New physics contributions to the forward-backward asymmetry at the Tevatron

TL;DR

This paper investigates potential new physics explanations for the Tevatron's observed forward-backward asymmetry in top quark pairs, using effective operators and statistical fitting to data.

Contribution

It introduces a comprehensive set of dimension six operators to model new physics effects and employs MCMC to identify the minimal set fitting the asymmetry data better than the Standard Model.

Findings

A small number of operators can improve the fit to the asymmetry data.

Standard Model perfectly fits total and differential cross sections.

New physics effects are constrained to specific operator combinations.

Abstract

The Tevatron has measured a discrepancy relative to the Standard Model prediction in the forward-backward asymmetry in top quark pair production. This asymmetry grows with the rapidity difference of the two top quarks. It also increases with the invariant mass of the tt pair, reaching, for high invariant masses, 3.4 standard deviations above the Next to Leading Order prediction for the charge asymmetry of QCD. However, perfect agreement between experiment and the Standard Model was found in both total and differential cross section of top quark pair production. As this result could be a sign of new physics we have parametrized this new physics in terms of a complete set of dimension six operators involving the top quark. We have then used a Markov Chain Monte Carlo approach in order to find the best set of parameters that fits the data, using all available data regarding top quark pair production at the Tevatron. We have found that just a very small number of operators are able to fit the data better than the Standard Model.