Constraining couplings of top quarks to the Z boson in t t ̄ + Z production at the LHC

Abstract

We study top quark pair production in association with a Z boson at the Large Hadron Collider (LHC) and investigate the prospects of measuring the couplings of top quarks to the Z boson. To date these couplings have not been constrained in direct measurements. Such a determination will be possible for the first time at the LHC. Our calculation improves previous coupling studies through the inclusion of next-to-leading order (NLO) QCD corrections in production and decays of all unstable particles. We treat top quarks in the narrow-width approximation and retain all NLO spin correlations. To determine the sensitivity of a coupling measurement we perform a binned log-likelihood ratio test based on normalization and shape information of the angle between the leptons from the Z boson decay. The obtained limits account for statistical uncertainties as well as leading theoretical systematics from residual scale dependence and parton distribution functions. We use current CMS data to place the first direct constraints on the tt ̄ Z couplings. We also consider the upcoming high-energy LHC run and find that with 300 fb-1 of data at an energy of 13 TeV the vector and axial tt ̄ Z coupling can be constrained at the 95% confidence level to CV = 0. 24-0.85+ 0.39 and CA = -0.60-0.18+0.14, where the central values are the Standard Model predictions. This is a reduction of uncertainties by 25% and 42%, respectively, compared to an analysis based on leading-order predictions. We also translate these results into limits on dimension-six operators contributing to the tt ̄ Z interactions beyond the Standard Model. © 2014 The Author(s).