Data driving the top quark forward--backward asymmetry with a lepton-based handle

Abstract

We propose that, within the standard model, the correlation between the $t\bar{t}$ forward--backward asymmetry $A_{t\bar t}$ and the corresponding lepton-based asymmetry $A_l$ -- at the differential level -- is strong and rather clean both theoretically and experimentally. Hence a combined measurement of the two distributions as a function of the lepton $p_T$, a direct and experimentally clean observable, would lead to a potentially unbiased and normalization-free test of the standard model prediction. To check the robustness of our proposal we study how the correlation is affected by mis-measurement of the $t\bar t$ system transverse momenta, acceptance cuts, scale dependence and compare the results of MCFM, POWHEG (with & without PYTHIA showering), and SHERPA's CSSHOWER in first-emission mode. We find that the shape of the relative differential distribution $A_{l} (p^{l}_{T}) [A_{t\bar{t}} (p^l_T)]$ is only moderately distorted hence supporting the usefulness of our proposal. Beyond the first emission, we find that the correlation is not accurately captured by lowest-order treatment. We also briefly consider other differential variables such as the system transverse mass and the canonical $t\bar t$ invariant mass. Finally, we study new physics scenarios where the correlation is significantly distorted and therefore can be more readily constrained or discovered using our method.