Search for long-lived heavy neutral leptons with displaced vertices in pp collisions at $\sqrt{s}=13\,\mathrm{TeV}$ with the CMS detector

A search for heavy neutral leptons (HNLs), the hypothetical right-handed Dirac or Majorana neutrinos, is performed in final states with three charged leptons (electrons or muons) using proton-proton collision data collected by the CMS experiment at a center-of-mass energy of $\sqrt{s}=13\,\mathrm{TeV}$. The data correspond to an integrated luminosity of $137\,\mathrm{fb}^{-1}$. The HNLs can be produced at the LHC through mixing with standard model (SM) neutrinos. For small values of the HNL mass (${<}20\,\mathrm{GeV}$) and of the HNL-SM neutrino mixing parameter squared ($10^{-7}{-}10^{-2}$), the decay length of these particles can be large enough to produce a resolved secondary vertex in the CMS silicon tracker. The study aims at identifying two leptons that form a displaced vertex with respect to the primary proton-proton collision vertex, while the third lepton is assumed to emerge from the primary vertex. No significant deviations from the SM expectations are observed, and constraints are obtained on the HNL mass and coupling strength parameters, extending the exclusion limits from previous searches in the mass range $1{-}18\,\mathrm{GeV}$ and mixing parameter values in the range of $10^{-7}{-}10^{-5}$.