Effects of nonstandard interactions for the energy spectrum of secondary leptons in e+e-→t(t)over-bar

The process of top-quark pair production followed by semileptonic decays at future high-energy e(+)e(-) linear colliders is investigated as a possible test of physics beyond the Standard Model. Assuming the most general nonstandard forms for gamma t(t)over-bar; Zt(t)over-bar and Wtb couplings, the energy spectrum of the single lepton l(+/-) and the energy correlation of l(+) and l(-) emerging from the process e(+)e(-) --> t(t)over-bar --> l(+/-) ... l(+)l(-) ... are calculated. Expected precision of the nonstandard-parameter determination is estimated adopting the recently-proposed optimal method.