| A search for the KS → pi 0mu+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( KS → pi0mu+mu -) = [ 2.9+1.5-1.2 (stat) +/- 0.2(syst)] x 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 KS 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 KS → pi0mu +mu- branching ratio was the missing piece to have a reliable Standard Model prediction for the CP violating KL → pi0mu+mu - decay, and will allow us to probe for new physics in future KL → pi0mu+mu - experiments. |
