First measurement of the low-momentum negative-muon beam for the muon-to-electron conversion experiment at J-PARC
The COMET Collaboration, K. Oishi, R. Abramishvili, G. Adamov, J. C. Angelique, K. Aoki, M. Aoki, Y. Arimoto, A. N. B. Az, I. Bagaturia, D. Bauer, T. Bouillaud, L. Del Buono, C. Cârloganu, N. Chadeau, Y. Chen, P. Chetry, D. Chokheli, T. Clouvel, P. D. Dauncey, S. Dekkers, R. Derveni, G. Devidze, P. J. Dornan, M. T. Dubouchet, L. B. Epshteyn, I. Evtoukhovitch, P. Evtoukhovitch, G. Faure, S. Fayer, G. V. Fedotovich, Y. Fujii, Y. Fukao, E. George, E. L. Gillies, M. Gouzevitch, D. N. Grigoriev, E. Hamada, I. H. Hashim, S. Hashimoto, M. Higashide, Y. Higuchi, N. Hiroshima, Y. Igarashi, F. V. Ignatov, M. Iio, M. Ikeno, K. Ishibashi, T. Itahashi, A. Jansen, R. Jeena, O. Jeremy, X. Jiang, P. M. Jonsson, Y. Kageyama, V. Kalinnikov, E. Kaneva, F. Kapusta, K. Kawagoe, R. Kawashima, N. Kazak, A. Khvedelidze, Y. Kojima, A. N. Kozyrev, B. Krikler, Y. Kuno, S. Kuribayashi, Y. Kurochkin, N. Kurosawa, A. Kurup, K. Ladhams, P. Lebrun, M. J. Lee, H. Li, W. Li, R. P. Litchfield, D. Lomidze, I. Lomidze, G. Macharashvili, Y. Makida, S. Makimura, G. Matsumoto, S. Matsumoto, A. Melnik, H. Miao, S. Mihara, A. Miles, M. Miyataki, Y. Miyazaki, T. Mizuno, M. Moritsu, R. Muto, R. Nagai, H. Nagashima, Y. Nakai, T. Nakamoto, Y. Nakamura, Y. Nakatsugawa, Y. Nakazawa, J. Nash, M. A. S. Nasrullah, T. M. Nguyen, V. Niess, Y. S. Ning, H. Nishiguchi, M. Nishihara, K. Noguchi, N. H. M. Nor, N. Norjoharuddeen, T. Ogitsu, H. Ohata, N. Ohuchi, K. Okabe, T. Okamura, H. Okurano, M. Oonaka, J. Pasternak, A. Paulau, D. Pieters, V. Raspal, R. Razali, A. Razquin, W. Roth, B. Sabirov, T. Saito, H. Sakamoto, P. Sarin, K. I. Sasaki, R. Sasaki, A. Sato, J. Sato, H. Sendai, N. Shigyo, M. Shinjo, Z. Shioya, M. Shoji, Dz. Shoukavy, W. Da Silva, A. Simonenko, D. Stöckinger, N. Sumi, A. Sumimura, S. Sun, J. Suzuki, S. Takami, S. Tanaka, J. Tang, J. Tojo, M. Tomizawa, T. Toriashvili, N. Tran, Z. Tsamalaidze, N. Tsverava, T. Uchida, Y. Uchida, Y. Uchiyama, S. Ueda, K. Ueno, Y. Uesaka, D. Vasileuskaya, E. Velicheva, M. L. Wong, T. S. Wong, C. Wu, T. Xing, Y. Xu, C. Yamada, H. Yamaguchi, M. Yamanaka, Y. Yang, W. Yao, B. Yeo, H. Yoshida, M. Yoshida, M. Yoshii, D. Yoshikawa, T. Yoshioka, Y. Yuan, Yu. V. Yudin, N. Yussup, I. Yusuff, J. Zhang, Y. Zhang, Z. Zhang, Y. Zhou, K. Zuber
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.
