Deviation from the standard model in the decay π + → e + νγ

Abstract

A precise investigation of radiative pion decay (π+ → e+νγ) in a pion beam from the meson factory of the Paul Scherrer Institute (Switzerland) was performed by the PIBETA Collaboration with the aid of the PIBETA detector. This resulted in finding 41 601 events of radiative pion decay in three kinematical regions. The absolute values of the branching ratio for radiative pion decay were determined in each of these regions. To a precision approximately four times higher than that known previously, the ratio of the axial-vector to the vector form factor was found to be γ ≡ FA/FV = 0.443 (15), the latter being fixed at FV = 0.0259. The number of events found in the kinematical region specified by photon energies of Eγ > 55.6 MeV, positron energies of Ee > 20.0 MeV, and angles of θγ, e > 40° between the momenta of the corresponding particles (B region) was 5233. In region B, the measured branching ratio for radiative pion decay, Rπ→evγ(expt) = 11.6(3) × 10−8, proved to be smaller by eight standard deviations than that which follows from the Standard Model, Rπ→evγ (theor) = 14.34(1) × 10−8.