Search for heavy right-handed Majorana neutrinos in the decay of top quarks produced in proton-proton collisions at s=13 TeV with the ATLAS detector

Ali, Hanadi and Alsolami, Zainab and Barton, A.E. and Borissov, G. and Bouhova-Thacker, E.V. and Ferguson, R.A.M. and Ferrando, James and Fox, H. and Hagan, Alina and Henderson, R.C.W. and Jones, R.W.L. and Kartvelishvili, V. and Love, P.A. and Marshall, E.J. and Meng, L. and Muenstermann, D. and Ribaric, N. and Sampson, Elliot and Smizanska, M. and Spinali, S. and Wharton, A.M. (2024) Search for heavy right-handed Majorana neutrinos in the decay of top quarks produced in proton-proton collisions at s=13 TeV with the ATLAS detector. Physical Review D, 110 (11): 112004. ISSN 2470-0010

Abstract

A search for heavy right-handed Majorana neutrinos is performed with the ATLAS detector at the CERN Large Hadron Collider, using the 140 fb−1 of proton–proton collision data at s=13 TeV collected during Run 2. This search targets tt¯ production, in which both top quarks decay into a bottom quark and a W boson, where one of the W bosons decays hadronically and the other decays into an electron or muon and a heavy neutral lepton. The heavy neutral lepton is identified through a decay into an electron or muon and another W boson, resulting in a pair of same-charge same-flavor leptons in the final state. This paper presents the first search for heavy neutral leptons in the mass range of 15–75 GeV using tt¯ events. No significant excess is observed over the background expectation, and upper limits are placed on the signal cross sections. Assuming a benchmark scenario of the phenomenological type-I seesaw model, these cross section limits are then translated into upper limits on the mixing parameters of the heavy Majorana neutrino with Standard Model neutrinos.