Measurement of the $W^{+}W^{-}$ diboson production cross-section with the ATLAS Experiment at the Large Hadron Collider

This thesis presents the first measurement of the $\ww$ production cross section in proton-proton collisions at $\sqrt{s} = 7 \TeV$ using the complete data set of $4.7 \ifb$ collected by the ATLAS detector at the LHC in the year 2011. Three di-leptonic decay channels of the $\ww$ pairs are used for this analysis: decays into two electrons, two muons, and decays into one electron and one muon. For each channel, a region with a high fraction of $\ww$ events is selected and contributions from background processes are estimated using Monte Carlo simulations and data-driven techniques. In total, 1524 $\ww$ candidate events are selected, with an estimated background of \mbox{$545 \pm 52$ events}. The result of the measurement is: $$\sigma (pp \rightarrow \ww) = 52.6 \pm 2.1 \text{(stat)} \pm 4.5 \text{(syst)} \pm 2.1 \text{(lumi)} \pb$$ This is compatible with the Standard Model cross-section prediction at next-to-leading order of $\sigma (pp \rightarrow \ww) = 47.0^{+2.0}_{-1.5}$ pb.