Measurement of $\Gamma(K_{e2})/\Gamma(K_{\mu2})$ ratio using stopped positive kaons at J-PARC

Abstract

The J-PARC E36 experiment is aiming at searching for the lepton universality violation by precisely measuring the ratio of the branching ratio of the K + → e + ν (K e2) to K + → µ + ν (K µ2) decays. The E36 experiment was performed at J-PARC employing a stopped K + beam 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 e + and µ + was carried out using an aerogel Cherenkov counters and a lead-glass Cherenkov counter, as well as by measuring the time-of-flight between TOF counters. The peak structure due to the K e2 decays was successfully observed in the e + momentum spectrum. The structuredependent radiative K + → e + νγ (K e2γ) events can be selected by requiring one photon hit in the CsI(Tl) calorimeter. The experimental spectra were reproduced by the Monte Carlo simulation, which indicates a correct understanding of the experimental conditions.

Global electroweak fit and vector-like leptons in light of the Cabibbo angle anomaly

“…Several experimental developments are foreseen which can improve the LFU tests in table 2. The J-PARC E36 experiment aims at measuring K → µν/K → eν [153]. A similar sensitivity as in R(V us ) may be possible for τ → µνν/τ → eνν at Belle II [154], where approximately one order of magnitude more τ leptons will be produced than at BELLE or BaBar.…”