Physics at tt̄ threshold in e+e- collisions

We present the results from quantitative studies of physics at tt̄ threshold, taking into account realistic experimental conditions expected at future linear e+e- colliders. A possible experimental strategy is illustrated for a sample case of mt=150 GeV, where the importance of the measurements of both total and differential cross sections is emphasized for precision determinations of various parameters. The studies are based on a recently developed theoretical formalism which includes full O(αs) corrections. An energy scan of 11 points with 1 fb-1 each allows us to measure the top mass and the strong coupling constant with statistical errors of Δmt=0.2 GeV and Δαs=0.005, respectively. As for the top width, ΔΓt/ Γt=0.2(stat) is expected, if both mt and αs are known. The measurement of the top momentum at some optimized energy point with 100 fb-1 reduces the error on αs to Δαs=0.0015, provided that the 1S peak position is known from the threshold scan. The momentum measurement also improves the precision on the top width to ΔΓt/Γt=0.04, if αs is known from other sources. The forward-backward asymmetry in the threshold region provides another interesting method to measure αs and Γt.