Multi-lepton probes of new physics and lepton-universality in top-quark interactions

We explore the sensitivity to new physics (NP) in the associated production of top-quarks with leptons pp -> t((t) over bar)l(+)l(-), which leads to the multi-leptons signals pp -> nl+jets+E?T, where n=2,3,4. The NP is parameterized via 4-Fermi effective t((t) over bar)l(+)l(-) contact interactions of various types, which are generated by multi-TeV heavy scalar, vector or tensor exchanges in t((t) over bar)-> l(+)l(-); we focus on the case of l=e,mu. We match the 4-Fermi ttll terms to the SMEFT operators and also give examples of specific underlying heavy physics that can generate such terms. Analysis of the SM signals and corresponding backgrounds shows that the di-lepton and tri-lepton channels are much better probes of the effective t((t) over bar)l(+)l(-) 4-Fermi terms than the four-lepton one at the 13 TeV LHC. Therefore, the best sensitivity is obtained in the di- and tri-lepton channels, for which the dominant background pp -> t((t) over bar) and pp -> WZ, respectively, can be essentially eliminated after applying the 2l and 3l selections and a sufficiently high invariant mass selection for the opposite sign same flavor (OSSF) lepton-pair. We explore two cases: lepton flavor universal (LFU) NP where the ttee and tt mu mu contact interactions are of same size and LFU violating (LFUV) NP, where the scale of the tt mu mu terms is assumed to be much lower. We show that in both cases it is possible to obtain new 95% CL bounds on the scale of the ttll contact interactions at the level lambda?2-3 TeV, which are considerably tighter than the current bounds on these 4-Fermi terms. (C) 2022 The Author(s). Published by Elsevier B.V.

Automated summary

In this study, we explore the sensitivity to new physics (NP) in the associated production of top-quarks with leptons. The analysis focuses on the four-Fermi effective t((t) over bar)l(+)l(-) contact interactions generated by multi-TeV heavy scalar, vector, or tensor exchanges. The results show that the di-lepton and tri-lepton channels are better probes for the effective t((t) over bar)l(+)l(-) interactions, and the dominant backgrounds can be eliminated through proper selections and invariant mass criteria. The study provides new and tighter constraints on the scale of the ttll contact interactions.