Improving the Top Quark Forward-Backward Asymmetry Measurement at the LHC

TL;DR

This paper develops a likelihood-based method using multiple variables to distinguish quark-initiated from gluon-initiated top quark events at the LHC, enhancing the measurement of the top quark forward-backward asymmetry.

Contribution

It introduces a combined variable approach and analytical methods to improve asymmetry measurement and analyze top quark polarization and spin correlation for new physics models.

Findings

Asymmetry measurement significance improved by 10-30%.

Central asymmetry values increased by 40-100%.

Derived optimal spin quantization axes for top quark studies.

Abstract

At the LHC, top quark pairs are dominantly produced from gluons, making it difficult to measure the top quark forward-backward asymmetry. To improve the asymmetry measurement, we study variables that can distinguish between top quarks produced from quarks and those from gluons: the invariant mass of the top pair, the rapidity of the top-antitop system in the lab frame, the rapidity of the top quark in the top-antitop rest frame, the top quark polarization and the top-antitop spin correlation. We combine all the variables in a likelihood discriminant method to separate quark-initiated events from gluon-initiated events. We apply our method on models including G-prime's and W-prime's motivated by the recent observation of a large top quark forward-backward asymmetry at the Tevatron. We have found that the significance of the asymmetry measurement can be improved by 10% to 30%. At the same time, the central values of the asymmetry increase by 40% to 100%. We have also analytically derived the best spin quantization axes for studying top quark polarization as well as spin-correlation for the new physics models.