  Recently the CDF and the D0 Collaborations presented the data on the top
forward-backward (FB) asymmetry A_FB as functions of M_(t t^bar) and \Delta y =
y_t - y_t^bar. We study these observables in the effective Lagrangian approach
with dimension-6 q q^bar t t^bar contact interactions, and compare with the CDF
and D0 data. When we stay within the validity region of the effective
Lagrangian approach, the mass dependent top FB asymmetry turns out to be
smaller than the CDF data, more than 2-sigma away. If this discrepancy remains
in the future data with better statistics, it would imply that the effective
Lagrangian approach is not adequate for the top FB asymmetry, and a new physics
scale around a few hundred GeV in the t- or u-channel may be responsible for
the observed top FB asymmetry.
