Rare leptonic decays of $B_{(s)}^0$ mesons are sensitive probes of New Physics effects. A combination of the CMS and LHCb analyses on the search of the rare decays $B_{s}^0 rightarrow mu^+mu^-$ and $B^0 rightarrow mu^+mu^-$ is presented. The branchin
g fractions of $B_{s}^0 rightarrow mu^+mu^-$ and $B^0 rightarrow mu^+mu^-$ are measured to be $mathcal{B}(B_{s}^0 rightarrow mu^+mu^-) = (2.8 ,^{+0.7}_{-0.6}) times 10^{-9}$ and $mathcal{B}(B^0 rightarrow mu^+mu^-) = (3.9 ,^{+1.6}_{-1.4}) times 10^{-10}$ respectively. A statistical significances of $6.2,sigma$ is evaluated for $B_{s}^0 rightarrow mu^+mu^-$ from the Wilks theorem while a significance of $3.0, sigma$ is measured for $B^0 rightarrow mu^+mu^-$ from the Feldman-Cousins procedure.
Rare lepton decays of the B(s), D and K mesons are sensitive probes of New Physics. In particular, the search for the decays $B^0_(s) -> mu^+ mu^-$ provides information on the presence of new (pseudo-)scalar particles. LHCb is well suited for these a
nalyses due to its large acceptance and trigger efficiency, as well as its excellent invariant mass resolution and lepton identification capabilities. The status of these analyses with $sim1$ fb$^{-1}$ of pp collisions collected by LHCb in 2011 at $sqrt{s}=7$ TeV is reviewed.
We present results of a direct search for the decay K_S->e+e- with the KLOE detector, obtained with a sample of e+e- -> phi -> K_S K_L events produced at DAFNE, the Frascati phi-factory, for an integrated luminosity of 1.9 fb-1. The Standard Model pr
ediction for this decay is BR(K_S->e+e-) = 2 x 10^-14. The search has been performed by tagging the K_S decays with simultaneous detection of a K_L interaction in the calorimeter. Background rejection has been optimized by using both kinematic cuts and particle identification. At the end of the analysis chain we find BR(K_S->e+e-) < 9.3 x 10^-9 at 90% CL, which improves by a factor of ~15 on the previous best result, obtained by CPLEAR experiment.
