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

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(Ke2)∕Γ(Kμ2)=Γ(K+→e+νe)∕Γ(K+→μ+νμ). Since the radiative K+→e+νeγ 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 Kμ2 decays. The delayed e+ signals from the muon decays were required in order to improve the S/N ratio of the Kμ2 peak by suppressing background events. In addition, a new energy calibration method of the CsI(Tl) calorimeter using stopped cosmic muons has been established.