Estimation of single event sensitivity of the COMET Phase-II muon conversion experiment

Abstract

In this study, we present the estimation of signal event sensitivity of COMET Phase-II muon-to-electron conversion ($\mu+N \to e+N$) experiment in J-PARC. The muon-to-electron conversion is one of CLFV (Charged Lepton Flavor Violation) process, which affects considerably to new physics theories beyond Standard Model in particle physics. The COMET experiment has two phases where the Phase-II experiment aims $10^{-17}$ level of sensitivity. The single event sensitivity and 90% confidence level upper limit of COMET Phase-II experiment with conversion electron signal at electromagnetic calorimeter and muon Decay-In-Orbit (DIO) background were estimated by using ICEDUST software framework. 100,000 conversion electron signals and 300,000 DIO background events were simulated and analyzed. The estimated signal event sensitivity is $1.59 \times 10^{-17}$, and 90% upper limit of null event is estimated to $2.03\times 10^{-17}$, which is around 100 times better sensitivity than Phase-I experiment even in the case of using calorimeter as main detector. The experimental sensitivity estimated in this study can be improved by considering straw tracking detector, which provides much better momentum measurements with narrower width.