New physics contributions to $A_{\mathrm{FB}}^{t\overline{t}}$ at the Tevatron

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 $\overline{t}t$ 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.

Figure 1

Feynman diagrams for $t\overline{t}$ production via anomalous $gt\overline{t}$ interaction.

Figure 2

Feynman diagrams with FCNC operators for $t\overline{t}$ production via gluon fusion.

Figure 3

FCNC Feynman diagrams for $t\overline{t}$ production via $q\overline{q}$ fusion.

Figure 4

Four-fermion Feynman diagrams for $t\overline{t}$ production.

Figure 5

The ${\chi }^2$ as a function of ${\alpha }_1$, ${\alpha }_3$, and ${\alpha }_7$ with each parameter taken nonzero at a time. The most favored values are ${\alpha }_1=0$ and $|{\alpha }_3|\ne 0$. There are no preferred values for ${\alpha }_7$.

Figure 6

One dimensional likelihood distribution of the parameters ${\alpha }_1$ to ${\alpha }_8$ after the fit.

Figure 7

One dimensional likelihood distribution of the total cross section and all asymmetries after the fit.

Figure 8

Two-dimensional correlation plots for the strong and electroweak FCNC parameters. Only the most relevant correlations are shown. The shaded areas are the ones where the values of ${\alpha }_i$ reach their highest probability—the darker ones represent 95% C.L. while the lighter ones are for 68% C.L.

Figure 9

Likelihood distributions for the most relevant combination of parameters.

Figure 10

Two-dimensional correlations between cross sections and asymmetries and between the different asymmetries.

Figure 11

Likelihood distribution for the parameters of the four-fermion Lagrangian, after the fit.

Figure 12

Likelihood distribution for the total cross section and for the asymmerties and for the six independent combinations of the 4F Lagrangian parameters, after the fit.

Figure 13

Two-dimensional correlations between the parameters that can give a significant contribution to the asymmetry. Also shown are typical examples of the correlations between cross section and asymmetries and between two asymmetry observables.

Figure 14

Two-dimensional correlations between the parameters $C_1$ and $C_2$ and the total cross section, total asymmetry, and the parameter $C_{22}$.