Measurement of structure dependent radiative $K^{+} \rightarrow e^{+} \nu \gamma$ decays using stopped positive kaons at t J-PARC

Abstract

The J-PARC E36 experiment aims at searching for a lepton universality violation by precisely measuring the ratio of the branching ratio of the + → + ( ) ( 2( ) ) to + → + ( 2 ) decays. The E36 experiment was performed in 2015 at J-PARC using a stopped + method in conjunction with a 12-sector iron-core superconducting toroidal spectrometer. Charged particle momenta were calculated by reconstructing the tracks in the spectrometer. Particle discrimination between + and + was carried out using an aerogel Cherenkov counter and a lead-glass Cherenkov counter, as well as by measuring the time-of-flight between TOF counters. The peak structure due to the 2( ) decays was successfully observed in the + momentum spectrum. The structuredependent radiative + → + ( SD 2 ) events were selected by requiring photon hits in the CsI(Tl) calorimeter or the GSC counter. The experimental spectra were reproduced by the Monte Carlo simulation, which indicates a correct understanding of the detector acceptance. The ( SD 2 ) value relative to ( 2( ) ) was obtained by calculating the ratio of the SD 2 and 2( ) yields corrected for their detector acceptances. The ( SD 2 )/ ( 2( ) ) value was obtained to be 1.22 ± 0.07 ± 0.04 in the CsI(Tl) analysis and 1.22 ± 0.13 ± 0.08 in the GSC analysis.