Physics impact of a precise determination of the top quark mass at an e+e- linear collider

At a prospective e + e - Linear Collider (LC) a very precise determination of the top quark mass with an accuracy of δm t ≤100 meV will be possible. This is to be compared with the envisaged accuracy of δm t = 1-2 GeV at the Tevatron and the LHC. We discuss the physics impact of such a precise determination of m t , focusing on the Standard Model (SM) and its minimal supersymmetric extension (MSSM). In particular, we show the importance of a precise knowledge of m t for electroweak precision observables, and for Higgs physics and the scalar top sector of the MSSM. Taking the mSUGRA model as a specific example, we furthermore demonstrate the importance of a precise m t value for the prediction of sparticle masses and for constraints on the parameter space allowed by the relic density. The uncertainty in m t also significantly affects the reconstruction of the supersymmetric high scale theory. We find that going from hadron collider to LC accuracy in m t leads to an improvement of the investigated quantities by up to an order of magnitude. (author)