An investigation of the very rare $K^+\rightarrow\pi^+\nu\bar{\nu}$ decay

A search for heavy neutral lepton (N) production in $K^+\rightarrow\pi^+\nu\bar{\nu}$ decays using the data sample collected by the NA62 experiment at CERN in 2017–2018 is reported. Upper limits of the extended neutrino mixing matrix element $|U_{e4}|^2$ are established at the level of $10^{−9}$ over most of the accessible heavy neutral lepton mass range 144–462 MeV/$c^2$, with the assumption that the lifetime exceeds 50 ns. These limits improve significantly upon those of previous production and decay searches. The $|U_{e4}|^2$ range favoured by Big Bang Nucleosynthesis is excluded up to a mass of about 340 MeV/$c^2$.