Subatomic Physics

Chapter 11
The Weak Interaction
This chapter explores the weak interaction part of the electro-weak theory. The
history of the weak interaction is a series of mystery stories. In each story, a puzzle
appears, at first only in a vague form and then more and more clearly. Clues to the
solution are present but are overlooked or discarded, usually for the wrong reason.
Finally, the hero comes up with the right explanation and everything is clear until
the next corpse is unearthed. In the treatment of the electromagnetic interaction,
the well-understood classical theory provided an example which, properly translated
and reformulated, guided the development of quantum electrodynamics. No such
classical analog is present in the weak interaction, and the correct features had to be
taken from experiment and from analogies to the electromagnetic interaction. We
shall describe some of the puzzles and their solutions. In doing so we are hampered
by the self-imposed constraint of not using the Dirac theory. We shall therefore not
be able to write the interaction properly but shall use other means to explain the
crucial concepts.
At low energies and to lowest order in perturbation theory the weak interaction
can be described semi-phenomenologically in a satisfactory way. At high energies,
however, problems appear that have no solution if the weak interaction is treated
alone. The unification of the weak interaction with the electromagnetic one, however,
leads to a deeper understanding and to a solution of these problems. In this
chapter, we review some of the experimental knowledge and the basic phenomenology
gained from a study of the weak interaction. In the next two chapters we lay
the groundwork for, and sketch, the electroweak theory.
11.1 The Continuous Beta Spectrum
The continuous β-spectrum would then be understandable under the
assumption that during β-decay a light neutral particle is emitted with
every electron such that the sum of energies of neutrino and electron are
constant.
W. Pauli
331