Three-Loop O(alpha_s² G_F M_t²) Corrections to Higgs Production and Decay at e^+e^- Colliders

We evaluate the next-to-leading-order QCD corrections of O(alpha_s^2 G_F M_t^2) to the Standard-Model l^+l^-H, ZZH, and W^+W^-H couplings in the heavy-top- quark limit. Exploiting these results together with knowledge of Delta rho to the same order, we analyze a variety of production and decay processes of low- mass Higgs bosons at e^+e^- colliders. Specifically, we consider H->l^+l^-, H->l^+l^-l'^+l'^-, e^+e^-->ZH, Z->ff-barH, and e^+e^-->ff-barH, with f=l,nu. We work in the electroweak on-shell scheme formulated with G_F and employ both the on-shell and MS-bar definitions of the top-quark mass in QCD. As expected, the scheme and scale dependences are greatly reduced when the next-to-leading- order corrections are taken into account. In the on-shell scheme of top-quark mass renormalization, the O(alpha_s^2 G_F M_t^2) corrections act in the same direction as the O(alpha_s G_F M_t^2) ones and further increase the screening of the O(G_F M_t^2) terms. The coefficients of (alpha_s/pi)^2 range from -6.847 for the ZZH coupling to -16.201 for the l^+l^-H coupling. This is in line with the value -14.594 recently found for Delta rho.