Physics at tt-bar threshold in e+e- collisions.

Abstract

We present the results from quantitative studies of physics at tt\ifmmode\bar\else\textasciimacron\fi{} threshold, taking into account realistic experimental conditions expected at future linear ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ colliders. A possible experimental strategy is illustrated for a sample case of ${\mathit{m}}_{\mathit{t}}$=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(${\mathrm{\ensuremath{\alpha}}}_{\mathit{s}}$) corrections. An energy scan of 11 points with 1 ${\mathrm{fb}}^{\mathrm{\ensuremath{-}}1}$ each allows us to measure the top mass and the strong coupling constant with statistical errors of \ensuremath{\Delta}${\mathit{m}}_{\mathit{t}}$=0.2 GeV and \ensuremath{\Delta}${\mathrm{\ensuremath{\alpha}}}_{\mathit{s}}$=0.005, respectively. As for the top width, \ensuremath{\Delta}${\mathrm{\ensuremath{\Gamma}}}_{\mathit{t}}$/${\mathrm{\ensuremath{\Gamma}}}_{\mathit{t}}$=0.2(stat) is expected, if both ${\mathit{m}}_{\mathit{t}}$ and ${\mathrm{\ensuremath{\alpha}}}_{\mathit{s}}$ are known. The measurement of the top momentum at some optimized energy point with 100 ${\mathrm{fb}}^{\mathrm{\ensuremath{-}}1}$ reduces the error on ${\mathrm{\ensuremath{\alpha}}}_{\mathit{s}}$ to \ensuremath{\Delta}${\mathrm{\ensuremath{\alpha}}}_{\mathit{s}}$=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 \ensuremath{\Delta}${\mathrm{\ensuremath{\Gamma}}}_{\mathit{t}}$/${\mathrm{\ensuremath{\Gamma}}}_{\mathit{t}}$=0.04, if ${\mathrm{\ensuremath{\alpha}}}_{\mathit{s}}$ is known from other sources. The forward-backward asymmetry in the threshold region provides another interesting method to measure ${\mathrm{\ensuremath{\alpha}}}_{\mathit{s}}$ and ${\mathrm{\ensuremath{\Gamma}}}_{\mathit{t}}$.