First Measurement of Time-Dependent CP Violation in the Flavor-Changing Neutral-Current Decay B0 → KS0 μ+μ-

A flavor-tagged time-dependent analysis of B0→KS0μ+μ- decays is performed across the full dimuon mass range excluding the J/ψ and ψ(2S) resonance regions. The analysis uses proton-proton collision data collected by the LHCb experiment in 2011-2018 at center-of-mass energies of 7, 8, and 13 TeV, corresponding to an integrated luminosity of 9 fb-1. The CP violation parameters are determined to be C=-0.13±0.32±0.04, S=+0.82±0.29±0.05, where the first uncertainties are statistical and the second are systematic. The results are consistent with the standard model prediction. This is the first experimental study of time-dependent CP violation in b→sℓ+ℓ- processes.