Measurement of the angular coefficients in Z-boson events using electron and muon pairs from data taken at s = 8 $$ \sqrt{s}=8 $$ TeV with the ATLAS detector

Abstract

Measurement of the angular coefficients in Z-boson events using electron and muon pairs from data taken at √ s = 8 TeV with the ATLAS detectorThe ATLAS collaboration E-mail: atlas.publications@cern.ch Abstract:The angular distributions of Drell-Yan charged lepton pairs in the vicinity of the Z-boson mass peak probe the underlying QCD dynamics of Z-boson production. This paper presents a measurement of the complete set of angular coefficients A 0−7 describing these distributions in the Z-boson Collins-Soper frame. The data analysed correspond to 20.3 fb −1 of pp collisions at √ s = 8 TeV, collected by the ATLAS detector at the CERN LHC. The measurements are compared to the most precise fixed-order calculations currently available (O(α 2 s )) and with theoretical predictions embedded in Monte Carlo generators. The measurements are precise enough to probe QCD corrections beyond the formal accuracy of these calculations and to provide discrimination between different parton-shower models. A significant deviation from the O(α 2 s ) predictions is observed for A 0 − A 2 . Evidence is found for non-zero A 5,6,7 , consistent with expectations. Keywords: Hadron-Hadron scattering (experiments)ArXiv ePrint: 1606.00689Open Access, Copyright CERN, for the benefit of the ATLAS Collaboration. Article funded by SCOAP 3 .doi:10.1007/JHEP08(2016)159 JHEP08(2016)159 JHEP08(2016)159E Quantifying A 5,6,7 58 F Additional results 61The ATLAS collaboration 83 IntroductionThe angular distributions of charged lepton pairs produced in hadron-hadron collisions via the Drell-Yan neutral current process provide a portal to precise measurements of the production dynamics through spin correlation effects between the initial-state partons and the final-state leptons mediated by a spin-1 intermediate state, predominantly the Z boson.In the Z-boson rest frame, a plane spanned by the directions of the incoming protons can be defined, e.g. using the Collins-Soper (CS) reference frame [1]. The lepton polar and azimuthal angular variables, denoted by cos θ and φ in the following formalism, are defined in this reference frame. The spin correlations are described by a set of nine helicity density matrix elements, which can be calculated within the context of the parton model using perturbative quantum chromodynamics (QCD). The theoretical formalism is elaborated in refs. [2][3][4][5].The full five-dimensional differential cross-section describing the kinematics of the two Born-level leptons from the Z-boson decay can be decomposed as a sum of nine harmonic polynomials, which depend on cos θ and φ, multiplied by corresponding helicity cross-sections that depend on the Z-boson transverse momentum (p Z T ), rapidity (y Z ), and invariant mass (m Z ). It is a standard convention to factorise out the unpolarised crosssection, denoted in the literature by σ U +L , and to present the five-dimensional differential cross-section as an expansion into nine harmonic polynomials P i (cos θ, φ) and dimensionless angular coefficients A 0−7 (p Z T , y Z , m Z ), whic...

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