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

TL;DR

The NA62 experiment measured the rare decay $K^+ ightarrow \pi^+ uar{ u}$, providing evidence with a 3.4 sigma significance and setting limits on related decay modes, advancing understanding of rare kaon decays.

Contribution

This work reports the first evidence for the very rare $K^+ ightarrow \pi^+ uar{ u}$ decay with a significant observation, using data collected at CERN SPS, and sets new limits on related decay channels.

Findings

Branching ratio measured as (10.6^{+4.0}_{-3.4} ext{(stat)} \pm 0.9_{ ext{syst}}) imes 10^{-11}

Evidence for decay with 3.4 sigma significance

Set limits on $K^+ o \pi^+ X$ decays

Abstract

The NA62 experiment reports the branching ratio measurement BR$(K^+ \rightarrow \pi^+ \nu\bar{\nu}) = (10.6^{+4.0}_{-3.4} |_{\rm stat} \pm 0.9_{\rm syst}) \times 10 ^{-11}$ 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 $K^+ \rightarrow \pi^+ \nu\bar{\nu}$ 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 $(8.4\pm1.0)\times10^{-11}$. 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.