New Result on K+→π+νν̄ from the NA62 Experiment

The NA62 experiment at the CERN SPS is designed to measure the branching ratio of the K + → π + ν ν ¯ with a decay-in-flight technique and took data in 2016–2018. The statistics collected in 2017 allows NA62 to achieve the best up–to–date single event sensitivity for K + → π + ν ν ¯ corresponding to more than 2 Standard Model K + → π + ν ν ¯ expected events. The analysis of the 2017 data has revealed 2 candidate K + → π + ν ν ¯ events, while 1.5 background were expected. Together with the candidate found on the 2016 data, this result leads to 1.85 × 10−10 as preliminary upper limit at 90% C.L. on the branching ratio of the K + → π + ν ν ¯ decay. The 2017 data also allow NA62 to achieve the best up–to–date sensitivity on the search for the π 0 → invisible decay.