ar X iv : h ep - p h / 02 01 18 0 v 1 1 9 Ja n 20 02 Threshold Top Quark Production

Predictions for the total cross section for e+e− → tt̄ near threshold are reviewed. The renormalization group improved results at NNLL order have improved convergence and reduced scale dependence relative to fixed order results at NNLO. Prospects for measurements of the topquark mass, width, and Yukawa coupling are discussed. INTRODUCTION The production of top quark pairs is among the important projects of a future linear collider. The large top quark width Γt ∼ 1.5 GeV makes the threshold cross section look quite different from that of charm or bottom pairs. The large value of the width prohibits the production of toponium states, and at the same time serves as an infrared cutoff from sensitivity to non-perturbative effects. Thus, perturbative methods can be used to describe the top-antitop dynamics to a very high degree of precision. This makes the threshold region an ideal place for extracting fundamental top quark parameters such as the top mass, width, and Yukawa coupling (for a light higgs). In the threshold region √ s ≃ 2mt ± 10GeV, the t and t̄ move with non-relativistic velocities. Defining the energy mtv = √ s−2mt we see that this region of s corresponds to velocities |v| < ∼ αs. In this region an exact treatment of QCD Coulomb singularities (αs/v) is required, ruling out a pure αs expansion. However, a combined expansion in powers of v and αs can be performed. Schematically, for the e+e− → tt̄ cross section, σtt̄(s), we want an expansion of the form R = σtt̄ σμ+μ− = v ∑ k (αs v k × [ 1 + {αs,v} + {αs ,αsv,v}+ . . . ] . (1) LO NLO NNLO The power counting for these corrections can be implemented in a simple and systematic way using the effective theory framework of Non-Relativistic QCD (NRQCD). The leading order (LO) prediction for R is shown in Fig. 1. The large top width gives the threshold cross section a smooth line-shape, with a single bump from the remnant of the 1S toponium state. The characteristics of the cross section are sensitive to the top parameters. In particular, the top mass determines the location of the rise/peak, 1 Plenary talk at the 9th International Symposium on Heavy Flavor Physics, Sept. 2001 340 345 350 355 !!! s HGeVL 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6