First observation of the decay $K_{S} \to \pi^{0}\mu^{+}\mu^{-}$ at the NA48 experiment at CERN

A search for the $K_{S} \to \pi^{0}\mu^{+}\mu^{-}$ decay was performed by the NA48/1 Collaboration at the CERN SPS accelerator. Six events were found with a background expectation of $0.22^{+0.18}_{-0.11}$ in the data collected in 2002 from the high-intensity $K_{S}$ run. The measured branching ratio is: $BR(K_{S} \to \pi^{0}\mu^{+}\mu^{-}) = [2.9^{+1.5}_{- 1.2} (stat) \pm 0.2 (syst)] \times 10^{-9}$, assuming that only vector currents contribute to the matrix element, and a unit form factor. Currently, this channel has the lowest branching ratio ever measured for $K_{S}$ decays. The understanding of the backgrounds and the development of an event selection that guaranteed a low background without killing the signal were the main challenges of this analysis. The determination of the $K_{S} \to \pi^{0}\mu^{+}\mu^{-}$ branching ratio was the missing piece to have a reliable Standard Model prediction for the CP violating $K_{L} \to \pi^{0}\mu^{+}\mu^{-}$ decay, and will allow us to probe for new physics in future $K_{L} \to \pi^{0}\mu^{+}\mu^{-}$ experiments.