Heavy Neutral Leptons, Searches for

We define searches for Heavy Neutral Leptons (HNLs) as searches for Dirac or Majorana fermions withsterile neutrino quantum numbers, that are heavy enough to not disrupt the simplestBig Bang Nucleosynthesis bounds and/or unstable on cosmological timescales:Typically HNLs have mass $\sim{}$ MeV or higher.nullSearches for these particles generically set bounds on the mixing between the HNL and the activeneutrinos, as parametrized by the extended 3${\times }$4 PMNS matrix elements ${{\mathit U}}_{ {{\mathit \ell}} {{\mathit x}} }$(see the "Neutrino mass, mixing and oscillations" review) where ${{\mathit \ell}}$ = ${{\mathit e}}$, ${{\mathit \mu}}$or ${{\mathit \tau}}$, and we denote the HNL as ${{\mathit \nu}_{{x}}}$. While many measurements may be interpreted to placebounds on various combinations of these matrix elements, we quote below limits only for thosecases in which one matrix element is assumed to be much larger than the other two, i.e.$\vert {{\mathit U}}_{ {{\mathit \ell}} {{\mathit x}} }\vert $ ${}\gg$ $\vert {{\mathit U}}_{ {{\mathit \ell}^{\,'}} {{\mathit x}} }\vert $ for ${{\mathit \ell}^{\,'}}{}\not=$ ${{\mathit \ell}}$ .nullExperimental searches make use of various different strategies, including e.g. resonance searchesin missing mass decay distributions or specific final states, searches for lepton number violatingdecays, and trilepton signatures. The resulting bounds on ${{\mathit U}}_{ {{\mathit \ell}} {{\mathit x}} }$ are typicallydependent on the HNL mass. The quoted limits below are either the best limit near an experimentalkinematic threshold, or a characteristic value in the mass range of the experimental sensitivity.