Development of the Range Counter for the COMET Phase-$\alpha$ Experiment

TL;DR

This paper reports the development and testing of a Range Counter for the COMET Phase-α experiment, designed to measure muon momentum spectra and detect decay electrons with high efficiency, aiding muon-to-electron conversion searches.

Contribution

The paper introduces a novel Range Counter system with high detection efficiency and demonstrates its performance through experimental evaluation and simulation for the COMET experiment.

Findings

Muon-trigger efficiency exceeded 99.9%.

Electron detection efficiency was over 99%.

Estimated acceptance for DIO electron reconstruction is about 47% with 60% purity.

Abstract

The COMET Phase-$\alpha$ experiment aims to evaluate the novel muon transport beamline for the muon-to-electron conversion search at J-PARC, Japan. A dedicated Range Counter (RC) was developed to measure the momentum spectrum of transported negative muons with momenta of 30--100 MeV/$c$. The RC consists of graphite momentum degraders, a muon absorber, and plastic scintillation counters ($\rm T_0$, $\rm T_1$, and $\rm T_2$) to detect decay-in-orbit (DIO) electrons from stopped muons. The number of muons stopped in the absorber is reconstructed from the decay time distribution. A copper absorber was selected due to the short lifetime of muonic atoms in copper, which enhances signal separation. The counters' performance was evaluated experimentally. The $\rm T_0$ Counter, made of a $200\times 200\times 0.5~{\rm mm^3}$ scintillator plate, achieved a muon-trigger efficiency exceeding 99.9%. The $\rm T_1$ and $\rm T_2$ Counters also demonstrated high electron-detection efficiencies of $>99$%. Based on these results, simulation studies estimate the acceptance for reconstructing the number of DIO electrons from the absorber to be approximately 47% with a corresponding signal purity of 60% against muon capture-induced backgrounds.