  We propose to measure the threshold lepton asymmetry, that is the
forward-backward asymmetry of the charged lepton in ttbar events near the
production threshold. At threshold top quark pairs are produced in an s-wave.
Angular momentum conservation then implies that the top spins equal the spin of
the initial state which - in the case of quarks - is uniquely fixed by the
chirality of the initial quarks. Thus measuring final state top spins
determines the chirality of the quarks which produced them. Information about
the top spins can be extracted by measuring the angular distribution of the
charged lepton in semileptonic or dileptonic decays of the top pair. One such
distribution, the threshold lepton asymmetry, vanishes in tree-level QCD but is
non-zero if new physics modifies the relative contribution of right-handed and
left-handed quarks to top pair production. This is interesting because
realistic models addressing the anomalous ttbar asymmetry have chiral couplings
to light quarks. Models with identical ttbar asymmetries at the Tevatron can be
distinguished by their threshold lepton asymmetries which range between plus
and minus 25% in realistic models.
