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arXiv · 2606.22373

Search for the charged lepton flavor violating decay $\eta \to e^{\pm}\mu^{\mp}$

M. Ablikim·M. N. Achasov·P. Adlarson·X. C. Ai·C. S. Akondi·R. Aliberti·A. Amoroso·Q. An·Y. H. An·Y. Bai·O. Bakina·H. R. Bao·X. L. Bao·M. Barbagiovanni·V. Batozskaya·K. Begzsuren·N. Berger·M. Berlowski·M. B. Bertani·D. Bettoni·F. Bianchi·E. Bianco·A. Bortone·I. Boyko·R. A. Briere·A. Brueggemann·D. Cabiati·H. Cai·M. H. Cai·X. Cai·A. Calcaterra·G. F. Cao·N. Cao·S. A. Cetin·X. Y. Chai·J. F. Chang·T. T. Chang·G. R. Che·Y. Z. Che·C. H. Chen·G. Chen·H. S. Chen·H. Y. Chen·M. L. Chen·S. J. Chen·S. M. Chen·T. Chen·W. Chen·X. R. Chen·X. T. Chen·X. Y. Chen·Y. B. Chen·Y. Q. Chen·Z. K. Chen·J. Cheng·L. N. Cheng·S. K. Choi·X. Chu·G. Cibinetto·F. Cossio·J. Cottee-Meldrum·H. L. Dai·J. P. Dai·X. C. Dai·A. Dbeyssi·R. E. de·D. Dedovich·C. Q. Deng·Z. Y. Deng·A. Denig·I. Denisenko·M. Destefanis·F. De·E. Di·X. X. Ding·Y. Ding·Y. X. Ding·J. Dong·L. Y. Dong·M. Y. Dong·X. Dong·Z. J. Dong·M. C. Du·S. X. Du·X. L. Du·Y. Q. Du·Y. Y. Duan·Z. H. Duan·P. Egorov·G. F. Fan·J. J. Fan·Y. H. Fan·J. Fang·S. S. Fang·W. X. Fang·Y. Q. Fang·L. Fava·F. Feldbauer·G. Felici·C. Q. Feng
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Abstract

Based on $(10.087 \pm 0.044) \times 10^{9}~J/\psi$ events collected at the center-of-mass energy $\sqrt{s} = 3.097$~GeV with the BESIII detector, we search for the charged lepton flavor violating decay $\eta \to e^{\pm}\mu^{\mp}$ through the process $J/\psi \to \gamma \eta'$ with $\eta' \to \pi^{+} \pi^{-} \eta$. No signal is observed, and an upper limit on the branching fraction is determined to be $\mathcal{B}(\eta \to e^{\pm}\mu^{\mp}) < 6.8 \times 10^{-7}$ at the 90\% confidence level. This result improves the previous best limit by one order of magnitude.

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M. Ablikim, M. N. Achasov, P. Adlarson, X. C. Ai, C. S. Akondi, R. Aliberti, A. Amoroso, Q. An, Y. H. An, Y. Bai, O. Bakina, H. R. Bao, X. L. Bao, M. Barbagiovanni, V. Batozskaya, K. Begzsuren, N. Berger, M. Berlowski, M. B. Bertani, D. Bettoni, F. Bianchi, E. Bianco, A. Bortone, I. Boyko, R. A. Briere, A. Brueggemann, D. Cabiati, H. Cai, M. H. Cai, X. Cai, A. Calcaterra, G. F. Cao, N. Cao, S. A. Cetin, X. Y. Chai, J. F. Chang, T. T. Chang, G. R. Che, Y. Z. Che, C. H. Chen, G. Chen, H. S. Chen, H. Y. Chen, M. L. Chen, S. J. Chen, S. M. Chen, T. Chen, W. Chen, X. R. Chen, X. T. Chen, X. Y. Chen, Y. B. Chen, Y. Q. Chen, Z. K. Chen, J. Cheng, L. N. Cheng, S. K. Choi, X. Chu, G. Cibinetto, F. Cossio, J. Cottee-Meldrum, H. L. Dai, J. P. Dai, X. C. Dai, A. Dbeyssi, R. E. de, D. Dedovich, C. Q. Deng, Z. Y. Deng, A. Denig, I. Denisenko, M. Destefanis, F. De, E. Di, X. X. Ding, Y. Ding, Y. X. Ding, J. Dong, L. Y. Dong, M. Y. Dong, X. Dong, Z. J. Dong, M. C. Du, S. X. Du, X. L. Du, Y. Q. Du, Y. Y. Duan, Z. H. Duan, P. Egorov, G. F. Fan, J. J. Fan, Y. H. Fan, J. Fang, S. S. Fang, W. X. Fang, Y. Q. Fang, L. Fava, F. Feldbauer, G. Felici, C. Q. Feng. 2026-06-21. Search for the charged lepton flavor violating decay $\eta \to e^{\pm}\mu^{\mp}$. https://arxiv.org/abs/2606.22373

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