Spinning the Top

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.

15 pages. V2: several minor improvements, new benchmark, comments and references added

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Cited by in corpus (17)

• Top quark and leptonic charge asymmetries for the Tevatron and LHC
• Asymmetries in top quark pair production at hadron colliders
• Zero and one jet combined NLO analysis of the top quark forward-backward asymmetry
• Single Vectorlike Quark Production at the LHC
• Discerning New Physics in Top-Antitop Production using Top Spin Observables at Hadron Colliders
• Top polarisation studies in $H^{-}t$ and $Wt$ production
• Enhanced $thj$ signal at the LHC with $h\to \gamma \gamma$ decay and $\mathcal{C}\mathcal{P}$-violating top-Higgs coupling
• Data driving the top quark forward--backward asymmetry with a lepton-based handle
• Light axigluon explanation of the Tevatron ttbar asymmetry and multijet signals at the LHC
• Transverse Top Quark Polarization and the ttbar Forward-Backward Asymmetry
• Better Hadronic Top Quark Polarimetry
• Renormalization group-induced phenomena of top pairs from four-quark effective operators
• Polarization of top produced in particle decays in an arbitrary frame
• Interpretations and Implications of the Top Quark Rapidity Asymmetries $A_{FB}^t$ and $A_{FB}^{\ell }$
• Quantum coherence, top transverse polarisation and the Tevatron asymmetry $A_{FB}^{\ell }$
• Multiple Z' -> t-tbar signals in a 4D Composite Higgs Model
• Top-quark Polarization and Asymmetries at the LHC in the Effective Description of Squark Interactions