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 $pp \to t \bar t \ell^+ \ell^-$, which leads to the multi-leptons signals $pp \to n \ell + {\tt jets} + \not\!\! E_T$, where $n = 2,3,4$. The NP is parameterized via 4-Fermi effective $t\bar{t} \ell^+ \ell^-$ contact interactions of various types, which are generated by multi-TeV heavy scalar, vector or tensor exchanges in $t \bar t \to \ell^+ \ell^-$; we focus on the case of $\ell=e,\mu$. We match the 4-Fermi $t t \ell \ell$ 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 $t\bar{t} \ell^+ \ell^-$ 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 $pp \to t \bar t$ and $pp \to WZ$, respectively, can be essentially eliminated after applying the $2\ell$ and $3 \ell$ 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 $t t e e$ and $t t \mu \mu$ contact interactions are of same size and LFU violating (LFUV) NP, where the scale of the $t t \mu \mu$ 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 $t t \ell \ell$ contact interactions at the level $\Lambda \gtrsim 2-3$ TeV, which are considerably tighter than the current bounds on these 4-Fermi terms.