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, $m_{ell^+ ell^-}^{tt min}(OSSF) > 1$ 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) $u t mu mu$ 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 $u t mu mu$ and $c t mu mu$ 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.
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