig. 5) pions produced by 15 Bev protons are 

 allowed to travel for some 70 feet before striking a 40-foot thick steel 

 shielding wall. During this interval, about 10 percent of the pions 

 decay, sending their neutrinos forward. The remaining pions, muons, 

 and all other debris are stopped by the wall, but the neutrinos pene- 

 trate it as though it were nonexistent. Behind the wall, in a well- 

 shielded room, stands a spark chamber (pi. 1, fig. 1) — an instrument 

 which can be made to show a track of sparks whenever a charged 

 particle passes through it. This spark chamber also acted as the tar- 

 get for the neutrinos, and consisted of 10 tons of aluminium in the 

 form of 1-inch thick plates. 



