Constraints on the K0S→μ+μ− Branching Fraction

A search for the decay K0S→μ+μ− is performed using proton-proton collision data, corresponding to an integrated luminosity of 5.6  fb−1 and collected with the LHCb experiment during 2016, 2017, and 2018 at a center-of-mass energy of 13 TeV. The observed signal yield is consistent with zero, yielding an upper limit of B(K0S→μ+μ−)<2.2×10−10 at 90% C.L.. The limit reduces to B(K0S→μ+μ−)<2.1×10−10 at 90% C.L. once combined with the result from data taken in 2011 and 2012.