The production of W boson pairs in proton-proton collisions at √ s = 8 TeV is studied using data corresponding to 20.3 fb −1 of integrated luminosity collected by the ATLAS detector during 2012 at the CERN Large Hadron Collider. The W bosons are reconstructed using their leptonic decays into electrons or muons and neutrinos. Events with reconstructed jets are not included in the candidate event sample. A total of 6636 W W candidate events are observed. Measurements are performed in fiducial regions closely approximating the detector acceptance. The integrated measurement is corrected for all acceptance effects and for the W branching fractions to leptons in order to obtain the total W W production cross section, which is found to be 71.1 ± 1.1(stat) 6 Determination of backgrounds 10 6.1 Background from top-quark production 10 6.2 Background from W +jets production 13 6.3 Other diboson processes and validation of diboson and W +jets backgrounds 15 6.4 Background from Drell-Yan production 15 6.5 Other background contributions 17 6.6 W W candidate events and estimated background yields 17 +5 JHEP09(2016)029A.1 Differential cross section measurements 45 A.2 Normalised differential cross sections 48 A.3 Bin-to-bin correlation matrices for the differential measurements 51 A.4 Bin-to-bin correlation matrices for the normalised differential measurements 54The ATLAS collaboration 63 IntroductionThe measurement of the production of pairs of electroweak gauge bosons plays a central role in tests of the Standard Model (SM) and in searches for new physics at the TeV scale [1]. The W W production cross section would grow arbitrarily large as a function of the centre-of-mass energy of the production process, √ŝ , were it not for the cancellations of s-and t-channel W + W − (henceforth denoted W W ) processes. New physics phenomena can occur as deviations from the gauge structure of the Standard Model in the triple-gauge-boson couplings ZW W or γW W [2], termed anomalous triple-gauge-boson couplings (aTGCs). As the cross section for W W production is one of the largest among those involving a triple-gauge-boson vertex, it allows tests of the self-interaction of the gauge bosons to be made with high precision through measurements of differential kinematic distributions. Studies of the W W production process are particularly important as it constitutes a large irreducible background to searches for physics beyond the SM as well as to resonant H → W + W − production.A precise measurement of W W production also tests the validity of the theoretical calculations. Perturbative quantum chromodynamics (pQCD) is the essential ingredient in all these calculations and a recent calculation of non-resonant W W production has been performed up to next-to-next-to-leading order (NNLO) [3]. However, fixed-order calculations may fail to describe effects that arise from restrictions imposed on the phase space of the measurement. In this analysis, it is required that there be no jets above a certain transverse momentum threshold, which i...