Measurement of $W^{+}W^{-}$ production in Proton-Proton Collisions at $\sqrt{s}=7$ TeV with the ATLAS Detector at the LHC

The ATLAS detector is a general purpose detector at the Large Hadron Collider (LHC) aimed at the discovery of new physics phenomena as well as furthering our understanding of the high energy behavior of the Standard Model (SM), the well established theoretical framework which describes the elementary particles and their interactions except gravity. The LHC is the world's largest hadron collider designed to provide head on proton proton (pp) collisions at 14 TeV center-of-mass (c.m.) energy and $10^{34} cm^{-1}s^{-1}$ peak luminosity. The LHC also runs in heavy ion mode with lead nuclei collisions at an energy of 574 TeV per nucleus (2.76 TeV per nucleon-pair) at the luminosity of 10$^{27}$ cm$^{-2}$s$^{-1}$. Both LHC and ATLAS are performing excitingly well since 2009. In 2011, the ATLAS experiment collected a 4.7 ${fb}^{-1}$ $pp$ collisions data at 7 TeV and is expecting to record another 25 ${fb}^{-1}$ of $pp$ collisions at 8 TeV by the end of 2012. This thesis presents a measurement of the SM WW production cross section and the determination of the corresponding limits on anomalous triple gauge boson couplings (aTGCs), using the 2011 4.7 ${fb}^{-1}$ $pp$ collisions data at 7 TeV collected in 2011. The measurement allows for a stringent test of the non-Abelian $SU(2) x U(1)$ SM electroweak sector and probes new physics that could manifest itself through aTGCs that may alter the observed production cross section or kinematic distributions. This measurement also provides a good understanding of the irreducible background in searches for the Higgs boson through the $H \rightarrow W^{+}W^{-}$ decay channel. The measurement of the WW production is based on the analysis of the purely leptonic decay channels with the final states of $e u e u$, $e u mu u$ and $mu u mu u$. The main background processes to the WW signal are $Z$+jets, $W$+jets, top quarks, and other diboson production, such as $WZ$, $ZZ$ and $W/Z+\gamma$. The $Z$+jets, $W$+jets and top background processes are estimated using dedicated data-driven techniques while the other diboson background is estimated from Monte-Carlo (MC) simulation. A total of 1325 signal candidates are selected from data with an overall estimated background of $368.5 \pm 60.9$. The corresponding measured total cross section is $51.9 \pm 2.0 (stat) \pm 3.0 (syst) \pm 0.9 (lumi) pb$. This result is compatible with the Standard Model Next-to-Leading Order (NLO) prediction of $44.7^{2.1}_{-1.9} pb$ and has surpassed the precision of results from experiments at the Tevatron accelerator. The fiducial cross section for each channel is also measured and a first differential distribution of the leading lepton transverse momentum spectrum is extracted. Finally, the TGCs of the WWZ and WW$\gamma$ vertices are studied by comparing the observed leading lepton transverse momentum spectrum of the $W^{+}W^{-}$ signal to the theoretical predictions with aTGCs included. For a cutoff scale of $\Lambda = 6TeV$, 95% Confidence Limits are set on $\Delta _Z$ and $\lambda_Z$ in the intervals $[-0.061, 0.093]$ and $[-0.062, 0.065 ]$, respectively for Equal Coupling Scenario. These are more stringent limits than those from experiments at the Tevatron accelerator and are competitive with results from experiments at the LEP accelerator. This thesis work has laid a solid foundation for further measurements of the $W^{+}W^{-}$ production with the $\sim 25 fb^{-1}$ integrated luminosity 8 TeV recorded data expected by the end of 2012, which will further improve the precision and yield more stringent limits on the aTGCs.