Same-Sign Dilepton Excesses and Vector-like Quarks

Multiple analyses from ATLAS and CMS collaborations, including searches for ttH production, supersymmetric particles and vector-like quarks, observed excesses in the same-sign dilepton channel containing b-jets and missing transverse energy in the LHC Run 1 data. In the context of little Higgs theories with T parity, we explain these excesses using vector-like T-odd quarks decaying into a top quark, a W boson and the lightest T-odd particle (LTP). For heavy vector-like quarks, decay topologies containing the LTP have not been searched for at the LHC. The bounds on the masses of the T-odd quarks can be estimated in a simplified model approach by adapting the search limits for top/bottom squarks in supersymmetry. Assuming a realistic decay branching fraction, a benchmark with a 750 GeV T-odd b-prime quark is proposed. We also comment on the possibility to fit excesses in different analyses in a common framework.Comment: 1+17 pages and 11 figure

The Same-Sign Dilepton Signature of RPV/MFV SUSY

The lack of observation of superpartners at the Large Hadron Collider so far has led to a renewed interest in supersymmetric models with R-parity violation (RPV). In particular, imposing the Minimal Flavor Violation (MFV) hypothesis on a general RPV model leads to a realistic and predictive framework. Naturalness suggests that stops and gluinos should appear at or below the TeV mass scale. We consider a simplified model with these two particles and MFV couplings. The model predicts a significant rate of events with same-sign dileptons and b-jets. We re-analyze a recent CMS search in this channel and show that the current lower bound on the gluino mass is about 800 GeV at 95% confidence level, with only a weak dependence on the stop mass as long as the gluino can decay to an on-shell top-stop pair. We also discuss how this search can be further optimized for the RPV/MFV scenario, using the fact that MFV stop decays often result in jets with large invariant mass. With the proposed improvements, we estimate that gluino masses of up to about 1.4 TeV can be probed at the 14 TeV LHC with a 100 fb^-1 data set.Comment: 18 pages, 6 figures; v2: References adde

A Simplified Model Approach to Same-sign Dilepton Resonances

We discuss same-sign dilepton resonances in the simplified model approach. The relevant SU(3}_Q^J quantum numbers are $1_2^{0,1,2}$. For simplicity, we only consider a spin 0 scalar, which is typically referred to as a doubly charged Higgs in the literature. We consider the three simplest cases where the doubly charged Higgs resides in a singlet, doublet or triplet $SU(2)_L$ representation. We discuss production and decay of such a doubly charged Higgs, summarize the current direct search limits, and obtain mass limits in the cases of singlet and doublet for the first time. We also present a complete set of updated indirect search limits. We study the discovery potential at the Large Hadron Collider (LHC) with center of mass energies 7 and 14 TeV for the dominant Drell-Yan pair production with $H^{\pm\pm}$ decay in the $ee$ and $\mu\mu$ channels. We find that at 7 TeV, the LHC with 10 $fb^{-1}$ luminosity can probe mass of the doubly charged Higgs up to 380 GeV assuming 100% decay to leptons. At 14 TeV, the LHC with 100 $fb^{-1}$ luminosity can reach a mass of up to 800 GeV.Comment: 25 pages, 7 figure

Same-sign top quarks as signature of light stops

We present a new method to search for a light scalar top (stop), decaying dominantly into $c\tilde\chi^0_1$, at the LHC. The principal idea is to exploit the Majorana nature of the gluino, leading to same-sign top quarks in events of gluino pair production followed by gluino decays into top and stop. We demonstrate the reach of our method in terms of the gluino mass and the stop-neutralino mass difference.Comment: 4 pages, 1 figure, to appear in Proceedings of SUSY06, the 14th International Conference on Supersymmetry and the Unification of Fundamental Interactions, UC Irvine, California, 12-17 June 200

Top Quark Forward-Backward Asymmetry and Same-Sign Top Quark Pairs

The top quark forward-backward asymmetry measured at the Tevatron collider shows a large deviation from standard model expectations. Among possible interpretations, a non-universal $Z^\prime$ model is of particular interest as it naturally predicts a top quark in the forward region of large rapidity. To reproduce the size of the asymmetry, the couplings of the $Z^\prime$ to standard model quarks must be large, inevitably leading to copious production of same-sign top quark pairs at the energies of the Large Hadron Collider (LHC). We explore the discovery potential for $tt$ and $ttj$ production in early LHC experiments at 7-8 TeV and conclude that if {\it no} $tt$ signal is observed with 1 fb$^{-1}$ of integrated luminosity, then a non-universal $Z^\prime$ alone cannot explain the Tevatron forward-backward asymmetry.Comment: Tevatron limit from same-sign tt search adde

Same-sign Tops: A Powerful Diagnostic Test for Models of New Physics

We study the connection between the same sign top (SST) and the top quark forward-backward asymmetry $A^t_{FB}$. We find that a large class of new physics models that have been proposed to account for the $A^t_{FB}$ lead to SST quark production rate much larger than the observed rate at the LHC and consequently are severely constrained or ruled out. Our model independent, general, operator analysis shows that none of the tree-level flavor-changing operators are able to explain $A^t_{FB}$ and simultaneously remain consistent with the same-sign top-quark production constraints from the LHC data.Comment: 19 LaTeX pages, 3 Tables and 4 png Figures, accepted version for publication in JHE