Non-factorizable effects in top quark production

TL;DR

This paper investigates the unexpectedly large forward-backward asymmetry in top-antitop production at Fermilab, exploring potential non-factorizable QCD effects and their possible impact on observed spectra.

Contribution

It provides an estimate that non-factorizable effects are negligible but suggests small spectral distortions, proposing tests for these effects to explain the asymmetry.

Findings

Observed asymmetry is three times larger than NLO QCD predictions.

Non-factorizable effects are estimated to be negligible.

Small spectral distortions could be present and are testable.

Abstract

The production of top-antitop pairs at the Fermilab Tevatron shows a forward-backward asymmetry in which the top quark tends to follow the proton direction, while the antitop tends to follow the antiproton direction. The effect grows with increasing effective mass $m_{t \bar t}$ of the top-antitop pair, and with increasing rapidity difference between the top and antitop. The observed effect is about three times as large as predicted by next-to-leading-order QCD, but with the same sign. An estimate of non-factorizable effects based on a QCD string picture finds they are negligible, but that small distortions of the $m_t$ spectrum are possible. Tests for such effects, both at and above the level of this estimate, are suggested.