Measurement of the branching ratio of the $K^{+}\to {\pi }^{+}\nu \bar{\nu }$ decay

Abstract

The NA62 experiment has measured the branching ratio of the rare decay $K^{+}\to {\pi }^{+}\nu \bar{\nu }$ using data collected in 2023 and 2024 at the CERN SPS, obtaining ${\mathrm{B}}_{2023-2024}(K^{+}\to {\pi }^{+}\nu \bar{\nu })=\left(7.2_{-2.1}^{+2.3}\right)\times 10^{-11}$. With results from data collected between 2016 and 2022, the analysis gives ${\mathrm{B}}_{2016-2024}(K^{+}\to {\pi }^{+}\nu \bar{\nu })=\left(9.6_{-1.8}^{+1.9}\right)\times 10^{-11}$, corresponding to a relative precision of $20\%$. This value is consistent with Standard Model predictions.