introduction-weak-interaction-volume-one

3 .7 The Nonconservation of Parity .
In 1956, Lee and Yang (50) suggested that the weak interactio n
might not be invariant under the parity operator, because of certai n
difficulties arising from the existence of two distinct decay modes fo r
the K0 with different intrinsic parities (see chapter 7) . They realized
that parity conservation in nuclear beta decay could be tested by measurin g
quantities which behaved as pseudoscalars under P . If parity were conserved ,
then these should vanish . In their original paper Lee and Yang (50 )
proposed that experiments should be performed to ascertain the value s
of the following parameters :
J 2, , (3 .7.la )
(3 .7 .1b )
J (6 x pe) , (3 .7 .10)
where J denotes the spin vector of the parent nucleus and 6 that of th e
emitted electron . We now examine each of the quantities (3 .7 .1) in turn .
Since J is an axial vector, it is P invariant, but pe
is a pure vector ,
and hence
it changes sign under P . A 0 --)O beta decay obviously cannot b e
used to evaluate (3 .7 .1a) because the nuclear spin J is zero, ensurin g
that J , pe is also zero, whether the weak interaction is parity invarian t
or not . As the testing of P invariance in forbidden transitions is very
difficult because of insufficient theoretical information on thei r
nature, we see that we must employ a Gamow-Teller decay of the for m
J-3 J * 1 for this purpose . We find (51) that the electron angula r
distribution, neglecting coulomb corrections,
is given by
I(8) = 4'r (1 + A pe
Ile) , (3 .7 .2a)
where 5 was defined in (3 .5 .13b) an d
- ICAI2
A =
I"07 I 2 R(J a , J b ) - 2 Re(CV M
CA F NGT * .
) x
Jarjb
x
3 J a /(J a + 1) (3 .7 .2b)
R ( Ja , J b) - + ( 1/(J a * 1) ) (2 + J a (Ja + 1) - Jb(Jb + 1)