Heavy Neutral Leptons in the NRSMEFT and the High-Luminosity LHC

We study heavy neutral leptons (HNLs) in the NRSMEFT, the effective field theory of the Standard Model extended with gauge singlet right-handed fermions. Four-fermion operators which contain two HNLs and two quarks can lead to a sizeable enhancement of the HNL pair-production cross section, compared to the minimal case where HNLs are produced only via their mixing with the active neutrinos. We calculate the expected sensitivities for the ATLAS detector and the future far-detector experiments at the high-luminosity LHC: AL3X, ANUBIS, CODEX-b, FASER, MATHUSLA and MoEDAL-MAPP. We find that new physics scales up to 22 TeV and active-heavy mixing squared as small as 10^{-24} could be probed by some of these experiments. Next, we consider four-fermion operators with a single HNL. Such operators might dominate both HNL production and decay, and we find that new physics scales in excess of 20 TeV could be tested at ATLAS.