Search for heavy neutral leptons in decays of W bosons produced in 13 TeV pp collisions using prompt signatures in the ATLAS detector

Ali, Hanadi and Alsolami, Zainab and Barton, A.E. and Borissov, G. and Bouhova-Thacker, E.V. and Fernandez Barbadillo, B. and Ferrando, James and Fox, H. and Hagan, Alina and Jones, R.W.L. and Kartvelishvili, V. and Love, P.A. and Marshall, Emma J. and McElhinney, Luke and Meng, L. and Sampson, Elliot and Smizanska, M. and Wharton, A.M. (2026) Search for heavy neutral leptons in decays of W bosons produced in 13 TeV pp collisions using prompt signatures in the ATLAS detector. European Physical Journal C: Particles and Fields, 86 (2): 153. ISSN 1434-6044

Abstract

The existence of right-handed neutrinos with Majorana masses below the electroweak scale could help address the origins of neutrino masses, the matter–antimatter asymmetry, and dark matter. In this paper, leptonic decays of W bosons from 140 fb $$^{-1}$$ - 1 of 13 TeV proton–proton collisions at the LHC, reconstructed in the ATLAS experiment, are used to search for heavy neutral leptons produced through their mixing with muon or electron neutrinos in a scenario with lepton number violation. The search is conducted using prompt leptonic decay signatures. The considered final states require two same-charge leptons or three leptons, while vetoing three-lepton same-flavour topologies. No significant excess over the expected Standard Model backgrounds is found, leading to constraints on the heavy neutral lepton’s mixing with muon and electron neutrinos for heavy-neutral-lepton masses. The analysis excludes $$|U_{e}|^2$$ | U e | 2 values above $$8\times 10^{-5}$$ 8 × 10 - 5 and $$|U_{\mu }|^2$$ | U μ | 2 values above $$5.0 \times 10^{-5}$$ 5.0 × 10 - 5 in the full mass range of 8–65 GeV. The strongest constraints are placed on heavy-neutral-lepton masses in the range 15–30 GeV of $$|U_{e}|^2 < 1.1 \times 10^{-5}$$ | U e | 2 < 1.1 × 10 - 5 and $$|U_{\mu }|^2 < 5 \times 10^{-6}$$ | U μ | 2 < 5 × 10 - 6 .