Measuring the Higgs Boson Yukawa Couplings at an NLC

The search for the Higgs boson is one of the primary objectives of present and future colliders. Once the Higgs boson has been discovered, it will be important to measure its couplings to fermions and to gauge bosons. These couplings are completely determined in the Standard Model and the process ee ! tth provides a direct mechanism for measuring the tth Yukawa coupling. Since this coupling can be signi cantly di erent in a supersymmetric model from that in the Standard Model, the measurement would provide a means of discriminating between di erent models. The associated production of a Higgs boson with a top quark pair in ee collisions has a small rate, around 1 fb for p s = 500 GeV and Mh 100 GeV . However, the signature, ee ! tth ! WW bbbb is distinctive and a precise measurement may be possible. A similar reaction in the b quark system, ee ! bbh, is suppressed in the Standard Model due to the smallness of the bbh Yukawa coupling. In a supersymmetric model, however, this coupling can be enhanced for large values of the parameter tan . In addition, a supersymmetric model contains resonant contributions not present in the Standard Model such as, for example, the process ee ! A0h0i ; A ! bb. In order to extract the Yukawa couplings, precise predictions for the rates, including QCD corrections, are necessary. The QCD corrections to the associated production of a Higgs boson with a heavy quark pair have been computed by two groups [1,2] and are the subject of this note.