Heavy quark contributions to Higgs production via gluon fusion

A primary mechanism for Higgs production at hadron colliders is gluon fusion through quark loops. It is generally believed that the cross section is independent of the mass of a sufficiently heavy quark: Measurement of the process would then bound the number of heavy quarks. However, two-loop corrections to the cross section due to internal Higgs exchange grow with the mass of the heavy quark. These corrections are calculated, and are found to be (mquaurk/520 GeV)2f(r) where r is the Higgs boson to top quark mass-squared ratio and f(r) varies from 0.10 to 1 over the region of interest. (orig.)