introduction-weak-interaction-volume-one

4 .3 The Two Neutrinos .
If the two neutrinos in the muon decay (4 .1 .4) are each other' s
antiparticles, then we might expect them to annihilate each other, makin g
the branching ratio for the decay mod e
/- > e '( (4 .3 .1 )
much higher than its experimental value of under
2 .2 x 10 —8 (4 .3 .2 )
Initially, there are a number of possible reasons for this behaviour . First ,
the two particles could be identical . However, Gell—Mann (10) and Feinberg (11 )
showed that this implied zero vector interaction in muon decay, which i s
obviously unreal . Second, the two neutrinos could be distinct particles . Th e
feasibility of an experiment to demonstrate this hypothesis was increased by
the suggestion of Lee and Yang (12) in 1960 that one of the
neutrinos in muon
decay was associated with the electron, while the other was associated wit h
the muon . The first problem encountered in setting up an experiment to tes t
this theory was the production of neutrinos of the desired energy . This was
overcome by Pontecorvo (13) who suggested the use of neutrinos in pi decay. The
pions would be produced by the interaction of a high—energy proton beam with
suitable target . In 1960, Schwartz, Steiberger and Lederman showed that th e
alternating gradient proton synchrotron2 at Brookhaven could provide a
sufficiently high—energy beam to produce the necessary energy and intensity o f
neutrinos . Thus, in 1962, Danby, Gaillard, Goulianos, Lederman, Mistry ,
Schwartz and Steiberger
(14) began to search for evidence of two distinct type s
of neutrino . The principle of their experiment was to determine whethe r
neutrinos produced in the decay
> t"* t v (4 .3 .3)
would, when absorbed by protons and neutrons, produce only muons, or bot h
muons and electrons according to the reaction s
v + p — n { e t , (4 .3 .4 )
V + p n + ~* . (4 .3 .5 )
The cross—sectio n 3 for the reactions (4 .3 .4) and (4 .3.5) was estimated t o
a
be about 1G
mm2 for neutrinos with an energy of 2000 PieV . Hence, it was