  A large forward-backward asymmetry in $t\bar t$ production, for large
invariant mass of the $t\bar t$ system, has been recently observed by the CDF
collaboration. Among the scalar mediated mechanisms that can explain such a
large asymmetry, only the t-channel exchange of a color-singlet weak-doublet
scalar is consistent with both differential and integrated $t\bar t$ cross
section measurements. Constraints from flavor changing processes dictate a very
specific structure for the Yukawa couplings of such a new scalar. No sizable
deviation in the differential or integrated $t\bar t$ production cross section
is expected at the LHC.
