Search for $K^{+}$ decays to a muon and invisible particles

Abstract

The NA62 experiment at CERN reports searches for $K^{+}\to {\mu }^{+}N$ and $K^{+}\to {\mu }^{+}\nu X$ decays, where $N$ and $X$ are massive invisible particles, using the 2016-2018 data set. The $N$ particle is assumed to be a heavy neutral lepton, and the results are expressed as upper limits of $\mathcal{O}(10^{-8})$ of the neutrino mixing parameter $|U_{\mu 4}{|}^2$ for $N$ masses in the range 200-384 MeV/$c^2$ and lifetime exceeding 50 ns. The $X$ particle is considered a scalar or vector hidden sector mediator decaying to an invisible final state, and upper limits of the decay branching fraction for $X$ masses in the range 10-370 MeV/$c^2$ are reported for the first time, ranging from $\mathcal{O}(10^{-5})$ to $\mathcal{O}(10^{-7})$. An improved upper limit of $1.0\times 10^{-6}$ is established at 90% CL on the $K^{+}\to {\mu }^{+}\nu \nu \bar{\nu }$ branching fraction.

Updated version published as Phys.Lett. B816 (2021) 136259

References in corpus (4)

• Search for heavy neutrinos in $K^{+}\to {\mu }^{+}{\nu }_H$ decays
• Search for heavy neutral lepton production in $K^{+}$ decays
• Search for heavy neutral lepton production in $K^{+}$ decays to positrons
• The integrated low-level trigger and readout system of the CERN NA62 experiment