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arXiv · 2610.37258

Precision measurement of the $J/ψ\toμ^+μ^-μ^+μ^-$ branching fraction and search for a new gauge boson $Z^{\prime}$ at BESIII

BESIII Collaboration·M. Ablikim·M. N. Achasov·P. Adlarson·X. C. Ai·C. S. Akondi·R. Aliberti·A. Amoroso·Q. An·Y. H. An·M. S. Anderson·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·Chao 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 Boer·D. Dedovich·C. Q. Deng·Z. Y. Deng·A. Denig·I. Denisenko·M. Destefanis·F. De Mori·E. Di Fiore·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·Shaoxu Du·X. L. Du·Y. Q. Du·Y. Y. Duan·Z. H. Duan·P. Egorov·G. F. Fan·J. J. Fan·K. X. Fan·Y. H. Fan·J. Fang·Jin Fang·S. S. Fang
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Abstract

We present a precision measurement of the branching fraction of the rare decay $J/ψ\toμ^+μ^-μ^+μ^-$, and a search for a new gauge boson $Z^{\prime}$ that couples only to the second and third lepton generations, using a data sample of $(8998 \pm 39) \times 10^6$ $J/ψ$ events collected by the BESIII detector. The branching fraction of the $J/ψ\toμ^+μ^-μ^+μ^-$ decay is measured to be $B(J/ψ\toμ^+μ^-μ^+μ^-)=[0.91\pm0.04~{\rm (stat.)}\pm0.07~{\rm (syst.)}]\times 10^{-6}$, representing the most precise measurement to date and consistent with the theoretical prediction within one standard deviation. Additionally, we search for $CP$ asymmetry in the $J/ψ\toμ^+μ^-μ^+μ^-$ decay and determine the T-odd observable to be $A_T = [0.05\pm 0.04~{\rm (stat.)}\pm0.01~{\rm (syst.)}]$, which is consistent with $CP$ conservation. In the search for $J/ψ\to Z^{\prime}μ^+μ^-$ via $Z^{\prime}\toμ^+μ^-$, no significant $Z^{\prime}$ signal is observed in the $μ^+μ^-$ invariant mass region of $(0.212,~2.6)$~GeV$/c^2$, and we set upper limits on the ${\rm U}(1)_{L_μ-L_τ}$ model coupling constant $g^{\prime}$ ranging from 0.001 to 0.294 at the 90\% confidence level, which is comparable to the upper limits set by Belle II for $Z^{\prime}$ masses below $0.7$~GeV$/c^2$.

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BESIII Collaboration, M. Ablikim, M. N. Achasov, P. Adlarson, X. C. Ai, C. S. Akondi, R. Aliberti, A. Amoroso, Q. An, Y. H. An, M. S. Anderson, 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, Chao 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 Boer, D. Dedovich, C. Q. Deng, Z. Y. Deng, A. Denig, I. Denisenko, M. Destefanis, F. De Mori, E. Di Fiore, 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, Shaoxu Du, X. L. Du, Y. Q. Du, Y. Y. Duan, Z. H. Duan, P. Egorov, G. F. Fan, J. J. Fan, K. X. Fan, Y. H. Fan, J. Fang, Jin Fang, S. S. Fang. 2026-10-27. Precision measurement of the $J/ψ\toμ^+μ^-μ^+μ^-$ branching fraction and search for a new gauge boson $Z^{\prime}$ at BESIII. https://arxiv.org/abs/2610.37258

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