Low–Energy Theorems in Higgs Physics

We present low–energy theorems for the calculation of loop amplitudes with external scalar or pseudoscalar Higgs bosons which are light compared to the loop particles. Starting from existing lowest–order versions of these theorems, we show how their applicability may be extended to the two–loop level. To illustrate the usefulness of these theorems, we discuss a number of applications to Higgs production and decay at and beyond the one–loop order. Permanent address: Max–Planck–Institut für Physik, Werner–Heisenberg–Institut, Föhringer Ring 6, 80805 Munich, Germany. Supported by Bundesministerium für Bildung und Forschung (BMBF), Bonn, Germany, under Contract 05 6 HH 93P (5) and by EU Program Human Capital and Mobility through Network Physics at High Energy Colliders under Contract CHRX–CT93–0357 (DG12 COMA).