Improved limit on the branching fraction of the rare decay K S 0 → μ + μ -

Abstract

A search for the decay $K_S^0\to {\mu }^{+}{\mu }^{-}$ is performed, based on a data sample of proton-proton collisions corresponding to an integrated luminosity of $3\text{fb}{}^{-1}$ , collected by the LHCb experiment at centre-of-mass energies of 7 and 8 $\mathrm{TeV}$ . The observed yield is consistent with the background-only hypothesis, yielding a limit on the branching fraction of $B(K_S^0\to {\mu }^{+}{\mu }^{-})<0.8\left(1.0\right)\times {10}^{-9}$ at $90\%(95\%)$ confidence level. This result improves the previous upper limit on the branching fraction by an order of magnitude.