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

TL;DR

This paper reports the most precise measurements of the rare $K^+ ightarrow\pi^+ uar{ u}$ decay, using data from the NA62 experiment, constraining new physics models that predict large enhancements.

Contribution

First combined analysis of 2016 and 2017 data from NA62 providing the most precise upper limit and measurement of the $K^+ ightarrow\pi^+ uar{ u}$ branching ratio.

Findings

Observed 2 candidate events with expected background of 1.5.

Set an upper limit of $1.78\times10^{-10}$ at 90% CL.

Measured branching ratio of $(0.48^{+0.72}_{-0.48})\times10^{-10}$ at 68% CL.

Abstract

The NA62 experiment reports an investigation of the $K^+\rightarrow\pi^+\nu\bar{\nu}$ mode from a sample of $K^+$ decays collected in 2017 at the CERN SPS. The experiment has achieved a single event sensitivity of $(0.389\pm 0.024)\times10^{-10}$, corresponding to 2.2 events assuming the Standard Model branching ratio of $(8.4\pm1.0)\times10^{-11}$. 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 $1.78\times10^{-10}$ for the $K^+\rightarrow\pi^+\nu\bar{\nu}$ branching ratio at 90\%\,CL. This, together with the corresponding 68\%\,CL measurement of $(0.48^{+0.72 }_{-0.48})\times10^{-10}$, 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.