  The physics motivation and the conceptual design of the PIONEER experiment, a
next-generation rare pion decay experiment testing lepton flavor universality
and CKM unitarity, are described. Phase I of the PIONEER experiment, which was
proposed and approved at Paul Scherrer Institut, aims at measuring the
charged-pion branching ratio to electrons vs.\ muons, $R_{e/\mu}$, 15 times
more precisely than the current experimental result, reaching the precision of
the Standard Model (SM) prediction at 1 part in $10^4$. Considering several
inconsistencies between the SM predictions and data pointing towards the
potential violation of lepton flavor universality, the PIONEER experiment will
probe non-SM explanations of these anomalies through sensitivity to quantum
effects of new particles up to the PeV mass scale. The later phases of the
PIONEER experiment aim at improving the experimental precision of the branching
ratio of pion beta decay (BRPB), $\pi^+\to \pi^0 e^+ \nu (\gamma)$, currently
at $1.036(6)\times10^{-8}$, by a factor of three (Phase II) and an order of
magnitude (Phase III). Such precise measurements of BRPB will allow for tests
of CKM unitarity in light of the Cabibbo Angle Anomaly and the theoretically
cleanest extraction of $|V_{ud}|$ at the 0.02\% level, comparable to the
deduction from superallowed beta decays.
