First Measurement of the Michel Parameter ζ′ in the τ- →μ- ν ¯ μντ Decay at Belle

D. Bodrov, P. Pakhlov, I. Adachi, H. Aihara, S. Al Said, D. M. Asner, H. Atmacan, T. Aushev, R. Ayad, V. Babu, Sw Banerjee, P. Behera, K. Belous, J. Bennett, M. Bessner, B. Bhuyan, T. Bilka, D. Biswas, A. Bobrov, A. BondarJ. Borah, A. Bozek, M. Bračko, P. Branchini, T. E. Browder, A. Budano, M. Campajola, D. Červenkov, M. C. Chang, B. G. Cheon, K. Chilikin, H. E. Cho, K. Cho, S. J. Cho, S. K. Choi, Y. Choi, S. Choudhury, D. Cinabro, S. Das, G. De Nardo, G. De Pietro, R. Dhamija, F. Di Capua, J. Dingfelder, Z. DoleŽal, T. V. Dong, D. Epifanov, T. Ferber, D. Ferlewicz, B. G. Fulsom, V. Gaur, A. Garmash, A. Giri, P. Goldenzweig, E. Graziani, D. Greenwald, T. Gu, Y. Guan, K. Gudkova, C. Hadjivasiliou, S. Halder, K. Hayasaka, H. Hayashii, M. T. Hedges, D. Herrmann, W. S. Hou, C. L. Hsu, T. Iijima, K. Inami, N. Ipsita, A. Ishikawa, R. Itoh, M. Iwasaki, W. W. Jacobs, E. J. Jang, Q. P. Ji, S. Jia, Y. Jin, K. K. Joo, D. Kalita, A. B. Kaliyar, T. Kawasaki, C. Kiesling, C. H. Kim, D. Y. Kim, K. H. Kim, Y. K. Kim, H. Kindo, K. Kinoshita, P. Kodyš, S. Korpar, P. KriŽan, P. Krokovny, T. Kuhr, M. Kumar, R. Kumar, K. Kumara, Y. J. Kwon, J. S. Lange, S. C. Lee, J. Li, L. K. Li, J. Libby, K. Lieret, Y. R. Lin, D. Liventsev, T. Luo, Y. Ma, M. Masuda, T. Matsuda, S. K. Maurya, F. Meier, M. Merola, F. Metzner, K. Miyabayashi, R. Mizuk, G. B. Mohanty, R. Mussa, M. Nakao, D. Narwal, Z. Natkaniec, A. Natochii, L. Nayak, M. Nayak, N. K. Nisar, S. Nishida, S. Ogawa, P. Oskin, G. Pakhlova, S. Pardi, H. Park, J. Park, S. H. Park, A. Passeri, S. Patra, S. Paul, R. Pestotnik, L. E. Piilonen, T. Podobnik, E. Prencipe, M. T. Prim, A. Rabusov, N. Rout, G. Russo, S. Sandilya, A. Sangal, L. Santelj, V. Savinov, G. Schnell, C. Schwanda, Y. Seino, K. Senyo, W. Shan, M. Shapkin, C. Sharma, J. G. Shiu, J. B. Singh, A. Sokolov, E. Solovieva, M. Starič, Z. S. Stottler, M. Sumihama, M. Takizawa, U. Tamponi, K. Tanida, F. Tenchini, R. Tiwary, K. Trabelsi, M. Uchida, T. Uglov, Y. Unno, K. Uno, S. Uno, S. E. Vahsen, G. Varner, A. Vinokurova, A. Vossen, D. Wang, E. Wang, M. Z. Wang, S. Watanuki, O. Werbycka, X. Xu, B. D. Yabsley, W. Yan, S. B. Yang, J. Yelton, J. H. Yin, C. Z. Yuan, Y. Yusa, Z. P. Zhang, V. Zhilich, V. Zhukova

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Abstract

We report the first measurement of the Michel parameter ζ′ in the τ-→μ-ν¯μντ decay with a new method proposed just recently. The measurement is based on the reconstruction of the τ-→μ-ν¯μντ events with subsequent muon decay in flight in the Belle central drift chamber. The analyzed data sample of 988 fb-1 collected by the Belle detector corresponds to approximately 912×106 τ+τ- pairs. We measure ζ′=0.22±0.94(stat)±0.42(syst), which is in agreement with the standard model prediction of ζ′=1. Statistical uncertainty dominates in this study, being a limiting factor, while systematic uncertainty is well under control. Our analysis proved the practicability of this promising method and its prospects for further precise measurement in future experiments.

Original languageEnglish
Article number021801
JournalPhysical Review Letters
Volume131
Issue number2
DOIs
StatePublished - 14 Jul 2023
Externally publishedYes

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