Subatomic Physics

11.4. A Variety of Weak Processes 343
leptonic processes : J l w · J l†
w
semileptonic processes : J l w
hadronic processes : J h w
h† · Jw + J h l†
w · Jw · J h†
w .
(11.26)
Weak processes of each of these three classes are known. In chapter 10, in the
treatment of the electromagnetic interaction, we have learned that life is easy as
long as only leptons are present. The story repeats itself in the weak interaction:
Leptonic processes can be calculated, and theory and experiment agree. Semileptonic
processes produce difficulties, and the weak processes involving only hadrons
cannot yet be calculated in detail from first principles. We shall now list processes
in each of the three classes.
Leptonic Processes The leptonic processes that are easiest to study are the
decay of the muon and tau
µ + −→ e + ¯νµνe, τ + −→ l + ¯ντνl. (11.27)
We will use muons here; muon decay also will be discussed in the following section,
where it will be seen that the maximum energy of the emitted electrons is about
53 MeV, the lifetime is 2.2 µ sec, and parity is not conserved. Investigations of
the decay of the tau are more difficult because the tau is mainly produced through
electromagnetic processes such as e + e − → τ + τ − , and not through the decay of a
heavier meson as in the case of the muon, where copiously produced pions give rise
to the muons.
The scattering of neutrinos with charged leptons also involves only leptons: The
processes
νee − −→ νee − , νµe − −→ νeµ − , ντe − −→ νeτ −
(11.28)
are without electromagnetic or hadronic complications, and they, and the corresponding
ones involving antineutrinos, are ideal for exploring the weak interaction
at high energies. Indeed, such reactions have been studied both at accelerators (9,10)
and at reactors. (11)
Semileptonic Processes In semileptonic processes, one current is leptonic and
the other one hadronic. Three semileptonic decays are listed in Table 11.2. The
π ± decays are similar to that of 14 O, Table 11.1, and the ft 1/2 values are closely
related.
Can these decays give sufficient information to study the semi-leptonic weak
interaction thoroughly? The maximum energy listed in Table 11.2 is 81 MeV, but
the electromagnetic interaction taught us that energies of the order of many GeV
11 J.M. Conrad, M.H. Shaevitz, and T. Bolton, Reviews of Modern Physics, 70, 1341 (1998).