AFBtmeets LHC

Abstract

The recent Tevatron measurement of the forward-backward asymmetry of the top quark shows an intriguing discrepancy with Standard Model expectations, particularly at large tt invariant masses.Measurements of this quantity are subtle at the LHC, due to its pp initial state, however, one can define a forward-central-charge asymmetry which captures the physics. We study the capability of the LHC to measure this asymmetry and find that within the SM a measurement at the 5σ level is possible with roughly 60 fb −1 at √ s = 14 TeV. If nature realizes a model which enhances the asymmetry (as is necessary to explain the Tevatron measurements), a significant difference from zero can be observed much earlier, perhaps even during early LHC running at √ s = 7 TeV. We further explore the capabilities of the 7 TeV LHC to discover resonances or contact interactions which modify the tt invariant mass distribution using recent boosted top tagging techniques. We find that TeV-scale color octet resonances can be discovered, even with small coupling strengths and that contact interactions can be probed at scales exceeding 6 TeV. Overall, the LHC has good potential to clarify the situation with regards to the Tevatron forward-backward measurement.

Charge asymmetries of top quarks at hadron colliders revisited

“…This is because the central region is dominated by gluon fusion processes, while the sample with large rapidities has a larger relative content of qq initiated events. The statistical significance of both observables is, however, very similar [51] because the larger size of the asymmetry A out C with respect to A in C is compensated by the lower rate of events at larger rapidities. We consider also the cut-independent charge asymmetries …”

Simple models for the top asymmetry: constraints and predictions

“…Specifically, we consider the central [7] and forward [46] charge asymmetries, which are defined, respectively, as…”

“…The tail of the invariant mass distribution is expected to be especially sensitive to new physics in qq → tt, since the q andq parton distribution functions become more important than the gluonic one for x ≥ 0.15. There are various predictions of new signals at the LHC in other processes, expected in the different models that explain the excess in the FB asymmetry [45,46]. However, in most cases the nonobservation of those signals would not rule out a given scenario, as a null result can usually be accomodated with suitable adjustments.…”

Z′ signals in polarised top-antitop final states

“…Invariant mass distributions of the total cross sections for tt production are also included for reference, with the statistical error normalised by the bin width. Although in this work we only estimate statistical uncertainties, systematics may be important as well [34,36]. However, although the mass window selection is expected to milden their actual contribution, their inclusion would require detailed detector simulations which are beyond the scope of this paper.…”