Measuring the Top Yukawa coupling to a heavy Higgs boson at future e+e- Linear Colliders

The determination of the Yukawa coupling of the top quark to the Higgs boson is one of the most important measurements that a future e^+e^- linear collider could provide. For a Higgs boson of mass greater than 350 GeV, this coupling can be determined using the Higgs resonant contribution to t \bar t production from W^+ W^- fusion at TeV energies. We have made a careful evaluation of the significance with which the signal of a Higgs decaying to t \bar t pairs could be observed at future e^+ e^- linear colliders, with center of mass energies close to 1 TeV and an integrated luminosity of 1 ab^{-1}. We find that a signal significance greater than 5 \sigma and a relative error in the top Yukawa measurement better than 10% can be achieved, for Higgs masses in the 350-500 GeV and 350-650 GeV ranges at facilities with 800 GeV and 1 TeV energies respectively.