introduction-weak-interaction-volume-one

much longer tracks than any of the particles from the accelerator . Thirt y,-fou r
photographs were eliminated because they contained tracks of this type . It wa s
possible that a number of neutrons might have triggered the anticoincidenc e
circuit, and a number of short-track events were discounted because of thi s
possibility . Finally, twenty-two photographs showing a vertex as expected ,
remained . The evidence against the fact that these might have been caused b y
neutrons was firstly that there was no attenuation in the number of event s
along the length of the spark chamber array, and secondly, that the remova l
of 1 .2 m of iron shielding, which would have been expected to cause a significan t
increase in the number of neutron events, caused no difference in the numbe r
of one-vertex events . By placing a lead block in front of the beam target ,
it was established that the spark chamber one-vertex photographs were th e
result of pion or kaon decay products, since this reduced the event rate fro m
1 .46 to 0 .3 events per 10 16 protons, since the pions and kaons were then no t
allowed to decay .
Finally, it was necessary to identify the secondary tracks produced b y
neutrino absorption . It was found that single tracks traversed about 8 .2 m of
aluminium, no case of nuclear interaction being observed . Had the particle s
been electrons or pions, a number of nuclear interactions would have bee n
expected, and these did not, in fact, occur. Furthermore, electron track s
tend to be erratic, but the tracks photographed were straight . In all, 5 1
photographs contained muon tracks, whereas only 6 photographs showed electron
showers . The latter were probably due to neutrinos produced in kaon decay .
Thus it was concluded that there exist two distinct types of neutrinos : on e
associated with the electron and one with the muon . On the basis of th e
'two neutrino experiment' it was suggested that an electron and a muon numbe r
should be attributed to every particle . The electron and e - neutrino ar e
assigned electron numbers of +1, while the positron and anti-e - neutrin o
are assigned electron numbers of -1 . All other particles are thought to hav e
zero electron number . Muon number is allotted analogously. Electron and muon
number appear to be conserved in all reactions, explaining why the deca y
> e + y (4 .3 .6 )
has never been observed .