Search for heavy neutral leptons in ${\pi }^{+}$ decays to positrons

A search for heavy neutral lepton ($N$) production in ${\pi }^{+}\to e^{+}N$ 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 $\mathrm{\mid }{U}_{e4}{\mathrm{\mid }}^2$ are established at the level of ${10}^{-8}$ for heavy neutral leptons with mass in the range 95-126 $MeV\mathrm{/}{c}^2$ and lifetime exceeding 50 ns.