  Motivated by a possible anomaly in the forward-backward (FB) asymmetry of top
quark (A_{\rm FB}) observed at the Tevatron, we perform a model independent
analysis on q \bar{q} \to t \bar{t} using an effective lagrangian with dim-6
four-quark operators. We derive necessary conditions on new physics structures
and the couplings that are consistent with the t\bar{t} production cross
section and A_{\rm FB} measured at the Tevatron, and discuss possible new
physics scenarios that could generate such dim-6 operators.
