Abstract
The problems of neutrino masses, matter-antimatter asymmetry, and dark matter could be successfully addressed by postulating right-handed neutrinos with Majorana masses below the electroweak scale. In this work, leptonic decays of W bosons extracted from 32.9 fb−1 to 36.1 fb−1 of 13 TeV proton–proton collisions at the LHC are used to search for heavy neutral leptons (HNLs) that are produced through mixing with muon or electron neutrinos. The search is conducted using the ATLAS detector in both prompt and displaced leptonic decay signatures. The prompt signature requires three leptons produced at the interaction point (either μμe or eeμ) with a veto on same-flavour opposite-charge topologies. The displaced signature comprises a prompt muon from the W boson decay and the requirement of a dilepton vertex (either μμ or μe) displaced in the transverse plane by 4–300 mm from the interaction point. The search sets constraints on the HNL mixing to muon and electron neutrinos for HNL masses in the range 4.5–50 GeV.
Highlights
Background processes were generated using PowhegBox [41,42,43] with the next-toleading order (NLO) CT10 parton distribution function (PDF) set [44] for top-quark pair and single top-quark production, in combination with Pythia [45] (v6.428, for parton showering) using the CTEQ6L1 PDF set [46] and Perugia 2012 tune [47]
Leptonic decays of W bosons extracted from 32.9 fb−1 to 36.1 fb−1 of 13 TeV proton–proton collisions at the LHC are used to search for heavy neutral leptons (HNLs) that are produced through mixing with muon or electron neutrinos
This is done by applying a transfer factor from 0-lepton displaced vertex (DV) to 2-lepton DVs obtained in the same-charge control region. This provides an unbiased estimate of all backgrounds remaining after the selection for which the ratio of 2-lepton background DVs to 0-lepton background DVs does not depend on the DV charge configuration. While this is the case for accidental crossings including those involving cosmic-ray muons, it does not include single cosmic-ray muons reconstructed as back-to-back muons nor decays of neutral hadrons which are either metastable or produced in material interactions, which can be neglected in the mDV > 4 GeV region as discussed above
Summary
This section details N production via mixing with an electron or muon neutrino originating from an on-shell W boson decay, as well as its leptonic decays via the same mixing, as illustrated in figure 1. The generation and simulation of Monte Carlo (MC) signal and background events is presented at the end of the section
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