Abstract
This paper describes a measurement of the $Z/\gamma^*$ boson transverse momentum spectrum using ATLAS proton-proton collision data at a centre-of-mass energy of $\sqrt{s}$ = 7 TeV at the LHC. The measurement is performed in the $Z/\gamma^* \rightarrow e^+e^-$ and $Z/\gamma^* \rightarrow \mu^+\mu^-$ channels, using data corresponding to an integrated luminosity of 4.7 fb$^{-1}$. Normalized differential cross sections as a function of the $Z/\gamma^*$ boson transverse momentum are measured for transverse momenta up to 800 GeV. The measurement is performed inclusively for $Z/\gamma^*$ rapidities up to 2.4, as well as in three rapidity bins. The channel results are combined, compared to perturbative and resummed QCD calculations and used to constrain the parton shower parameters of Monte Carlo generators.
Highlights
Background processes includeW ± → ±ν, Z → τ +τ − and bb, cc → ± + X and are generated with Pythia6-AUET2B
The total uncertainty is between 0.5% and 1% for pZT < 30 GeV, below 1.5% per bin up to pZT = 150 GeV and rises to 7% at the end of the spectrum
In order to compensate for the changes in energy and particle flow induced by the modifications in the parton shower and primordial kT parameters, the pT threshold for the QCD 2 → 2 scattering processes used in multiple parton interactions (MPI) is changed from 2.085 GeV to 2.18 GeV for Pythia8, following the underlying event data measured in Drell-Yan events by ATLAS [54]
Summary
The measurements are compared to a representative set of theoretical predictions They rely on perturbative QCD (pQCD) only, or include resummation of soft-gluon emissions. The ResBos calculation relies on soft-gluon resummation at low pZT and matches the O(αS2) cross section at high pZT. It simulates the vector boson decays but does not include a description of the hadronic activity in the event. The Pythia [2] and Herwig [3] generators use the parton shower approach to describe the low-pZT region and include an O(αS) matrix element for the emission of one hard parton.
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