We report on the first search for electron-muon lepton flavor violation (LFV) in the decay of a $b$ quark and $b$ antiquark bound state. We look for the LFV decay $\mathrm{\Upsilon }(3S)\to e^{\pm }{\mu }^{\mp }$ in a sample of 118 million $\mathrm{\Upsilon }(3S)$ mesons from $27{\mathrm{fb}}^{-1}$ of data collected with the BABAR detector at the SLAC PEP-II $e^{+}{e}^{-}$ collider operating with a 10.36 GeV center-of-mass energy. No evidence for a signal is found, and we set a limit on the branching fraction $\mathcal{B}[\mathrm{\Upsilon }(3S)\to e^{\pm }{\mu }^{\mp }]<3.6\times {10}^{-7}$ at 90% C. L. This result can be interpreted as a limit ${\mathrm{\Lambda }}_{\mathrm{NP}}/g_{\mathrm{NP}}^2>80\mathrm{TeV}$ on the energy scale ${\mathrm{\Lambda }}_{\mathrm{NP}}$ divided by the coupling-squared $g_{\mathrm{NP}}^2$ of relevant new physics (NP).

• Received 10 September 2021
• Accepted 27 January 2022

DOI:https://doi.org/10.1103/PhysRevLett.128.091804

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP³.

Published by the American Physical Society