New flavor physics in di- and tri-lepton events from single-top at the LHC and beyond

Abstract

The associated production of a single-top with opposite-sign same-flavor (OSSF) di-leptons, $pp\to t{\ell }^{+}{\ell }^{-}$ and $pp\to t{\ell }^{+}{\ell }^{-}+j$ ($j=$light jet), can lead to striking tri-lepton $pp\to {\ell }^{\prime }{\ell }^{+}{\ell }^{-}+X$ and di-lepton $pp\to {\ell }^{+}{\ell }^{-}+j_b+X$ ($j_b=b$-jet) events at the LHC, after the top decays. Although these rather generic multi-lepton signals are flavor-blind, they can be generated by new 4-Fermi flavor changing (FC) $u_{i}t\ell \ell$ scalar, vector and tensor interactions ($u_i\in u,c$), which we study in this paper; we match the FC $u_{i}t\ell \ell$ 4-Fermi terms to the SMEFT operators and also to different types of FC underlying heavy physics. The main backgrounds to these di- and tri-lepton signals arise from $t\bar{t}$, $Z$+jets and $VV$ ($V=W,Z$) production, but they can be essentially eliminated with a sufficiently high invariant mass selection on the OSSF di-leptons, $m_{{\ell }^{+}{\ell }^{-}}^{\mathtt{min}}(OSSF)>1$ TeV; the use of $b$-tagging as an additional selection in the di-lepton final state case also proves very useful. We find, for example, that the expected 95\% CL bounds on the scale of a tensor(vector) $ut\mu \mu$ interaction, with the current $\sim 140$ fb${}^{-1}$ of LHC data, are $\Lambda <5(3.2)$ TeV or $\Lambda <4.1(2.7)$ TeV, if analyzed via the di-muon ${\mu }^{+}{\mu }^{-}+j_b$ signal or the $e{\mu }^{+}{\mu }^{-}$ tri-lepton one, respectively. The expected reach at the HL-LHC with 3000 fb${}^{-1}$ of data is $\Lambda <7.1(4.7)$ TeV and $\Lambda <2.4(1.5)$ TeV for the corresponding $ut\mu \mu$ and $ct\mu \mu$ operators. We also study the potential sensitivity at future 27 TeV and 100 TeV high-energy LHC successors and also discuss the possible implications of this class of FC 4-Fermi effective interactions on lepton non-universality tests at the LHC.

Comments: 26 pages, 10 figures, 11 tables. Final version as published in PRD