In the original Yukawa formulation of meson theory,
the π meson (as we now believe Yukawa's particle to
be) was to provide a natural explanation for β decay.
The process π → e + ν[bar] was regarded as an elementary
interaction and nuclear β decay was imagined to proceed
by the route n → p + π → μ + e + ν[bar]. There are a variety
of reasons why this scheme fails. Just the opposite point
of view is now generally adopted, namely, that the
nuclear β decay is fundamental and that the observed
decay of the π meson is to be explained in terms of it.
We do not exclude the possibility that β decay be
described in terms of an as yet unknown heavy intermediate.
Nevertheless, the nuclear β decay is to be
regarded as essentially primary. In order to describe the
actual dominant π-meson decay mode π → μ + ν[bar] it is
necessary to assume the existence of another β decay
like process, μ-meson capture. The elementary process
may be described as μ + p → n + ν, or equally well as
n + p[bar] → μ + ν[bar]; the first is the experimentally observed
μ-meson absorption reaction, whereas, the second, the
annihilation of a neutron and an antiproton, plays an
important role in π-meson decay
