Long-lived heavy neutral leptons at the LHC: four-fermion single-$N_R$ operators

TL;DR

This paper investigates the potential of the high-luminosity LHC to detect heavy neutral leptons through four-fermion operators within an effective field theory framework, extending the search beyond minimal mixing scenarios.

Contribution

It introduces a model-independent analysis of HNLs using the $N_R$SMEFT, focusing on four-fermion operators that could dominate HNL production and decay at the LHC.

Findings

ATLAS could probe new physics scales over 20 TeV

Four-fermion operators significantly impact HNL phenomenology

Sensitivity exceeds minimal mixing scenario expectations

Abstract

Interest in searches for heavy neutral leptons (HNLs) at the LHC has increased considerably in the past few years. In the minimal scenario, HNLs are produced and decay via their mixing with active neutrinos in the Standard Model (SM) spectrum. However, many SM extensions with HNLs have been discussed in the literature, which sometimes change expectations for LHC sensitivities drastically. In the $N_R$SMEFT, one extends the SM effective field theory with operators including SM singlet fermions, which allows to study HNL phenomenology in a "model independent" way. In this paper, we study the sensitivity of ATLAS to HNLs in the $N_R$SMEFT for four-fermion operators with a single HNL. These operators might dominate both production and decay of HNLs, and we find that new physics scales in excess of 20 TeV could be probed at the high-luminosity LHC.