Measurement of the very rare [formula omitted] decay

Abstract

The decay K+→π+νν¯, with a very precisely predicted branching fraction of (8.4±1.0)×10−11 [1], is among the best candidate processes to reveal indirect effects of new physics. The NA62 experiment at CERN SPS is designed to study the K+→π+νν¯ decay and to measure its branching fraction using a decay-in-flight technique. NA62 took data in 2016, 2017 and 2018, reaching the sensitivity of the Standard Model for the K+→π+νν¯ decay by the analysis of the 2016 and 2017 data, and providing the most precise measurement of the branching fraction to date by the analysis of the 2018 data. The final result from the analysis of the full 2016-2017-2018 data set is B(K+→π+νν¯)=(10.6−3.4+4.0|stat±0.9syst)×10−11 [2]. This measurement is also used to set the most precise upper limits on B(K+→π+X), where X is a scalar or pseudo-scalar particle. The analyses leading to the final results are discussed and future plans and prospects are reviewed.