First measurement of the low-momentum negative-muon beam for the muon-to-electron conversion experiment at J-PARC
Abstract
The COMET Phase-α experiment, the initial beam-operation stage of the COMET muon-to-electron conversion programme at J-PARC, took place in 2023 and marked the first beam operation at the COMET facility. It served to commission the delivery of a bunched, slow-extracted 8 GeV proton beam and the muon beamline. Its objective was to demonstrate experimentally that low-momentum negative muons could be transported to the experimental area through the Transport Solenoid (TS). We report a measurement of the transported muon beam using the Range Counter, a dedicated detector that identifies stopped negative muons by detecting delayed electrons emitted after the muons stop in a copper absorber. From the measured decay-time spectra, we reconstruct the number of stopped muons, the corresponding momentum spectrum, and the transverse beam profile. The observation of the stopped-muon component demonstrates that slow negative muons were transported through the TS to the experimental area. The reconstructed momentum spectrum indicates that the transported beam is concentrated in the low-momentum region. The non-uniform transverse profile is qualitatively consistent with charge drift in the curved solenoidal magnetic field. A Geant4-based simulation reproduces the overall dependence on degrader thickness and detector position; the measured yields are systematically higher by roughly 30%. These results provide the first experimental benchmark of low-momentum negative-muon transport in COMET and establish the basis for future beam characterisation and production studies.
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BibTeX
@misc{oishi2026first,
title = {{First measurement of the low-momentum negative-muon beam for the muon-to-electron conversion experiment at J-PARC}},
author = {K Oishi and R Abramishvili and G Adamov and J.C Angelique and K Aoki and M Aoki and Y Arimoto and A.N.B Az and I Bagaturia and D Bauer and T Bouillaud and L del Buono and C Carloganu and N Chadeau and Y Chen and P Chetry and D Chokheli and T Clouvel and P.D Dauncey and S Dekkers and R Derveni and G Devidze and P.J Dornan and M.T Dubouchet and L.B Epshteyn and I Evtoukhovitch and P Evtoukhovitch and G Faure and S Fayer and G.V Fedotovich and Y Fujii and Y Fukao and E George and E.L Gillies and M Gouzevitch and D.N Grigoriev and E Hamada and I.H Hashim and S Hashimoto and M Higashide and Y Higuchi and N Hiroshima and Y Igarashi and F.V Ignatov and M Iio and M Ikeno and K Ishibashi and T Itahashi and A Jansen and R Jeena and O Jeremy and X Jiang and P.M Jonsson and Y Kageyama and V Kalinnikov and E Kaneva and F Kapusta and K Kawagoe and R Kawashima and N Kazak},
howpublished = {HAL},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.02855},
eprint = {2610.02855},
archivePrefix = {arXiv},
url = {https://hal.science/hal-05829677v1},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - First measurement of the low-momentum negative-muon beam for the muon-to-electron conversion experiment at J-PARC
AU - K Oishi
AU - R Abramishvili
AU - G Adamov
AU - J.C Angelique
AU - K Aoki
AU - M Aoki
AU - Y Arimoto
AU - A.N.B Az
AU - I Bagaturia
AU - D Bauer
AU - T Bouillaud
AU - L del Buono
AU - C Carloganu
AU - N Chadeau
AU - Y Chen
AU - P Chetry
AU - D Chokheli
AU - T Clouvel
AU - P.D Dauncey
AU - S Dekkers
AU - R Derveni
AU - G Devidze
AU - P.J Dornan
AU - M.T Dubouchet
AU - L.B Epshteyn
AU - I Evtoukhovitch
AU - P Evtoukhovitch
AU - G Faure
AU - S Fayer
AU - G.V Fedotovich
AU - Y Fujii
AU - Y Fukao
AU - E George
AU - E.L Gillies
AU - M Gouzevitch
AU - D.N Grigoriev
AU - E Hamada
AU - I.H Hashim
AU - S Hashimoto
AU - M Higashide
AU - Y Higuchi
AU - N Hiroshima
AU - Y Igarashi
AU - F.V Ignatov
AU - M Iio
AU - M Ikeno
AU - K Ishibashi
AU - T Itahashi
AU - A Jansen
AU - R Jeena
AU - O Jeremy
AU - X Jiang
AU - P.M Jonsson
AU - Y Kageyama
AU - V Kalinnikov
AU - E Kaneva
AU - F Kapusta
AU - K Kawagoe
AU - R Kawashima
AU - N Kazak
PY - 2026
DA - 2026/10/02
PB - HAL
DO - 10.48550/arxiv.2610.02855
UR - https://hal.science/hal-05829677v1
AB - The COMET Phase-α experiment, the initial beam-operation stage of the COMET muon-to-electron conversion programme at J-PARC, took place in 2023 and marked the first beam operation at the COMET facility. It served to commission the delivery of a bunched, slow-extracted 8 GeV proton beam and the muon beamline. Its objective was to demonstrate experimentally that low-momentum negative muons could be transported to the experimental area through the Transport Solenoid (TS). We report a measurement of the transported muon beam using the Range Counter, a dedicated detector that identifies stopped negative muons by detecting delayed electrons emitted after the muons stop in a copper absorber. From the measured decay-time spectra, we reconstruct the number of stopped muons, the corresponding momentum spectrum, and the transverse beam profile. The observation of the stopped-muon component demonstrates that slow negative muons were transported through the TS to the experimental area. The reconstructed momentum spectrum indicates that the transported beam is concentrated in the low-momentum region. The non-uniform transverse profile is qualitatively consistent with charge drift in the curved solenoidal magnetic field. A Geant4-based simulation reproduces the overall dependence on degrader thickness and detector position; the measured yields are systematically higher by roughly 30%. These results provide the first experimental benchmark of low-momentum negative-muon transport in COMET and establish the basis for future beam characterisation and production studies.
ER -