Measurement of the very rare $K^+ \to \pi^+ \nu \bar\nu$ decay

Abstract

The decay $K^+\to\pi^+\nu\bar{\nu}$, 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 at CERN SPS is designed to study the $K^+\to\pi^+\nu\bar{\nu}$ decay and to measure its branching ratio using a decay-in-flight technique. NA62 took data in 2016, 2017 and 2018, reaching the sensitivity of the Standard Model for the $K^+\to\pi^+\nu\bar{\nu}$ decay by the analysis of the 2016 and 2017 data, and providing the most precise measurement of the branching ratio to date by the analysis of the 2018 data. This measurement is also used to set limits on BR($K^+\to\pi^+X$), where $X$ is a scalar or pseudo-scalar particle. The final result of the BR($K^+\to\pi^+\nu\bar{\nu}$) measurement and its interpretation in terms of the $K^+\to\pi^+ X$ decay from the analysis of the full 2016-2018 data set is presented, and future plans and prospects are reviewed.