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 ttbZ couplings. We also consider the upcoming high-energy LHC run and find that with 300 inverse fb of data at an energy of 13 TeV the vector and axial ttbZ couplings can be constrained at the 95% confidence level to C_V=0.24^{+0.39}_{-0.85} and C_A=-0.60^{+0.14}_{-0.18}, 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 ttbZ interactions beyond the Standard Model.
Highlights
JHEP07(2014)091 which distinguishes it from other indirect probes such as the LEP measurements of the ρ-parameter [7] and the Z → bb branching ratio [8]
In this article we studied top quark pair production in association with a Z boson
We study the process pp → ttZ in the tri-lepton final state, which provides the best compromise between clean signature and large enough cross section
Summary
We consider the tri-lepton signature pp → tt+Z → t(→ νb) t(→ jjb) Z(→ ) which profits from a large cross section due to the hadronic decay of one W boson and the lepton multiplicities from the remaining W and Z bosons. Application of the narrow-width approximation for top quarks and the Z boson allows us to separate production and decays stage according to dσpp→ νbbjj = dσpp→tt+Z dBt→b ν dBt→ ̄bjj dBZ→ + O(Γt/mt, ΓZ /MZ ), (2.1). The use of the narrow width approximation neglects contributions which are parametrically suppressed by O(Γ/m), arising from a largely off-shell top quark or Z boson. In our analysis we aim for a large cross section and only place mild cuts required by experimental detector acceptance. We believe the narrow-width is an excellent approximation for our study. We neglect the contribution from the decay t → W b + Z since the available phase space for on-shell top quarks is tiny and Bt→W bZ ≈ 3 × 10−6 [33,34,35,36]
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