Subatomic Physics

346 The Weak Interaction
ones for hypercharge-conserving decays. We shall return to this discrepancy in Section
11.9 and show that it has an explanation within the framework of the weak
current–current interaction.
Parity violation has already been treatedinSection9.3;theelectromagnetic and
the hadronic force conserve parity, but a violation appears in the weak one. The
example discussed in Section 9.3 was a semileptonic decay. The original evidence
for parity nonconservation came from the decay of the charged kaons into two and
three pions; these weak decays involve hadrons. In the next section we shall show
that the purely leptonic decay of the muon also does not conserve parity. These
examples indicate that the various processes all violate parity conservation. This
fact alone would not justify classing them all into one category. However, it indicates
a similarity in the form of the interaction that causes these decays, and it supports
the conclusion already reached from a consideration of the lifetimes.
Conservation of strangeness or hypercharge in the hadronic and the electromagnetic
interaction was postulated in Eq. (7.42). The examples of weak decays
discussedinSection7.5andinthepresent section indicate that many cases are
known where the strangeness changes by one unit; no case has been found where a
change of two units occurs. The selection rule for strangeness,
∆S = 0 in hadronic and electromagnetic interaction
∆S =0, ±1 in the weak interaction, (11.33)
thus establishes another characteristic feature of the weak interaction.
11.5 The Muon Decay
In the previous section we have surveyed weak processes, and we have partially answered
the first question posed at the end of Section 11.3, namely what phenomena
are described by Hw. The form of the weak current and the value of the weak
coupling constant remain to be studied. We can expect that the fundamental features
of the weak interaction will be easiest to explore in purely leptonic processes
because no serious interference from the hadronic force is present there. In this
section, the salient features of the much studied muon decay will be described. The
decay of the tau is very similar, but it can decay into either muons or electrons.
Muons and taus do not interact strongly, and it is consequently not possible
to produce them directly and copiously through a reaction. However, the decay of
charged pions is a convenient source of muons. Assume, for instance, that positive
pions are produced at an accelerator. The pions are selected in a pion channel and
slowed down in an absorber (Fig. 11.7). If their energy is not too high they usually
come to rest before decaying through the mode
π + −→ µ + νµ. (11.34)