Subatomic Physics

344 The Weak Interaction
Table 11.2: Decay Properties of Three Semileptonic Decays.
Spin-parity t 1/2 † Emax ft 1/2
Decay Sequence (sec) (MeV) (sec)
π ± → π0eν 0− → 0− 1.76 4.1 3.1 × 103 n0 → pe¯ν
1 + 1 +
→ 2 2 612 0.78 1.1 × 103 Σ− → Λ0e− ¯ν
1 + 1 +
→ 2 2 1.8 × 10−6 81 6 × 103 † Partial half-life.
are necessary to explore some of the properties. Weak decays with such energies are
very difficult to observe because a state with very high excitation generally decays
hadronically or electromagnetically, so that the weak interaction cannot compete.
An example is the ψ/J and its excited states with energies in excess of 3 GeV. Even
though selection rules slow down the decay into hadrons, the contribution from the
weak interaction to the decay is so small that it has not yet been observed. At much
higher energies, the situation is even more unfavorable.
One of the best ways of studying the high energy behavior of the weak interaction
is through semileptonic neutrino-induced reactions such as
and deep inelastic scattering
νµn −→ µ − p, νµp −→ νµp
¯νµp −→ µ + n, νµp −→ νµnπ + ,
νµp −→ νµX,
−→ µ −X, (11.29)
(11.30)
where X is any particle or particles.
The reactions in the first column of Eq. (11.29) involve charged weak currents
and the exchange of a W ± , the ones in the right column require neutral weak
currents and the exchange of a Z 0 . The reactions of the types shown in Eqs. (11.29)
and (11.30) have helped to validate the Weinberg–Salam (WS) theory, and have
been used to obtain structure functions. They will be discussed in more detail
below and in Section 11.14.
In the semileptonic processes listed so far, the weak decays have not involved a
change of strangeness. True, the decay Σ + → Λ 0 e + ν in Table 11.2 involves strange
particles, but the hadrons in the initial and final states have the same strangeness.
We have, however, mentioned in Section 7.5 that strangeness or hypercharge is not
necessarily conserved in the weak interaction. Indeed, strangeness-changing weak
decays exist, and three are listed in Table 11.3. They are all mediated by charged
currents. No strangeness-changing decays or reactions that occur through neutral
weak currents have been observed; for instance, the decay Λ 0 → ne + e − is absent.