COMET Phase-α Experiment to Investigate COMET's New Muon Beamline at J-PARC

The COMET Phase-I and Phase-II experiments will search for muon-to-electron conversion, a process that violates the charged lepton flavour conservation, with the goal of significantly improving upon the current upper limit for the branching ratio. It involves the construction of dedicated facilities and a beamline at J-PARC, Japan, necessitating comprehensive assessments.

The Phase-Alpha experiment took place in February and March 2023, representing a crucial milestone in commissioning the primary proton beam and investigating the secondary muon beam. For the purposes, we developed simple and effective detectors, including TiO2 sensors for proton beam monitoring, a plastic scintillating fibre hodoscope, and an assembly to count negative muons. Additionally, a tracking detector originally designed for Phase-I and -II was employed.

The commissioning of the proton beam within the COMET facility was a significant achievement, accompanied by precise measurements of its beam size. Furthermore, the successful transport of the muon beam through the COMET Muon Transport Solenoid, a pivotal beamline component, was confirmed. Notably, the momentum spectrum and beam profile of the transported negative muons are generically aligned with estimations derived from our simulation software.

This article provides a comprehensive overview of Phase-Alpha, encompassing its detectors, preliminary results, and future prospects.