Magnetic Field Measurement of Superconducting Transport Solenoid for COMET

The COMET experiment, currently under construction at J-PARC, aims to explore the process of muon-to-electron conversion in a nucleus. This phenomenon, known as charged-lepton flavor violation, is an elementary process beyond the Standard Model. The muon beam is produced from pion decays generated by bombarding a proton beam on a target and corrected and transported from the production target to a stopping target in a detector with superconducting solenoid magnets. Their momentum is selected by a curved solenoid field (3 T) and dipole field (0.05 T) in the Muon Transport Solenoid (MTS), which has a diameter of 468 mm and a curvature radius of 3 m with a bending angle of 90 degrees. We energized the solenoid up to 105 A and the dipole to $\pm$175 A in the summer of 2022. Although the solenoid was operated at a half of its design value (210 A) due to the limitation of the support structure, this commissioning provided a sufficient current for the subsequent engineering beam operation (Phase-$\alpha$) to be conducted successfully. We performed in-situ magnetic measurements of the curved solenoid with Hall probes and compared them to 3D calculations. The paper describes the scheme and results of the magnetic filed measurement in MTS.

I. Introduction

A new superconducting muon beamline is under construction at J-PARC in Japan for the COMET experiment [1]. This experiment aims to search for muon-to-electron conversion processes that indicate new physics beyond the Standard Model of elementary particles. The search needs a high-intensity muon beam with an intensity of to achieve sensitivity on the branching ratio of the processes down to . A series of superconducting solenoid magnets is employed to capture and transport muons to the detector system. The COMET experiment plans a phased approach to upgrading the superconducting magnet system, and the magnets for COMET Phase-I are now under construction at J-PARC.