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2. The Pion Beta Decay Experiment

The goal of this experiment is the measurement of the rate of the rare decay
p ^{+}-> p ^{0}e^{+} n _{e} ( p b )
with an accuracy of 0.5%. The b -decay of the pion is analogous to the
superallowed pure Fermi transition in nuclear b -decay since pions are
spinless. It is a fundamental semileptonic weak interaction processes and thus
calculable with minimal model-dependent corrections. The accuracy of the
theoretical description allows a very precise test of the standard model of
electroweak interactions if the experimental result is of comparable precision.
This will allow to prove theoretical calculations of the decay rate including
radiative corrections and consequently probe the unitarity of the
Cabibbo-Kobayashi-Maskawa mixing matrix.
Before the description of the experimental technique the motivation and the
importance of a precise measurement shall be explained. The theoretical
description shall start with an introduction to the nuclear b -decay using
the Fermi theory of point interaction including the conserved vector current
(CVC) hypothesis, followed by the description of the electroweak interaction
and the CKM-matrix.