Search for long-lived heavy neutral leptons in proton-proton collision events with a lepton-jet pair associated with a secondary vertex 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 CMS data, setting new exclusion limits on their properties with no observed excess.

Contribution

The study introduces a novel search strategy for long-lived HNLs involving lepton-jet pairs and secondary vertices, providing the most stringent limits to date in certain mass ranges.

Findings

No excess events observed above the Standard Model expectation.

Exclusion limits set for HNL masses between 1 and 16.5 GeV.

Limits surpass previous results for masses above 11 GeV.

Abstract

A search for long-lived heavy neutral leptons (HNLs) using proton-proton collision data corresponding to an integrated luminosity of 138 fb$^{-1}$ collected at $\sqrt{s}$ = 13 TeV with the CMS detector at the CERN LHC is presented. Events are selected with a charged lepton originating from the primary vertex associated with the proton-proton interaction, as well as a second charged lepton and a hadronic jet associated with a secondary vertex that corresponds to the semileptonic decay of a long-lived HNL. No excess of events above the standard model expectation is observed. Exclusion limits at 95% confidence level are evaluated for HNLs that mix with electron and/or muon neutrinos. Limits are presented in the mass range of 1-16.5 GeV, with excluded square mixing parameter values reaching as low as 2 $\times$ 10$^{-7}$. For masses above 11 GeV, the presented limits exceed all previous results in the semileptonic decay channel, and for some of the considered scenarios are the strongest to date.