The top-pair forward-backward asymmetry beyond NLO

TL;DR

The paper addresses the Tevatron top-quark forward-backward asymmetry by employing RG-improved perturbation theory with NNLL soft-gluon resummation in two kinematic schemes, delivering NLO+NNLL predictions for both lab- and $t\bar{t}$-frame observables and providing differential and binned results. It finds that NNLL corrections reduce scale and PDF uncertainties and stabilize predictions, though the high-mass bin tension with data persists. The study also extends to the LHC, computing a partially integrated charge asymmetry as a function of rapidity cuts to offer complementary probes for new physics. Overall, the work refines SM predictions for top-quark asymmetries, clarifies the status of the Tevatron anomaly, and outlines high-rapidity LHC observables that could reveal new interactions in the top-quark sector.

Abstract

We make use of recent results in effective theory and higher-order perturbative calculations to improve the theoretical predictions of the QCD contribution to the top-quark pair production forward-backward asymmetry at the Tevatron. In particular, we supplement the fixed-order NLO calculation with higher-order corrections from soft gluon resummation at NNLL accuracy performed in two different kinematic schemes, which allows us to make improved predictions for the asymmetry in the $p\bar p$ and $t\bar t$ rest frames as a function of the rapidity and invariant mass of the $t\bar t$ pair. Furthermore, we provide binned results which can be compared with the recent measurements of the forward-backward asymmetry in events with a large pair invariant mass or rapidity difference. Finally, we calculate at NLO+NNLL order the top-quark charge asymmetry at the LHC as a function of a lower rapidity cut-off for the top and antitop quarks.

Figures (6)

• Figure 1: Left: The asymmetric differential cross section $d\Delta\sigma^{p \bar{p}}_{\text{FB}}/dy_t$. Right: The asymmetry $A^{p \bar{p}}_{\text{FB}}(y_t)$. The bands show the uncertainties related to scale variation as explained in the text.
• Figure 2: Left: The asymmetric cross section $d\Delta\sigma^{t\bar{t}}_{\text{FB}}/dM_{t\bar{t}}$ as a function of the invariant mass at NLO and NLO+NNLL order. Right: The asymmetry $A^{t \bar{t}}_{\text{FB}}(M_{t\bar{t}})$. The bands show the uncertainties related to scale variation as explained in the text.
• Figure 3: The asymmetry in the high and low invariant-mass region as measured in Aaltonen:2011kc, compared to our predictions at NLO+NNLL order. The bands in the NLO+NNLL results are related to uncertainties from scale variation, while the NLO result in the higher bin is evaluated at $\mu_f= m_t$.
• Figure 4: Left: The asymmetric differential cross section $d\Delta\sigma^{t \bar{t}}_{\text{FB}}/d\Delta y$. Right: The asymmetry $A^{t \bar{t}}_{\text{FB}}(\Delta y)$. The bands show the errors related to scale variation as explained in the text.
• Figure 5: The asymmetry for $\Delta y < 1$ and $\Delta y \ge 1$ as measured in Aaltonen:2011kc, compared to our predictions at NLO+NNLL order. The bands in the NLO+NNLL results are related to uncertainties from scale variation, while the NLO result in the higher bin is evaluated at $\mu_f= m_t$.