Discerning New Physics in Top-Antitop Production using Top Spin Observables at Hadron Colliders

TL;DR

This paper investigates various new physics models to explain the Tevatron top-antitop forward-backward asymmetry anomaly, finding that only an axigluon model aligns with all current measurements, while scalar models face significant constraints.

Contribution

It systematically compares simplified resonance models against collider data, identifying which models can explain the Tevatron anomaly without conflicting with LHC results.

Findings

Only axigluon models fit all Tevatron and LHC data.

W' and Z' models are excluded by LHC charge asymmetry measurements.

Scalar models predict deviations in top spin observables, constrained by recent measurements.

Abstract

Copious production of top-antitop quark pairs at hadron colliders has enabled various probes into the properties and interactions of top quarks. Among the various presently measured observables, the forward-backward asymmetry (FBA) in t tbar production measured at the Tevatron significantly deviates from the standard model predictions, and many models of new physics have been invented to explain the puzzle. We consider the consistency of the simplified single-resonance models containing a color octet axial-vector ("axigluon"), color triplet or sextet weak singlet scalars, weak isodoublet scalar, flavor-changing neutral Z', or charged W' vector boson with existing t tbar production measurements. Among the considered models only an axigluon can reproduce all Tevatron observables, without being in severe tension with the recent LHC results on t tbar production cross section, charge asymmetry and top-spin correlations. The LHC charge asymmetry measurements exclude the W' and Z' explanations of the Tevatron FBA anomaly. On the other hand, all scalar models predict notable deviations in several top spin observables, and the recent top spin correlation measurement using the "helicity" spin quantization axis by ATLAS already provides a significant constraint on possible explanations of the Tevatron FBA anomaly. Future precise measurements of top spin correlations and especially top polarization could differentiate between scalar t(u)-channel models, while they are less sensitive to pure axigluon contributions.