Search for the rare $K^0_S \to \mu^+ \mu^-$ decay

A search for the decay $K^0_S \to \mu^+ \mu^-$ is performed, based on a data sample corresponding to an integrated luminosity of 1.0 fb$^{−1}$ of pp collisions at $\sqrt s = 7$ TeV collected by the LHCb ex- periment at the Large Hadron Collider. The observed number of candidates is consistent with the background–only hypothesis, yielding an upper limit of $\cal B(K_S^0 \to \mu^+ \mu^−) < 11(9) × 10^{−9}$ at $95~ (90) \%$ confidence level. This limit is below the previous measurement by more than a factor of thirty.