First search for K+→π+νν¯ using the decay-in-flight technique

The NA62 experiment at the CERN SPS reports the first search for K + →π + νν¯ using the decay-in-flight technique, based on a sample of 1.21×10 11 K + decays collected in 2016. The single event sensitivity is 3.15×10 −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 of 14×10 −10 on the K + →π + νν¯ branching ratio at 95% CL.