Search for the rare decays $B^0_{(s)} \to \mu^{+} \mu^{-}$ with 300 pb$^{-1}$ at LHCb

A search for the $B^0_{s} \to \mu^{+} \mu^{-}$ and $B^0 \to \mu^{+} \mu^{-}$ is performed in $\sim$ 300 pb$^{-1}$ of $pp$ collisions at $\sqrt{s}$ = 7 TeV collected by the LHCb experiment at the Large Hadron Collider at CERN. The measured upper limit for the branching ratio of the $B^0_{s} \to \mu^{+} \mu^{-}$ decay is $\cal{B}(B^0_s \to \mu^{+} \mu^{-})$ < 1.3 (1.6) x 10$^{-8}$ at 90% (95%) confidence level (C.L.), while in the case of the $B^0 \to \mu^{+} \mu^{-}$ decay the measured upper limit is $\cal{B}(B^0 \to \mu^{+} \mu^{-})$ < 4.2 (5.1) x 10$^{-9}$ at 90% (95%) C.L. A combination with the LHCb observations on the 2010 dataset results in $\cal{B}(B^0_s \to \mu^{+} \mu^{-})$ < 1.2 (1.5) x 10$^{-8}$ at 90% (95%) C.L.