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arXiv · 2503.22195

Search for lepton-flavor-violating tau decays to $\ell\alpha$ at Belle

Belle Collaboration·K. Uno·K. Hayasaka·K. Inami·H. Aihara·R. Ayad·Sw. Banerjee·K. Belous·J. Bennett·M. Bessner·D. Biswas·D. Bodrov·M. Bračko·P. Branchini·T. E. Browder·A. Budano·M. Campajola·K. Cho·S. -K. Choi·Y. Choi·S. Choudhury·G. De Nardo·G. De Pietro·F. Di Capua·J. Dingfelder·Z. Doležal·P. Ecker·D. Epifanov·T. Ferber·B. G. Fulsom·V. Gaur·A. Giri·P. Goldenzweig·E. Graziani·Y. Guan·K. Gudkova·C. Hadjivasiliou·T. Hara·H. Hayashii·D. Herrmann·C. -L. Hsu·N. Ipsita·A. Ishikawa·R. Itoh·M. Iwasaki·W. W. Jacobs·Y. Jin·C. Kiesling·C. H. Kim·D. Y. Kim·K. -H. Kim·K. Kinoshita·P. Kodyš·A. Korobov·S. Korpar·E. Kovalenko·P. Krokovny·R. Kumar·K. Kumara·A. Kuzmin·Y. -J. Kwon·T. Lam·L. K. Li·Y. B. Li·L. Li Gioi·J. Libby·D. Liventsev·Y. Ma·M. Masuda·T. Matsuda·D. Matvienko·F. Meier·M. Merola·K. Miyabayashi·G. B. Mohanty·M. Nakao·H. Nakazawa·A. Natochii·M. Niiyama·S. Nishida·S. Ogawa·H. Ono·S. Pardi·J. Park·S. -H. Park·S. Patra·S. Paul·T. K. Pedlar·L. E. Piilonen·T. Podobnik·S. Prell·E. Prencipe·M. T. Prim·G. Russo·S. Sandilya·L. Santelj·V. Savinov·G. Schnell·C. Schwanda·Y. Seino
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Abstract

We report a search for the lepton-flavor-violating decays $\tau^{\pm}\to\ell^{\pm}\alpha$~($\ell=e,\mu$), where $\alpha$ is an undetected spin-0 particle, such as an axion-like particle using $736\times10^{6}$ tau lepton pairs collected by the Belle detector at the KEKB asymmetric-energy $e^{+}e^{-}$ collider. We find no evidence of signal and obtain the most stringent upper limits on the branching fractions at 95\% confidence level: $\mathcal{B}(\tau^{\pm}\rightarrow e^{\pm}\alpha)$ $<$ $(0.4$--$6.4)\times10^{-4}$ and $\mathcal{B}(\tau^{\pm}\rightarrow \mu^{\pm}\alpha)$ $<$ $(0.2$--$3.5)\times10^{-4}$ at 95\% confidence level for an $\alpha$ mass in the range $0.0\leq m_{\alpha}\leq 1.6$~GeV/$c^{2}$.

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Belle Collaboration, K. Uno, K. Hayasaka, K. Inami, H. Aihara, R. Ayad, Sw. Banerjee, K. Belous, J. Bennett, M. Bessner, D. Biswas, D. Bodrov, M. Bračko, P. Branchini, T. E. Browder, A. Budano, M. Campajola, K. Cho, S. -K. Choi, Y. Choi, S. Choudhury, G. De Nardo, G. De Pietro, F. Di Capua, J. Dingfelder, Z. Doležal, P. Ecker, D. Epifanov, T. Ferber, B. G. Fulsom, V. Gaur, A. Giri, P. Goldenzweig, E. Graziani, Y. Guan, K. Gudkova, C. Hadjivasiliou, T. Hara, H. Hayashii, D. Herrmann, C. -L. Hsu, N. Ipsita, A. Ishikawa, R. Itoh, M. Iwasaki, W. W. Jacobs, Y. Jin, C. Kiesling, C. H. Kim, D. Y. Kim, K. -H. Kim, K. Kinoshita, P. Kodyš, A. Korobov, S. Korpar, E. Kovalenko, P. Krokovny, R. Kumar, K. Kumara, A. Kuzmin, Y. -J. Kwon, T. Lam, L. K. Li, Y. B. Li, L. Li Gioi, J. Libby, D. Liventsev, Y. Ma, M. Masuda, T. Matsuda, D. Matvienko, F. Meier, M. Merola, K. Miyabayashi, G. B. Mohanty, M. Nakao, H. Nakazawa, A. Natochii, M. Niiyama, S. Nishida, S. Ogawa, H. Ono, S. Pardi, J. Park, S. -H. Park, S. Patra, S. Paul, T. K. Pedlar, L. E. Piilonen, T. Podobnik, S. Prell, E. Prencipe, M. T. Prim, G. Russo, S. Sandilya, L. Santelj, V. Savinov, G. Schnell, C. Schwanda, Y. Seino. 2025-03-28. Search for lepton-flavor-violating tau decays to $\ell\alpha$ at Belle. https://arxiv.org/abs/2503.22195

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