Search for heavy neutral leptons in π + decays to positrons

A search for heavy neutral lepton (N) production in π + → 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.