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 NRSMEFT, 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 NRSMEFT 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.
Highlights
Interest in searches for heavy neutral leptons (HNLs) at the LHC has increased considerably in the past few years
Ref. [38] studied single-NR operators for various proposed LLP “far” detectors, such as MATHUSLA [3, 42, 43], CODEXb [44], AL3X [45], FASER [46], and ANUBIS [47], as well as ATLAS, for HNLs produced from charm and bottom meson decays and with mass below 5 GeV.3
The Standard Model (SM) effective field theory (EFT) extended with sterile neutrinos, known as the NRSMEFT, provides a framework to systematically study sterile neutrinos associated with a high new-physics (NP) scale in ultra-violet complete models beyond the SM
Summary
We briefly introduce the NRSMEFT, focusing on the operators of interest for the current work. We are interested in the effects of the lepton-number-conserving four-fermion interactions containing one NR and three SM fermions. The single-NR operators including quarks can lead to enhanced HNL production cross section at the LHC, but they trigger the decay of NR to a lepton and two quarks. Neglecting the masses of the lepton and light quarks, the partial decay width to charged leptons plus quarks is given by. With mN being the HNL mass, cO the Wilson coefficient of the operator O, and fO the numerical factor depending on the operator type. For Majorana neutrinos, one has to add the charged conjugated channels, leading to another factor of 2 for the widths
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