Abstract
The angular distributions of Drell-Yan charged lepton pairs around 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 $\sqrt{s} = 8$ TeV, collected by the ATLAS detector at the CERN LHC. The measurements are compared to the most precise fixed-order calculations currently available ($\mathcal{O}(\alpha^{2}_{s})$) 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 $\mathcal{O}(\alpha^{2}_{s})$ predictions is observed for $A_{0}-A_{2}$. Evidence is found for non-zero $A_{5,6,7}$, consistent with expectations.
Highlights
The 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
The high granularity and precision of the specific measurements presented in this paper provide a stringent test of the most precise perturbative quantum chromodynamics (QCD) predictions for Z-boson production in pp collisions and of Monte Carlo (MC) event generators used to simulate Z-boson production
PowhegBox (v1/r2129) sample [14,15,16,17], which uses the CT10 next-to-leading order (NLO) set of parton distribution functions (PDFs) [33], is interfaced to Pythia 8 (v.8.170) [34] with the AU2 set of tuned parameters [35] to simulate the parton shower, hadronisation and underlying event, and to Photos (v2.154) [36] to simulate quantum electrodynamics (QED) final-state radiation (FSR) in the Z-boson decay
Summary
The 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. The measurement is performed in the Z-boson invariant mass window of 80–100 GeV, as a function of pZT, and in three bins of yZ These results are based on 20.3 fb−1 of pp collision data. |η| > 2.5 (referred to hereafter as the central-forward or eeCF channel) is used to extend the measurement to the high-yZ region encompassed by the third yZ bin In this case, because of the fewer events available for the measurement itself and to evaluate the backgrounds (see section 4), the measurement is only performed for pZT up to 100 GeV using projections of cos θ and φ, making A1 and A6 inaccessible in the 2 < |yZ| < 3.5 bin.
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