introduction-weak-interaction-volume-one

isospin—formulated fields, but this study has little or no application i n ,
weak interaction theory. A simple treatment of the subject may be found i n
Kallen : Elementary Particle Physics, Addison—Wesley 1964, pp . 67—144 .
We now mention two other quantum numbers which are commonly used i n
both strong and weak interaction theory . The first of these is hypercharge, Y ,
which is defined by
Y 2 (5 .1 .25 )
where Q is the average charge of the particles in a particular multiplet .
We know that
Q/e = T 3 — i . (5 .1 .26 )
Following the suggestion of Pais (3), Gell—Kann (4) and Nishijima (5) postulatep ,
in 1953 and 1955 respectively, that (5 .1 .26) could be expressed more elegantl y
by introducing a new quantum number called strangenes s3 , S, according to th e
formul a
Q/e = (B +. 8)/2 + 1 3 . (5 .1 .2.7 )
Alternatively, we may say that strangeness is twice the distance by which a give n
multiplet is displaced from the standard multiplet . For baryons, this is taken
as the nucleons, and for mesons, the pions . The displacement of a particula r
multiplet is found by taking the difference between its centre of charge o r
average charge, Q, and the centre of charge of the standard multiplet . Thus ,
empirically, we find tha t
S = Y — B . (5 .1 .28 )
5 .2 The Conserved Vector Current Hypothesis .
We recall that the vector coupling constants for neutron beta deca y
and for muon decay are almost equal, whereas the axial vector coupling
constants for these two processes differ by a factor of about 20 ,: . A posaibl e
explanation for this fact is that,initially,both the vector and the axia l
vector coupling constants for the two processes are equal, but that then ,
strong interaction effects occur in the neutron decay, and much weaker electro -
magnetic effects occur in the muon decay . The process affecting the couplin g
constant is 'renormalization ' 4 . ire know that the vector current in neutron
decay may be written :