Testing Lepton Flavor Universality and CKM Unitarity with Rare Pion Decays in the PIONEER experiment

Abstract

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/µ , 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), π + → π 0 e + ν(γ), currently at 1.036(6) × 10 −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.

High-precision muon decay predictions for ALP searches

“…The same investigation has been performed by PIENU [22], with resulting limits at the level of 10 −4 − 10 −5 in the mass range 47.8 MeV < m X < 95.1 MeV. The forthcoming experiment PIONEER [23,24] is expected to improve such limits by 1 − 2 orders of magnitude, in a similar mass range as PIENU. Furthermore, the Muχe experiment [25] has been proposed to explore the mass range 86 MeV < m X < 105 MeV with a branching ratio sensitivity of 10 −6 − 10 −7 .…”

“…as all VP terms of order α 2 that are not included in (24). This is a slight abuse of our notation, as h vp 2 is already pointwise finite.…”

The remarkable role of the vector-like quark doublet in the Cabibbo angle and $W$-mass anomalies

“…In order to settle the CKM unitary puzzle, improved experimental inputs for neutron decay time [124,125], g A parameter [126][127][128], and pion β decay [129,130] are expected in the foreseeable future. The leading hadronic uncertainties at both super-allowed 0 + − 0 + and neutron decays can be reduced by lattice QCD calculations, which improve the estimation of γW box diagrams (and are executable with the state of the art techniques) [131].…”

Some Theoretical Aspects of Searches for Heavy Neutrino Emission in Kaon Decays

“…The uncertainty in the PIENU measurement of R (π) e/µ is ≃ 2 × 10 −3 , while the estimated uncertainty in the theoretical calculation of R (π) e/µ is 1.2 × 10 −4 . There are plans for a new experiment called PIONEER at PSI, which will measure BR(π + → e + ν e ) to an accuracy of ∼ 10 −4 , matching the accuracy of theoretical calculation [22,23]. This will enable a more stringent test for heavy neutrino effects, as well as testing e − µ universality to higher precision.…”