Measuring the Top Yukawa Coupling to a Heavy Higgs Boson at Future e⁺e^- Linear Colliders

If the Higgs boson mass is greater than 350 GeV, the top quark Yukawa coupling can be determined using the Higgs resonant contribution to tt̅ production from W⁺W^- fusion at high energy e⁺e^- linear colliders. We have evaluated the significance with which the signal of a Higgs decaying to tt̅ pairs could be observed at future e⁺e^- colliders, with center of mass energies of 800 GeV and 1 TeV, and an integrated luminosity of 1 ab^-1. We find that a signal significance greater than 5σ and a relative error in the top Yukawa measurement better than 10% can be achieved at these facilities, for Higgs boson masses in the ranges of 350–500 GeV and 350–650 GeV, respectively.