Measurement Of The Very Rare K(+)->Pi+Nu Nu Decay

The NA62 experiment reports the branching ratio measurement BRK+-> pi+nu nu =10.6-3.4+4.0stat+/- 0.9systx10-11 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+-> pi+nu nu decay, observed with a significance of 3.4 sigma. The experiment achieves a single event sensitivity of (0.839 +/- 0.054) x 10(-11), corresponding to 10.0 events assuming the Standard Model branching ratio of (8.4 +/- 1.0) x 10(-11). This measurement is also used to set limits on BR(K+-> pi X+), where X is a scalar or pseudo-scalar particle. Details are given of the analysis of the 2018 data sample, which corresponds to about 80% of the total data sample.