Abstract
Spin correlations in the production of top-antitop quark (t{bar{t}}) pairs at the Large Hadron Collider (LHC) are an experimentally verified prediction of the Standard Model. In this paper, we compute the full spin density matrix for t{bar{t}}Z production at next-to-leading order precision in QCD, for center-of-mass energies of 13 and 14 text {TeV}. We find that the additional emission of a Z boson leads to significantly different spin correlations with respect to the t{bar{t}} case, and induces small longitudinal polarisations of the top quarks. We further propose an analysis strategy that could lead to the observation of spin correlations in t{bar{t}}Z events at the end of Run 3 of the LHC, or possibly earlier by combining the ATLAS and CMS datasets. In addition, we show that the pure angular information contained in the spin density matrix provides novel constraints on the dimension-6 effective field theory operators relevant to the t-Z interaction, without any reference to the total production rates.
Highlights
We have for the first time studied the phenomenology of spin correlations in the ttZ process at the Large Hadron Collider (LHC)
We have produced a complete set of predictions for the ttZ spin density matrix at NLO precision in QCD, for center-of-mass energies of 13 and 14 TeV. We have found these results to be significantly different from those previously obtained for the ttprocess; as such, a measurement by the ATLAS and CMS experiments of spin and polarisation observables in ttZ events would be a further test of the important connections between the top quark and the electroweak sectors of the Standard Model, in one of the heaviest processes that can be produced at the LHC
By performing a template fit to the coefficients of the ttZ spin density matrix, readily extracted from a set of distinct angular observables, it is estimated that the existing LHC Run 2 dataset could be enough to claim evidence of spin correlation in ttZ events; combining these results with our predictions for Run 3, an observation could be conclusively made
Summary
Following the formalism of Ref. [20], we recall that hadronic ttproduction in the narrow-width approximation depends on a spin density matrix R, decoupled from the decay of the top quarks (which is hereafter assumed to proceed entirely via t → W b). While the function A encodes the ttcross section and parton kinematics, the 3-vectors B± represent the top polarisations and the 3×3 matrix C the correlation between the spins of the top and anti-top quarks. To define these three axes, we adopt the basis of Ref. The extra factor of sign(y) in the definitions of rand nbreaks the Bose symmetry of the gg initial state by identifying a forward direction and ensures non-zero values of the relevant spin density matrix elements. The spin correlation matrix C can be expanded in terms of the basis (2) as. Crk and the three diagonal correlations (crr , ckk and cnn) are all C-, CP- and CPT-even, and significantly non-zero at LO in QCD1
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