Search for the $D^0 \to \mu^+ \mu^-$ decay with 0.9 $\rm{fb}^{−1}$ at LHCb

A search for the very rare decay $D^0 \to \mu^+ \mu^-$ is presented using data collected in 2011 corresponding to an integrated luminosity of 0.9 $\rm{fb}^{−1}$. No significant excess of events with respect to the background expectations was found and therefore, using the asymptotic CLs method, an upper limit on the branching fraction was derived: ${\cal B}(D^0 \to \mu^+ \mu^-) < 1.3 \cdot 10^{-8}$ at 95% CL, with a median expected upper limit, assuming no signal, of ${\cal B}(D^0 \to \mu^+ \mu^-)< 8.4^{+3.6}_{-2.4} \cdot 10^{-9}$ at 95% CL. This result improves the current world best limit by more than an order of magnitude.