Observation of the \( {K}^{+}\to {\pi}^{+}\nu \overline{\nu} \) decay and measurement of its branching ratio

Abstract

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