  The top quark has a large Yukawa coupling with the Higgs boson. In the usual
extensions of the standard model the Higgs sector includes extra scalars, which
also tend to couple strongly with the top quark. Unlike the Higgs, these fields
have a natural mass above 2m_t, so they could introduce anomalies in ttbar
production at the LHC. We study their effect on the ttbar invariant mass
distribution at sqrt{s}=7 TeV. We focus on the bosons (H,A) of the minimal SUSY
model and on the scalar field (r) associated to the new scale f in Little Higgs
(LH) models. We show that in all cases the interference with the standard
amplitude dominates over the narrow-width contribution. As a consequence, the
mass difference between H and A or the contribution of an extra T-quark loop in
LH models become important effects in order to determine if these fields are
observable there. We find that a 1 fb^{-1} luminosity could probe the region
tan beta \le 3 of SUSY and v/(sqrt{2}f) \ge 0.3 in LH models.

High Energy Physics - Phenomenology