First search for K + → π + ν ν ˉ using the decay-in-flight technique

摘要

The NA62 experiment at the CERN SPS reports the first search forK+→π+ννˉusing the decay-in-flight technique, based on a sample of1.21×1011K+decays collected in 2016. The single event sensitivity is3.15×10?10, corresponding to 0.267 Standard Model events. One signal candidate is observed while the expected background is 0.152 events. This leads to an upper limit of14×10?10on theK+→π+ννˉbranching ratio at 95% CL.