We consider extensions of the Standard Model with a number of additional
heavy quarks which couple to the Higgs boson via top-like Yukawa interactions.
We construct an effective theory valid for a Higgs boson mass which is lighter
than twice the lightest heavy quark mass and compute the corresponding Wilson
coefficient through NNLO. We present numerical results for the gluon fusion
cross-section at the Tevatron for an extension of the Standard Model with a
fourth generation of heavy quarks. The gluon fusion cross-section is enhanced
by a factor of roughly 9 with respect to the Standard Model value. Tevatron
experimental data can place stringent exclusion limits for the Higgs mass in
this model.Comment: 14 pages, 1 tabl
