  A next-generation rare pion decay experiment, PIONEER, is strongly motivated
by several inconsistencies between Standard Model (SM) predictions and data
pointing towards the potential violation of lepton flavor universality. It will
probe non-SM explanations of these anomalies through sensitivity to quantum
effects of new particles even if their masses are at very high scales.
Measurement of the charged-pion branching ratio to electrons vs. muons
$R_{e/\mu}$ is extremely sensitive to new physics effects. At present, the SM
prediction for $R_{e/\mu}$ is known to 1 part in $10^4$, which is 15 times more
precise than the current experimental result. An experiment reaching the
theoretical accuracy will test lepton flavor universality at an unprecedented
level, probing mass scales up to the PeV range. Measurement of pion beta decay,
$\pi^+\to \pi^0 e^+ \nu (\gamma)$, with 3 to 10-fold improvement in
sensitivity, will determine $V_{ud}$ in a theoretically pristine manner and
test CKM unitarity, which is very important in light of the recently emerged
tensions. In addition, various exotic rare decays involving sterile neutrinos
and axions will be searched for with unprecedented sensitivity. The experiment
design benefits from experience with the recent PIENU and PEN experiments at
TRIUMF and the Paul Scherrer Institut (PSI). Excellent energy and time
resolutions, greatly increased calorimeter depth, high-speed detector and
electronics response, large solid angle coverage, and complete event
reconstruction are all critical aspects of the approach. The PIONEER experiment
design includes a 3$\pi$ sr 25 radiation length calorimeter, a segmented low
gain avalanche detector stopping target, a positron tracker, and other
detectors. Using intense pion beams, and state-of-the-art instrumentation and
computational resources, the experiments can be performed at the PSI ring
cyclotron.
