  A measurement of angular correlations in Drell-Yan lepton pairs via the
phistar observable is presented. This variable probes the same physics as the
Z/gamma* boson transverse momentum with a better experimental resolution. The
Z/gamma*->e+e- and Z/gamma*->mu+mu- decays produced in proton--proton
collisions at a centre-of-mass energy of sqrt(s) = 7 TeV are used. The data
were collected with the ATLAS detector at the LHC and correspond to an
integrated luminosity of 4.6 fb-1. Normalised differential cross sections as a
function of phistar are measured separately for electron and muon decay
channels. These channels are then combined for improved accuracy. The cross
section is also measured double differentially as a function of phistar for
three independent bins of the Z boson rapidity. The results are compared to QCD
calculations and to predictions from different Monte Carlo event generators.
The data are reasonably well described, in all measured Z boson rapidity
regions, by resummed QCD predictions combined with fixed-order perturbative QCD
calculations. Some of the Monte Carlo event generators are also able to
describe the data. The measurement precision is typically better by one order
of magnitude than present theoretical uncertainties.
