Measurement of the very rare K + → π + ν ν ¯ decay at the NA62 experiment

The K+→π+νν¯ decay, with a very precisely predicted branching ratio of less than 10-10, is among the best processes to reveal indirect effects of new physics. The NA62 experiment reports the branching ratio measurement BR( K+→π+νν¯ ) = 1.06-0.34+0.40|stat ± 0.009| syst ) · 10-10 at 68% CL, based on the observation of 20 signal candidates with an expected background of 7.0 events from the total data sample collected at the CERN SPS during 2016-2018. This provides evidence for the very rare K+→π+νν¯ decay, observed with a significance of 3.4σ. This measurement is also used to set limits on BR(K + → π +X), where X is a new feebly interacting scalar or pseudo-scalar particle, foreseen in several new physics scenarios.