Performance check of the CsI(Tl) calorimeter for the J-PARC E36 experiment by observing e+ from muon decay

Abstract The J-PARC E36 experiment is searching for lepton universality violation with a stopped kaon beam by measuring the ratio of the K + decay widths Γ ( K e 2 ) ∕ Γ ( K μ 2 ) = Γ ( K + → e + ν e ) ∕ Γ ( K + → μ + ν μ ) . Since the radiative K + → e + ν e γ decays are backgrounds to be removed in this measurement, the radiated γ rays were detected in a CsI(Tl) calorimeter. The energy calibration for the 768 CsI(Tl) modules was performed using mono-chromatic μ + s from the K μ 2 decays. The delayed e + signals from the muon decays were required in order to improve the S/N ratio of the K μ 2 peak by suppressing background events. In addition, a new energy calibration method of the CsI(Tl) calorimeter using stopped cosmic muons has been established.