Standard Model Top Quark Asymmetry at the Fermilab Tevatron

TL;DR

This paper analyzes the forward–backward asymmetry in Standard Model tt̄ production at the Tevatron, focusing on how asymmetries originate from NLO QCD effects and how they correlate with extra jet activity. It performs recalculations of inclusive and jet-tagged samples, and uses matrix-element tools to predict leptonic asymmetries in dilepton and lepton+jets channels, including spin correlations. It highlights that while the inclusive asymmetry is small and challenging to measure at Run II, differential asymmetries in tt̄+1j and tt̄+0j samples offer additional handles, albeit with limited statistics. The work also compares fixed-order results with Parton Shower Monte Carlo models to study color-coherence effects and the potential of MCs to mimic or obscure the true QCD-induced asymmetries. Overall, it underscores the importance of precise measurements and improved theory/tools to extract meaningful insights about QCD dynamics and shower modeling at the Tevatron.

Abstract

Top quark pair production at proton-antiproton colliders is known to exhibit a forward-backward asymmetry due to higher-order QCD effects. We explore how this asymmetry might be studied at the Fermilab Tevatron, including how the asymmetry depends on the kinematics of extra hard partons. We consider results for top quark pair events with one and two additional hard jets. We further note that a similar asymmetry, correlated with the presence of jets, arises in specific models for parton showers in Monte Carlo simulations. We conclude that the measurement of this asymmetry at the Tevatron will be challenging, but important both for our understanding of QCD and for our efforts to model it.

Figures (10)

• Figure 1: Differential cross section distributions as a function of the top (left) and anti-top (right) quark rapidities, produced in $p\bar{p}$ collisions at the Tevatron in Run II, $\sqrt{s}=1.96\;$TeV. Shown are the LO $t\bar{t}$ inclusive (dotted), $t\bar{t}$ NLO inclusive (solid), LO $t\bar{t}j$ inclusive (dot-dashed) and $t\bar{t}0j$ exclusive (dashed) predictions. We define the LO $t\bar{t}j$ inclusive rate as that where the additional final-state parton has $p_{T}(j)>20$ GeV and $|\eta(j)|<3$. The $t\bar{t}0j$ exclusive rate is then the NLO inclusive rate minus the LO $t\bar{t}j$ inclusive rate.
• Figure 2: Forward-backward lepton asymmetry in production-radiation $t\bar{t}j$ dilepton events as a function of the transverse momentum of the additional hard jet. The $\ell^{+}$ ($\ell^{-}$) distribution is the solid (dashed) curve. The two curves are $CP$-invariant up to the level of Monte Carlo statistical uncertainty.
• Figure 3: Forward-backward lepton asymmetries for each subprocess in production-radiation $t\bar{t}j$ dilepton events as a function of the pseudorapidity of the additional hard jet. The $\ell^{+}$ ($\ell^{-}$) distributions are shown by the solid (dashed) curves. The left panel shows the dominant $q\bar{q}$ contribution to the asymmetry (the curves with the slightly larger magnitude for both $\ell^{+}$ and $\ell^{-}$) and the asymmetry for both charges for the total rate (the curves with the smaller magnitude). The small difference between $q\bar{q}$ and total arises from the contributions of the other parton channels., whose asymmetries are indicated in the right panel. Note the absence of any charge dependence in the curves in the right panel.
• Figure 4: Normalized differential cross section with respect to the extra jet pseudorapidity (dashed), overlaid with the total forward-backward positive lepton asymmetry (solid), in dilepton production-radiation $t\bar{t}j$ events with loose cuts as described in the text.
• Figure 5: Forward-backward lepton asymmetries for each subprocess in production-radiation $t\bar{t}j$ lepton+jets events as a function of the pseudorapidity of the additional hard jet. The $\ell^{+}$ ($\ell^{-}$) distributions are shown by the solid (dashed) curves. The left panel shows the dominant $q\bar{q}$ contribution to the asymmetry (the curves with the slightly larger magnitude for both $\ell^{+}$ and $\ell^{-}$) and the asymmetry for both charges for the total rate (the curves with the smaller magnitude). The small difference between $q\bar{q}$ and total arises from the contributions of the other parton channels., whose asymmetries are indicated in the right panel. Note the absence of any charge dependence in the curves in the right panel.