Spinning the top quark

We propose to measure the threshold lepton asymmetry, that is the forward-backward asymmetry of the charged lepton in tt̄ 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 nonzero 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 tt̄ asymmetry have chiral couplings to light quarks. Models with identical tt̄ asymmetries at the Tevatron can be distinguished by their threshold lepton asymmetries, which range between plus and minus 25% in realistic models.