The COMET Experiment Searching for Muon-to-Electron Conversion at J-PARC

Title: The COMET Experiment Searching for Muon-to-Electron Conversion at J-PARC

The COMET experiment searches for coherent muon-to-electron conversion in aluminium nuclei, $\mu^- + {\rm Al} \rightarrow e^- + {\rm Al}$, at J-PARC in Ibaraki, Japan. This charged-lepton-flavour-violating process is extremely suppressed in the Standard Model, with an expected rate of order $10^{-54}$, even when neutrino oscillations are taken into account. In contrast, a wide range of theories beyond the Standard Model (BSM) predict experimentally accessible rates, at most $10^{-15}$. Its observation would therefore provide evidence of BSM, while improved limits would place stringent constraints on flavour models at high energy scales.

COMET will be carried out in two stages. Phase-I aims to reach a single-event sensitivity at the $\mathcal{O}(10^{-15})$ level, followed by Phase-II with a target sensitivity of $\mathcal{O}(10^{-17})$. The experiment uses a pulsed 8-GeV proton beam slowly extracted from the J-PARC Main Ring. This beam structure provides an intense source of stopped muons while allowing prompt beam-related backgrounds to be strongly suppressed by delayed timing measurements. A dedicated muon beamline based on superconducting solenoids is being constructed to efficiently transport low-momentum negative muons to the aluminium stopping target.

We have been constructing a dedicated experimental facility at J-PARC. Its first commissioning, Phase-$\alpha$, was successfully performed in 2023. In this campaign, proton beam extraction into the COMET area and muon transport to the detector region were demonstrated using a simplified beamline configuration.

Significant progress has also been made toward the completion of the Phase-I beamline with installation of all the required superconducting. Magnet commissioning is currently in progress, and a detailed magnetic-field measurement in the detector region is also planned. These activities are essential for establishing the beam optics and detector acceptance required for the first physics run.

The main Phase-I detector, the CyDet system, consists of the Cylindrical Drift Chamber (CDC) and the Cylindrical Trigger Hodoscope (CTH). Detector construction, trigger development, data acquisition, and analysis frameworks have been progressing through cosmic-ray and bench tests. Preparations are now underway for an integrated dry run of CyDet.

In this talk, I will introduce the physics motivation and experimental concept of COMET, report the latest progress of the Phase-I facility and detector preparation, and discuss the prospects for the forthcoming commissioning and physics programme.