Search for heavy neutral lepton production in $K^+$ decays

TL;DR

This paper reports a search for heavy neutral leptons in $K^+$ decays using CERN's NA62 data, setting new upper limits on their mixing parameters and extending the mass range compared to previous studies.

Contribution

It provides the first search results for heavy neutral leptons in $K^+$ decays with improved limits and extended mass coverage, using data from the NA62 experiment.

Findings

Upper limits on $|U_{e4}|^2$ and $|U_{\mu4}|^2$ at $10^{-7}$ to $10^{-6}$ levels.

Extended the mass range for heavy neutral lepton searches up to 448 MeV/$c^2$.

Improved constraints over previous $K^+$ decay searches.

Abstract

A search for heavy neutral lepton production in $K^+$ decays using a data sample collected with a minimum bias trigger by the NA62 experiment at CERN in 2015 is reported. Upper limits at the $10^{-7}$ to $10^{-6}$ level are established on the elements of the extended neutrino mixing matrix $|U_{\ell 4}|^2$ ($\ell=e,\mu$) for heavy neutral lepton mass in the range $170-448~{\rm MeV}/c^2$. This improves on the results from previous production searches in $K^+$ decays, setting more stringent limits and extending the mass range.