Subatomic Physics

342 The Weak Interaction
Figure 11.6: Weak neutral currents mediated by Z 0 .
Weak neutral currents were
discovered at CERN through
the observation of the elastic
scattering of neutrinos
and antineutrinos on electrons,
νµe → νµe and ¯νµe →
¯νµe. (9)
These reactions are forbidden by muon number conservation if only charged weak
currents exist. Weak neutral currents now have been verified in many other experiments.
(10)
The concepts of a weak current and a weak charge require some reassuring
remarks. We are used to electric charges and currents: They can be observed and
measured, and they form part of our everyday surroundings. Weak currents and
weak charges, on the other hand, have no classical analog. The only way to become
familiar with them is to assume their existence and explore the consequences. Since
all experiments agree with the predictions of the standard model based on a weak
current–current interaction, confidence in the existence of weak charges and currents
is justified. In the following sections, we shall inquire into three questions related
to Hw: (1) What phenomena are described by Hw? (2) What is the form of the
weak current Jw? (3) What is the value of the coupling constant GF ?
11.4 A Variety of Weak Processes
The discussion so far has been restricted to beta decay, the oldest and best known
example of a weak interaction. If it were the only manifestation of the weak force,
interest would be limited. However, a surprising variety of weak processes is known.
Weak reactions have been a rich source of unexpected new phenomena, such as the
violation of parity and CP conservation as well as numerous other phenomena associated
with the neutral kaons and other systems. Moreover, the unification of the
weak and electromagnetic interactions (chapter 13) has had a profound influence on
our understanding of fundamental forces. In the present section, we shall categorize
the weak processes, list a few examples, and state why they all are called weak.
A classification of weak processes can be based on the separation of the weak
current into a leptonic and a hadronic part, as in Eq. (11.22). Inserting Eq. (11.22)
in the form Jw = J l w + J h w into the weak Hamiltonian (11.25) produces four scalar
products; one involves only leptons and one only hadrons, and two couple lepton
and hadron currents. The classification is performed according to these terms:
9F. J. Hasert et al., Phys. Lett. B46, 121 (1973); H. Faissner et al., Phys. Rev. Lett. 41, 213
(1978); R.C. Allen, Phys. Rev. Lett. 22,2401 (1985).
10V. Nguyen-Khac and A. M. Lutz, eds Neutral Currents: 20 Years Later, World Scientific,
Singapore, 1993.