PHYSICS AT T(T)OVER-BAR THRESHOLD IN E+E- COLLISIONS

We present the results from quantitative studies of physics at ttBAR 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 m(t) = 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(alpha(s)) 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 DELTAm(t)=0.2 GeV and DELTAalpha(s)=0.005, respectively. As for the top width, DELTGAMMA(t)/GAMMA(t)=0.2 (stat) is expected, if both m(t) and alpha(s) are known. The measurement of the top momentum at some optimized energy point with 100 fb-1 reduces the error on alpha(s) to DELTAalpha(s)=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 DELTAGAMMA(t)/GAMMA(t)=0.04, if alpha(s) is known from other sources. The forward-backward asymmetry in the threshold region provides another interesting method to measure alpha(s) and GAMMA(t).