COMETBMP · Designing, Optimising and Validating the Beam Measurement Programme at the New Intense Muon Beam Line for the COMET Muon-to-Electron Search Experiment

Designing, Optimising and Validating the Beam Measurement Programme at the New Intense Muon Beam Line for the COMET Muon-to-Electron Search Experiment

The COMET experiment will search for muon-to-electron conversion in aluminium, Al + μ- -> Al + e-, at J-PARC in Japan, with a single-event sensitivity of up to O(10^-17) [1]. The conversion process violates charged lepton flavour conservation. Its branching ratio is estimated to be lower than O(10^-54) in the standard model, including neutrino scintillation. However, several models beyond the standard model (BSM) estimate it to be enhanced to maximally O(10^-15). Since the standard model's branching ratio is negligibly small, any observation of the process implies a new BSM physics discovery. The experiment aims to make the first observation or further improve upon the current upper limit provided by the previous experiment [2]. The synchrotron accelerator at J-PARC provides the world's most intense proton beam to a fixed target, generating a large number of muons. Particularly, the low-momentum portion should be transported to a muon stopping target to be measured by detectors. However, the intense proton beam also introduces a significant number of background particles into the detectors. To address both challenges, we have installed the Muon Transport Solenoid (TS) after the proton-beam target section. This component of the beamline is curved and surrounded by a series of solenoid magnets. The curved solenoidal magnetic field enables us to transport the secondary muon beam while effectively eliminating other background particles at the same time. The COMET collaboration plans to execute Beam Measurement Programmes (BMP) differently from Phase-I, which is the main phase for measuring the conversion process, in order to test muon transport and study beam backgrounds. The BMP consist of two phases: (1) Phase-α focuses on TS commissioning with a simple setup, and (2) Phase-I BMP aims to investigate beam backgrounds using the same beamline as in Phase-I. One crucial aspect of the Phase-I BMP is the design of a beam blocker (BB), which will be installed before the detectors to shield them from intense background beam flux and assist in effectively measuring important particles to assess TS performance. The project contributed to both Phase-α and the Phase-I BMP. I served as the Phase-α working group leader. One of the most significant accomplishments was the successful demonstration that the TS transports beam muons as anticipated. For the Phase-I BMP, we tested the performance of a tungsten BB using pion, muon, and electron beams at the Paul Scherrer Institut (PSI) in Switzerland. The experimental data are currently being compared with our simulation tool's expectations. This comparison aims to validate the simulation tool for further refinement of the BB design. Additionally, the experiment included successful tests for two other detectors for Phase-α and Phase-I. [1] The COMET Collaboration, 2020, Progress of Theoretical and Experimental Physics. [2] Bertl et al., The European Physical Journal C - Particles and Fields.

Data: CORDIS, © European Union