(t W)(anti-t W) Same-Charge Dilepton Search

We search for events with two high-momentum, same-charge leptons (e or μ) and many jets, that are characteristic of fermion-pair decays to t quarks and W bosons. This particular signature—two same-charge leptons with transverse momentum typically greater than 50 GeV/c in events with at least four jets—has only rare standard-model sources, and it is an excellent channel for seeking new phenomena with the first run of the LHC. The physics motivation is compelling: the hypothetical fermions could be the down-type quarks for a fourth standard-model generation (d₄), or they could be Kaluza-Klein fermions (B, T_5/3) manifesting warped extra dimensions.  In particular, recent CP-violation measurements are very suggestive of the existence of fourth-generation quarks. These and other models that predict quark-like objects are allowed by current experimental constraints, provided that the masses are lighter than about 500 GeV/c². Thus, these models can be explored or excluded with the data from the first Atlas run.

Status and plans

In order to obtain reliable results from data recorded during the first Atlas run, possible background sources must be sufficiently understood using both simulations and recorded data. In the near term, we intend to produce a public Atlas note documenting the expected sensitivity and dominant backgrounds; this document will be a roadmap for studies to undertake with the first data. In particular, sources of background leptons must be understood. For electrons, the probability of mis-identifying the charge or of mis-reconstructing a jet is significant. The charge misidentification rate is related to the amount of material in the tracking detectors in front of the calorimeters; this material distribution can be studied using photon conversions in recorded data. The jet-fake rate must be obtained from data, and there are several ways to approach this. On the one hand, high-statistics processes are needed to obtain precision, such as γ + jet events. On the other hand, an event sample with a representative number of heavy-quark jets needs to be studied to estimate possible systematic biases in the background estimate. Processes with electrons can be compared to the same processes with muons, which are efficiently identified in the Atlas detector. Moreover, the contribution from t-quark pairs producing events with two leptons needs to be understood, which will be one of the main goals of Atlas during the first data run.

References

[1] G. D. Kribs, T. Plehn, M. Spannowsky and T. M. P. Tait, “Four generations and Higgs physics,” Phys. Rev. D 76, 075016 (2007). (Spires)

This excellent paper neatly reviews all the constrains on fourth-generation masses and discusses the implications for Higgs physics, including cross sections and decay modes.

[2] P. H. Frampton, P. Q. Hung and M. Sher, “Quarks and leptons beyond the third generation,” Phys. Rept. 330, 263 (2000). (Spires)

[3] P. Q. Hung and M. Sher, “Experimental constraints on fourth generation quark masses,” Phys. Rev. D 77, 037302 (2008). (Spires)

[4] H. J. He, N. Polonsky and S. f. Su, “Extra families, Higgs spectrum and oblique corrections,” Phys. Rev. D 64, 053004 (2001). (Spires)