introduction-weak-interaction-volume-one

Experiments give (26 )
1/Tc = 480 ± 70 s 1 , (5 .5 .40 )
which is within the limits of Weinberg's prediction, once again favourin g
weak magnetism, which was used to determine coupling constants, but not producin g
conclusive evidence for it .
Finally, we consider the evidence for weak magnetism from high-energ y
neutrino scattering :
v + p ----> antilepton t n , (5 .5 .41 )
v + n > lepton + p . (5 .5 .42 )
We may predict the differential cross-section s9 for the processes (5 .5 .41 )
and (5 .5 .42) using the isovector electromagnetic form factors FV( g 2 ) and
GV ( g2 ) and the weak form factors fA ( g2), fV( g 2 ), gA( g2 ), and g V ( g 2) . High-energy
electron-nucleon scattering experiments (26) have shown tha t
FV(g2 ) (GV(12))/(GV(0)) = 1/(1 + (g 2/ mv 2 ) 2 )
== gV (g2 ) = (ff(g2))/(fV(0)) , (5 .5 .43 )
where
mV 0 .84 GeV . (5 .5 .44)
The equation (5 .5 .43) has been verified up to a momentum transfer, q, o f
5 GeV/c . From similar experiments, and from the so-called 'double-pole' mode l
in dispersion theory, we hav e
(gA(g2))/(gA(0)) =
1/(1+
q2/ 6 A2 ) 2 , (5 .5 .45 )
where mA is a mass parameter which must be determined by experiment . By
graphing inferred neutrino spectra, the best value for mA is (27 )
mA = 0 .8 ± 0 .15 GeV. (5 .5 .46 )
Within the limits of experimental error, m V (5 .5 .44) and mA (5 .5 .46) appear
to be equal, demonstrating that, up to about 4 GeV, vector and axial vecto r
form factors have a similar q 2 dependence . Thus we see that the weak magneti c
form factor is nonzero, in accordance with the hypothesis of weak magnetism .
This constitutes the best experimental evidence in favour of weak magnetis m
obtained to date .