[Paper Review] The ttbar asymmetry in the Standard Model and beyond

Germán Rodrigo|arXiv (Cornell University)|Jul 2, 2012

Particle physics theoretical and experimental studies29 references3 citations

TL;DR

This paper reviews the top quark pair charge asymmetry in the Standard Model (SM) and beyond, analyzing discrepancies between SM predictions and Tevatron measurements that show a persistent 3–3.4σ excess. It introduces a new observable, $ A_{t\bar{t}}(Y) $, and evaluates the impact of $ t\bar{t} $ transverse momentum cuts, concluding that current LHC data show a 1σ tension with SM predictions, while new physics models are increasingly constrained by LHC results.

ABSTRACT

A sizable charge asymmetry in top quark pair production has been observed at the Tevatron. The experimental results seem to exceed systematically the Standard Model theory predictions by a significant amount and have triggered a large number of suggestions for 'new physics'. The effect is also visible at the LHC, and preliminary results have already been presented by the ATLAS and CMS collaborations. In this talk, we review the present status of the theoretical predictions, and their comparison with the experimental measurements.

Motivation & Objective

• To assess the persistent discrepancy between SM predictions and Tevatron measurements of the top quark charge asymmetry, which show a 3–3.4σ excess.
• To introduce and validate a new observable, $ A_{t\bar{t}}(Y) $, that measures charge asymmetry with respect to the average rapidity of top and antitop quarks, suitable for both Tevatron and LHC.
• To evaluate whether cuts on $ t\bar{t} $ transverse momentum could explain the observed asymmetry discrepancy.
• To examine the compatibility of LHC measurements with SM predictions and assess the viability of new physics models proposed to explain the Tevatron anomaly.
• To update the tension between experimental results and SM predictions, considering recent data from CDF, D0, ATLAS, and CMS.

Proposed method

• Theoretical calculation of the top quark charge asymmetry in the SM using QCD corrections, focusing on interference between Born amplitudes and antisymmetric one-loop corrections.
• Use of cut diagrams (Fig. 1) to represent absorptive contributions to the asymmetry, emphasizing the role of $ d_{abc}^2 = 40/3 $ color factor.
• Introduction of $ A_{t\bar{t}}(Y) $, a rapidity-based asymmetry measure defined with respect to the average rapidity of the $ t\bar{t} $ pair, to standardize comparisons across colliders.
• Analysis of the effect of $ p_\perp^{t\bar{t}} $ cuts on the asymmetry, considering potential mismodeling in $ p_\perp^{t\bar{t}} $ distributions and logarithmic corrections.
• Comparison of theoretical predictions with experimental data from CDF, D0, ATLAS, and CMS, including updated results from the 2012 LHC run.
• Evaluation of the impact of higher-order QCD corrections (NLL, NNLL), confirming their small effect on the leading-order asymmetry prediction.

Experimental results

Research questions

• RQ1 Does the observed top quark charge asymmetry excess at the Tevatron exceed SM predictions, and if so, by how much?
• RQ2 Can the $ t\bar{t} $ transverse momentum distribution explain the observed asymmetry discrepancy?
• RQ3 How does the new observable $ A_{t\bar{t}}(Y) $ improve the comparison of asymmetry measurements between the Tevatron and LHC?
• RQ4 What is the current level of tension between LHC measurements and SM predictions for the charge asymmetry?
• RQ5 How do recent LHC results constrain models proposed to explain the Tevatron anomaly?

Key findings

• The Tevatron experiments CDF and D0 observe a persistent positive charge asymmetry excess over SM predictions, with a 3.4σ significance in the high $ m_{t\bar{t}} $ region, later reduced to less than 3σ in updated CDF analysis.
• LHC measurements by ATLAS and CMS show central values below the SM prediction, indicating a 1σ tension with the SM, though experimental errors remain large.
• The new observable $ A_{t\bar{t}}(Y) $ provides a consistent framework for comparing asymmetries across colliders, with theoretical predictions matching experimental data within uncertainties.
• Theoretical analysis confirms that NLL and NNLL corrections do not significantly alter the leading-order SM prediction for the asymmetry.
• Cuts on $ t\bar{t} $ transverse momentum may affect asymmetry estimates due to potential mismodeling of $ p_\perp^{t\bar{t}} $ distributions and large logarithmic corrections, requiring further study.
• LHC results, including same-sign top production and $ t\bar{t} $+jet measurements, are increasingly constraining the parameter space of new physics models proposed to explain the Tevatron anomaly.