A search for the very rare $D^0\to {\mu }^{+}{\mu }^{-}$ decay is performed using data collected by the LHCb experiment in proton-proton collisions at $\sqrt{s}=7$, 8, and 13 TeV, corresponding to an integrated luminosity of $9{\mathrm{fb}}^{-1}$. The search is optimized for $D^0$ mesons from $D^{*+}\to D^0{\pi }^{+}$ decays but is also sensitive to $D^0$ mesons from other sources. No evidence for an excess of events over the expected background is observed. An upper limit on the branching fraction of this decay is set at $\mathcal{B}(D^0\to {\mu }^{+}{\mu }^{-})<3.1\times {10}^{-9}$ at a 90% C.L. This represents the world’s most stringent limit, constraining models of physics beyond the standard model.

• Received 21 December 2022
• Accepted 18 May 2023

DOI:https://doi.org/10.1103/PhysRevLett.131.041804

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.

© 2023 CERN, for the LHCb Collaboration