Search for rare decays of $D^0$ mesons into two muons

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 $13\mathrm{T}\mathrm{e}\mathrm{V}$, corresponding to an integrated luminosity of $9\mathrm{f}{\mathrm{b}}^{-1}$. The search is optimised for $D^0$ mesons from $D^{\ast +}\to D^0{\pi }^{+}$ 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 $90\mathrm{\%}$ CL. This represents the world’s most stringent limit, constraining models of physics beyond the Standard Model.