Measurement of the very rare K⁺ → π⁺ν(Equation presented) decay at the NA62 experiment at CERN

The NA62 experiment reports the branching ratio measurement BR(K⁺ → π⁺ν(Equation presented)) 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⁺ → π⁺ν(Equation presented) decay, observed with a significance of 3.4σ. The experiment achieves a single event sensitivity of (0.839 ± 0.054) × 10⁻¹¹, corresponding to 10.0 events assuming the Standard Model branching ratio of (8.4 ± 1.0) × 10⁻¹¹. The result represents the most accurate measurement achieved so far of this ultra-rare decay. Future prospects and plans for data taking from 2021 will also be presented.