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 NRSMEFT, 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.
Highlights
A simple and popular class of neutral longlived particles (LLPs) models are the so-called ‘portal’ models
We present the effects of a possible timing cut at ANUBIS on our analysis, while the second discusses the reinterpretation of a CMS search for Heavy neutral leptons (HNLs) [27] within our theoretical setup
We have treated the mixing with the active neutrinos |VαN |2 with α = e, μ, τ, the HNL mass mN, and the operator coefficients cO/Λ2 as independent parameters
Summary
We present some basic theoretical aspects of our setup. First, as a motivation, we briefly discuss different variants of electroweak seesaw mechanisms. In our model implementations (to be detailed below), the calculation of production cross sections with MadGraph considers both possibilities: HNLs as Dirac or Majorana states. Operators in the right part of table 1 contain only one NR They can lead to sizeable production cross sections for NR too, and mediate decays of ( the lightest) NR to SM fermions at the same time. Since we are mostly interested in studying the EFT limit, here we will only discuss the Z option and three particular scalar LQs. For a complete list of all possible LQ states for generating d = 6 four-fermion operators see ref. For the EFT description to be valid, mZ should be significantly larger than 2mN , contrary to the LQ case, where the NR production occurs via t-channel diagrams
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