Search for heavy neutral leptons in $\ddot{I}{€}^{+}$ decays to positrons

Abstract

A search for heavy neutral lepton ($N$) production in $\ddot{I}{€}^{+}\to e^{+}N$ in-flight decays using data collected by the NA62 experiment at CERN in 2017-2024 is reported. Upper limits for the extended neutrino mixing matrix element $|U_{e4}{|}^2$ are established at the level of $10^{-8}$ for heavy neutral leptons with mass in the range 95-126 $MeV/c^2$ and lifetime exceeding 50 ns.

Updated to match the published version in Physics Letters B. arXiv admin note: text overlap with arXiv:2005.09575