A search for the decay KS -> mu+ mu- is performed, based on a data sample of 1.0 fb^-1 of pp collisions at sqrt{s}=7 TeV collected by the LHCb experiment at the Large Hadron Collider. The observed number of candidates is consistent with the backgroun
d-only hypothesis, yielding an upper limit of BR(KS -> mu+ mu-) < 11 (9) x 10^-9 at 95 (90)% confidence level. This limit is a factor of thirty below the previous measurement.
