Observation of the $K^{+}\to {\pi }^{+}\nu \bar{\nu }$ decay and measurement of its branching ratio

A measurement of the $K^{+}\to {\pi }^{+}\nu \bar{\nu }$ 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 $B(K^{+}\to {\pi }^{+}\nu \bar{\nu })=\left(13.0_{-3.0}^{+3.3}\right)\times {10}^{-11}$ is reported. With $51$ signal candidates observed and an expected background of $18_{-2}^{+3}$ events, $B(K^{+}\to {\pi }^{+}\nu \bar{\nu })$ becomes the smallest branching ratio measured with a signal significance above $5\sigma$.