Heavy flavor production at high energyep colliders

The total production rates for heavy quark pairs due to gauge boson fusion processes at high energyep colliders are evaluated. At HERA,b (bar t) production dominates overt (bar t) production form _t ≧60 GeV and is observable up tom _t ?80(90)GeV where the number of expectedb (bar t) events is about 15(10) for ∝L=200pb^?1. Including the contributions fromep→WX→btX the total number of expectedbt events amounts to about 50 events form _t ?80GeV. The influence of thresholds for heavy quark pair production is also studied for the relevant structure functionsF _i (x,Q ²) and shown to contribute to the measured scaling violations. All these effects are sensitive to the heavy quark massesand to the shape of the gluon distribution which can thus be tested experimentally by analyzing heavy quark pair signals.