  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.
