Heavy neutral leptons in effective field theory and the high-luminosity LHC

TL;DR

This paper explores the potential for detecting long-lived heavy neutral leptons at the LHC and future experiments within an effective field theory framework, highlighting enhanced production mechanisms and extended sensitivity ranges.

Contribution

It introduces a comprehensive EFT model with dimension-6 operators for HNLs, analyzing their production and detection prospects at high-energy colliders and future detectors.

Findings

Enhanced production cross sections due to non-renormalizable operators.

Extended sensitivity to HNL mass and mixing angles beyond minimal models.

Differences in phenomenology between Dirac and Majorana HNLs above 100 GeV.

Abstract

Heavy neutral leptons (HNLs) with masses around the electroweak scale are expected to be rather long-lived particles, as a result of the observed smallness of the active neutrino masses. In this work, we study long-lived HNLs in $N_R$SMEFT, a Standard Model (SM) extension with singlet fermions to which we add non-renormalizable operators up to dimension-6. Operators which contain two HNLs can lead to a sizable enhancement of the production cross sections, compared to the minimal case where HNLs are produced only via their mixing with the SM neutrinos. We calculate the expected sensitivities for the ATLAS detector and the future far-detector experiments: AL3X, ANUBIS, CODEX-b, FASER, MATHUSLA, and MoEDAL-MAPP in this setup. The sensitive ranges of the HNL mass and of the active-heavy mixing angle are much larger than those in the minimal case. We study both, Dirac and Majorana, HNLs and discuss how the two cases actually differ phenomenologically, for HNL masses above roughly 100 GeV.