introduction-weak-interaction-volume-one

interactions. The currents (4 .4 .1) and (4 .4 .2) are both charged, and we now
consider the possibility that neutral currents also exist . First, there are
terms of the typ e
(e e) (e e) , (4 .4 .11 )
(}^ r) (t" /'') , (4 .4 .12 )
(e e) ( M) . (4 .4 .13)
If these have coupling constants of the same order as that for muon decay, the n
it is obvious that their effects will be almost unobservable, because of th e
fact that all the reactions (4 .4 .11), (4 .4 .12) and (4 .4 .13) can occur via
second—order electromagnetic processes with a much higher transition probability .
For example, (4 .4 .11) would occur electromagnetically a s
ē e >r- ē e (4 .4 .14 )
Thus, at present, there is no experimental evidence concerning the neutral
current terms (4 .4 .11) , (4 .4 .12) and (4 .4 .13) . However, if the same current —
current interaction occurs in semileptonic as well as pure leptonic processes ,
then the failure to observe decays such as
K° 1-A + (4 .4 .15 )
might indicate that these neutral current terms were not in fact present .
The term
(j e) (e e) (4 .4 .16 )
appears to be absent, if it is of the same strength as the muon decay tern ,
since decays such as
N r > e l e r e (4 .4 .17 )
have a branching ratio of less than (16 )
1 .5 x 10-7 . (4 .4.18 )
The coupling
(e r) (f^ H (4 .4.19 )
which would cause such decays a s
—fie y (4 .4 .20 )
if it is present, must have a very small amplitude, since the branching rati o
for (4 .4 .20) is less than (4 .3 .2) . The similar term
(/a e ) (f" e) , (4 .4 .2.1 )
because of the failure to observe such processes a s