Search for long-lived heavy neutral leptons with displaced vertices in proton-proton collisions at $\sqrt{s} =$ 13 TeV

TL;DR

This paper reports a search for long-lived heavy neutral leptons in proton-proton collisions at 13 TeV, using displaced vertex signatures to set new constraints on their mass and mixing parameters.

Contribution

It introduces a novel search strategy for HNLs with displaced vertices in CMS data, extending the exclusion limits for masses 1-20 GeV and mixing parameters down to 10^{-7}.

Findings

No significant deviations from the standard model observed.

Constraints set on HNL mass and coupling, excluding new parameter space.

Extended exclusion limits for HNLs in the 1-20 GeV mass range.

Abstract

A search for heavy neutral leptons (HNLs), the 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 $\sqrt{s} =$ 13 TeV at the CERN LHC. The data correspond to an integrated luminosity of 138 fb$^{-1}$. The HNLs could be produced through mixing with standard model neutrinos $\nu$. For small values of the HNL mass ($\lt$ 20 GeV) and the square of the HNL-$\nu$ mixing parameter (10$^{-7}$-10$^{-2}$), the decay length of these particles can be large enough so that the secondary vertex of the HNL decay can be resolved with the CMS silicon tracker. The selected final state consists of one lepton emerging from the primary proton-proton collision vertex, and two leptons forming a displaced, secondary vertex. No significant deviations from the standard model expectations are observed, and constraints are obtained on the HNL mass and coupling strength parameters, excluding previously unexplored regions of parameter space in the mass range 1-20 GeV and squared mixing parameter values as low as 10$^{-7}$.