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

Abstract

The NA62 experiment reports the branching ratio measurement BR$(K^{+}\to {\pi }^{+}\nu \bar{\nu })=(10.6_{-3.4}^{+4.0}{|}_{\mathrm{stat}}\pm 0.9_{\mathrm{syst}})\times 10^{-11}$ 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 $K^{+}\to {\pi }^{+}\nu \bar{\nu }$ decay, observed with a significance of 3.4$\sigma$. The experiment achieves a single event sensitivity of $(0.839\pm 0.054)\times 10^{-11}$, corresponding to 10.0 events assuming the Standard Model branching ratio of $(8.4\pm 1.0)\times 10^{-11}$. This measurement is also used to set limits on BR($K^{+}\to {\pi }^{+}X$), where $X$ is a scalar or pseudo-scalar particle. Details are given of the analysis of the 2018 data sample, which corresponds to about 80% of the total data sample.

References in corpus (8)

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