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

Abstract

The NA62 experiment reports an investigation of the $K^{+}\to {\pi }^{+}\nu \bar{\nu }$ mode from a sample of $K^{+}$ decays collected in 2017 at the CERN SPS. The experiment has achieved a single event sensitivity of $(0.389\pm 0.024)\times 10^{-10}$, corresponding to 2.2 events assuming the Standard Model branching ratio of $(8.4\pm 1.0)\times 10^{-11}$. Two signal candidates are observed with an expected background of 1.5 events. Combined with the result of a similar analysis conducted by NA62 on a smaller data set recorded in 2016, the collaboration now reports an upper limit of $1.78\times 10^{-10}$ for the $K^{+}\to {\pi }^{+}\nu \bar{\nu }$ branching ratio at 90\%\,CL. This, together with the corresponding 68\%\,CL measurement of $(0.48_{-0.48}^{+0.72})\times 10^{-10}$, are currently the most precise results worldwide, and are able to constrain some New Physics models that predict large enhancements still allowed by previous measurements.

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