Top-quark physics in six-quark final states at the Next Linear Collider

The processes of six-quark production with one bb̄ pair are studied by means of a complete tree-level electroweak calculation. The top-quark signal is examined: the importance of electroweak backgrounds, of the order of 10% above the tt̄ threshold and of about 30% of the purely electroweak signal at threshold, is further stressed by studying the dependence of the cross-section at threshold on the Higgs mass in the range between 100 GeV and 185 GeV, and finding variations of the order of 10%. In the study of some event-shape variables, a strong effect of initial-state radiation is found, in particular for the thrust distribution, which is studied for several centre-of-mass energies at the TeV scale. The effectiveness of cuts on the thrust for isolating QCD backgrounds, as pointed out by some authors, is confirmed also in the presence of electroweak backgrounds and initial-state radiation.