Physics Impact of GigaZ

By running the high-energy e^+ e^- collider TESLA in the GigaZ mode on the Z resonance, experiments can be performed on the basis of more than 10^9 Z events. They will allow the measurement of the effective electroweak mixing angle to an accuracy of approximately +- 10^-5. Likewise the W boson mass is expected to be measurable with an error of about 6 MeV near the W^+ W^- threshold. In this note, we study the accuracy with which the Higgs boson mass can be determined from loop corrections to these observables in the Standard Model. The comparison with a directly observed Higgs boson may be used to constrain new physics scales affecting the virtual loops. We also study constraints on the heavy Higgs particles predicted in the Minimal Supersymmetric Standard Model, which are very difficult to observe directly for large masses. Similarly, it is possible to constrain the mass of the heavy scalar top particle.