Speaker
Description
The first observation of the $K^+\to\mu^+\nu\mu^+\mu^-$ decay with the 2016-2024 dataset collected by the NA62 experiment is presented. The branching ratio of the process is measured relative to the normalisation channel $K^+\to\pi^+\mu^+\mu^-$, taking advantage of similar final states and both decays being collected by the same trigger stream. The dominant background is the $K^+\to \pi^+\pi^+\pi^-$ decay with three in-flight $\pi^\pm\to \mu^\pm\nu$ decays. The observation of the signal mode is possible due to complete kinematic information about the event provided by a beam spectrometer for the kaon and a downstream spectrometer for the muons. This, together with sub-ns timing resolution of the NA62 detector systems, allows to control a background process with a branching ratio higher by six orders of magnitude than the signal.
