  Precision measurements in allowed nuclear beta decays and neutron decay are
reviewed and analyzed both within the Standard Model and looking for new
physics. The analysis incorporates the most recent experimental and theoretical
developments. The results are interpreted in terms of Wilson coefficients
describing the effective interactions between leptons and nucleons (or quarks)
that are responsible for beta decay. New global fits are performed
incorporating a comprehensive list of precision measurements in neutron decay,
superallowed $0^+\rightarrow 0^+$ transitions, and other nuclear decays that
include, for the first time, data from mirror beta transitions. The results
confirm the $V$-$A$ character of the interaction and translate into updated
values for $V_{ud}$ and $g_A$ at the $10^{-4}$ level. We also place new
stringent limits on exotic couplings involving left-handed and right-handed
neutrinos, which benefit significantly from the inclusion of mirror decays in
the analysis.
