Spinning the Top

TL;DR

This paper introduces a method to measure the threshold lepton asymmetry in top quark pair production, which can reveal the chirality of initial quarks and distinguish new physics models affecting the ttbar asymmetry.

Contribution

It proposes using the threshold lepton asymmetry as a novel observable to probe the chirality of quark couplings in top pair production near threshold.

Findings

Threshold lepton asymmetry vanishes in tree-level QCD.

New physics can induce a non-zero asymmetry up to ±25%.

Different models with similar ttbar asymmetries can be distinguished by this measurement.

Abstract

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.