RECENT ADVANCES IN NRQCD

Abstract

We discuss recent theoretical developments concerning the description of the production and decay of heavy quarks and colored scalars in the framework of nonrelativistic QCD. 1. Threshold Physics at the ILC The e + e center-of-mass (c.m.) energy at a linear collider (LC) can be very precisely monitored, allowing for an accurate exploration of the threshold regime. The top-quark mass can be determined from a measurement of σ(e + e → Z � , γ � → t¯ t) line shape at a LC operating at c.m. energies around the tthreshold ( p q 2 ∼ 350 GeV). The rise of the cross section with increasing c.m. energy is directly related to the mass of the top quark. Assuming a total integrated luminosity of 300 fb 1 , LC simulations of a threshold scan of the top-antitop total cross section have demonstrated that experimental uncertainties below 100 MeV for the top-quark mass determination can be obtained 1,2 , even when beam effects, which lead to some smearing of the effective c.m. energy, are taken into account. If the normalization of the cross section line shape is well under control, it is possible to determine the strong coupling, the total top quark width and, if the Higgs boson is light, the top Yukawa coupling. In view of the accuracy obtainable at the LC the theoretical uncertainties for the total cross section should be lowered to a level of a few percent 3,4 . A precise knowledge of the top mass would also improve the analysis of electroweak precision observables and put indirect constraints on New Physics. It has � Talk given at 7th Workshop on Continuous Advances in QCD, Minneapolis, Minnesota,