Observation of the K + → π + ν ν ¯ decay and measurement of its branching ratio

A measurement of the K+→π+νν¯ decay by the NA62 experiment at the CERN SPS is presented, using data collected in 2021 and 2022. This dataset was recorded, after modifications to the beamline and detectors, at a higher instantaneous beam intensity with respect to the 2016–2018 data taking. Combining NA62 data collected in 2016–2022, a measurement of BK+→π+νν¯=13.0−3.0+3.3×10−11 is reported. With 51 signal candidates observed and an expected background of 18−2+3 events, BK+→π+νν¯ becomes the smallest branching ratio measured with a signal significance above 5σ.