Study of the muon decay-in-flight in the $τ^-\toμ^-\barν_μν_τ$ decay to measure the Michel parameter $ξ^\prime$
Creators: name: Belle Collaboration
name: Bodrov, D.
name: Pakhlov, P.
name: Adachi, I.
name: Aihara, H.
name: Said, S. Al
name: Asner, D. M.
name: Atmacan, H.
name: Aushev, T.
name: Ayad, R.
name: Babu, V.
name: Banerjee, Sw.
name: Behera, P.
name: Belous, K.
name: Bennett, J.
name: Bessner, M.
name: Bhuyan, B.
name: Bilka, T.
name: Biswas, D.
name: Bobrov, A.
name: Bondar, A.
name: Borah, J.
name: Bozek, A.
name: Bračko, M.
name: Branchini, P.
name: Browder, T. E.
name: Budano, A.
name: Campajola, M.
name: Červenkov, D.
name: Chang, M. -C.
name: Cheon, B. G.
name: Chilikin, K.
name: Cho, K.
name: Cho, S. -J.
name: Choi, S. -K.
name: Choi, Y.
name: Choudhury, S.
name: Cinabro, D.
name: Das, S.
name: De Nardo, G.
name: De Pietro, G.
name: Dhamija, R.
name: Di Capua, F.
name: Dingfelder, J.
name: Doležal, Z.
name: Dong, T. V.
name: Epifanov, D.
name: Ferber, T.
name: Ferlewicz, D.
name: Fulsom, B. G.
name: Gaur, V.
name: Garmash, A.
name: Giri, A.
name: Goldenzweig, P.
name: Graziani, E.
name: Greenwald, D.
name: Gu, T.
name: Guan, Y.
name: Gudkova, K.
name: Hadjivasiliou, C.
name: Halder, S.
name: Hayasaka, K.
name: Hayashii, H.
name: Hedges, M. T.
name: Herrmann, D.
name: Hou, W. -S.
name: Hsu, C. -L.
name: Iijima, T.
name: Inami, K.
name: Ipsita, N.
name: Ishikawa, A.
name: Itoh, R.
name: Iwasaki, M.
name: Jacobs, W. W.
name: Jang, E. -J.
name: Ji, Q. P.
name: Jia, S.
name: Jin, Y.
name: Joo, K. K.
name: Kalita, D.
name: Kaliyar, A. B.
name: Kang, K. H.
name: Kawasaki, T.
name: Kiesling, C.
name: Kim, C. H.
name: Kim, D. Y.
name: Kim, K. -H.
name: Kim, Y. -K.
name: Kindo, H.
name: Kinoshita, K.
name: Kodyš, P.
name: Korobov, A.
name: Korpar, S.
name: Križan, P.
name: Krokovny, P.
name: Kuhr, T.
name: Kumar, M.
name: Kumar, R.
name: Kumara, K.
name: Kwon, Y. -J.
name: Lange, J. S.
name: Lee, S. C.
name: Li, J.
name: Li, L. K.
name: Li, Y.
name: Libby, J.
name: Lieret, K.
name: Lin, Y. -R.
name: Liventsev, D.
name: Ma, Y.
name: Masuda, M.
name: Matsuda, T.
name: Maurya, S. K.
name: Meier, F.
name: Merola, M.
name: Metzner, F.
name: Miyabayashi, K.
name: Mizuk, R.
name: Mohanty, G. B.
name: Mussa, R.
name: Nakamura, I.
name: Nakao, M.
name: Narwal, D.
name: Natkaniec, Z.
name: Natochii, A.
name: Nayak, L.
name: Nayak, M.
name: Nisar, N. K.
name: Nishida, S.
name: Ogawa, S.
name: Ono, H.
name: Oskin, P.
name: Pakhlova, G.
name: Pardi, S.
name: Park, H.
name: Park, J.
name: Park, S. -H.
name: Patra, S.
name: Paul, S.
name: Pedlar, T. K.
name: Pestotnik, R.
name: Piilonen, L. E.
name: Podobnik, T.
name: Prencipe, E.
name: Prim, M. T.
name: Rabusov, A.
name: Russo, G.
name: Sandilya, S.
name: Sangal, A.
name: Santelj, L.
name: Savinov, V.
name: Schnell, G.
name: Schwanda, C.
name: Seino, Y.
name: Senyo, K.
name: Shan, W.
name: Shapkin, M.
name: Sharma, C.
name: Shiu, J. -G.
name: Singh, J. B.
name: Sokolov, A.
name: Solovieva, E.
name: Starič, M.
name: Stottler, Z. S.
name: Sumihama, M.
name: Takizawa, M.
name: Tamponi, U.
name: Tanida, K.
name: Tenchini, F.
name: Tiwary, R.
name: Trabelsi, K.
name: Uchida, M.
name: Uglov, T.
name: Unno, Y.
name: Uno, K.
name: Uno, S.
name: Vahsen, S. E.
name: Varner, G.
name: Vinokurova, A.
name: Vossen, A.
name: Wang, D.
name: Wang, E.
name: Wang, M. -Z.
name: Watanuki, S.
name: Xu, X.
name: Yabsley, B. D.
name: Yan, W.
name: Yang, S. B.
name: Yelton, J.
name: Yin, J. H.
name: Yuan, C. Z.
name: Yusa, Y.
name: Zhang, Z. P.
name: Zhilich, V.
name: Zhukova, V.
Publisher: name: arXiv
Year: 2023
Subjects: subject: High Energy Physics - Experiment (hep-ex)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: We present the first measurement of the Michel parameter $ξ^\prime$ in the $τ^-\toμ^-\barν_μν_τ$ decay using the full data sample of $988\,\text{fb}^{-1}$ collected by the Belle detector operating at the KEKB asymmetric energy $e^+ e^-$ collider. The method is based on the reconstruction of the $μ^- \to e^- \barν_eν_μ$ decay-in-flight in the Belle central drift chamber and relies on the correlation between muon spin and its daughter electron momentum. We study the main sources of the background that can imitate the signal decay, such as kaon and pion decays-in-flight and charged particle scattering on the detector material. Highly efficient methods of their suppression are developed and applied to select 165 signal-candidate events. We obtain $ξ^\prime=0.22\pm0.94\pm0.42$ where the first uncertainty is statistical, and the second one is systematic. The result is in agreement with the Standard Model prediction of $ξ^\prime=1$.
description: 16 pages, 14 figures, published in Phys. Rev. D