Measurement of the very rare ${K}^{+}\to {\pi }^{+}\nu \bar{\nu }$ decay at the NA62 experiment

The ${K}^{+}\to {\pi }^{+}\nu \bar{\nu }$ 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(${K}^{+}\to {\pi }^{+}\nu \bar{\nu }$) = ${1.06}_{-0.34}^{+0.40}{|}_{stat}$ ± 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 ${K}^{+}\to {\pi }^{+}\nu \bar{\nu }$ 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.