Comment on measuring the t-tbar forward-backward asymmetry at ATLAS and CMS

We suggest a new possibility for ATLAS and CMS to explore the t-tbar forward-backward asymmetry measured at the Tevatron, by attempting to reconstruct t-tbar events, with one of the tops decaying semileptonically in the central region (|\eta| < 2.5) and the other decaying hadronically in the forward region (|\eta| > 2.5). For several models which give comparable Tevatron signals, we study the charge asymmetry at the LHC as a function of cuts on |\eta| and on the t-tbar invariant mass, m_{t-tbar}. We show that there is an interesting complementarity between cuts on |\eta| and m_{t-tbar} to suppress the dominant and symmetric gg -> t-tbar rate, and different combinations of cuts enhance the distinguishing power between models. This complementarity is likely to hold in other new physics scenarios as well, which affect the t-tbar cross section, so it motivates extending t-tbar reconstruction to higher |\eta|.