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The NA62 experiment reports an investigation of the$K^+\rightarrow\pi^+\nu\bar{\nu}$ mode from a sample of $K^+$ decays collectedin 2017 at the CERN SPS. The experiment has achieved a single event sensitivityof $(0.389\pm 0.024)\times10^{-10}$, corresponding to 2.2 events assuming theStandard Model branching ratio of $(8.4\pm1.0)\times10^{-11}$. Two signalcandidates are observed with an expected background of 1.5 events. Combinedwith the result of a similar analysis conducted by NA62 on a smaller data setrecorded in 2016, the collaboration now reports an upper limit of$1.78\times10^{-10}$ for the $K^+\rightarrow\pi^+\nu\bar{\nu}$ branching ratioat 90\%\,CL. This, together with the corresponding 68\%\,CL measurement of$(0.48^{+0.72 }_{-0.48})\times10^{-10}$, are currently the most precise resultsworldwide, and are able to constrain some New Physics models that predict largeenhancements still allowed by previous measurements.

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