Measurement of the WW production cross-section in Proton-Proton Collisions at √s = 8 TeV with the ATLAS Detector

Abstract : The Standard Model (SM), as the fundamental theory for particle physics, provides an description for fundamental interactions: the eletro-magnetic, weak and strong forces. So far it has been tested by various world-wide experiments. There are questions yet to be answered for the completeness of this theory. We've built up high energy physics experiments to create the particle interactions up to high energy scales. The data from those experiments will offer us chance for testing our understanding of the SM and searching for physics beyong the SM. At the European Organization for Nuclear Research (CERN), the physicists and engineers from all over the world are devoted to discover the fundamental element in the universe. The world's largest and most sophisticated experimental instruments are built up, which accelerate particles to the energy of 3.5-4 \TeV and let them collide at the center of detectors. In this way the physicists are able to find tracks about the way particles interact, and further inspired towards the laws of the nature. A Toroidal LHC ApparatuS (ATLAS), as one of the four main detectors of Large Hadron Collider (LHC) at CERN, aims at a wide range of physics, including the precision measurement of the SM processes, the search for the Higgs bosons, extra dimensions, dark matter and so on. It has the similar purpose as the CMS experiment at the LHC. In ATLAS, di-boson production is one of the most important electro-weak processes. Among the massive vector boson pair production processes, ${W^+W^-}$ has larger cross section compared to ${WZ}$ and ${ZZ}$ production. The electro-weak sector of the SM, as well as the strong interactions, can be tested through precision measurements of the ${W^+W^-}$ production cross section. A measurement of $W^+W^-$ production cross section is performed with a data sample collected in proton-proton collisions at center-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 20.3 fb$^{-1}.$ The $W^{+}W^{-}$ events are selected with 3 final states: $ee$, $e\mu$, and $\mu\mu$. In order to suppress the background contamination, mainly from the Drell-Yan and $t\bar{t}$ processes, we cut on missing transverse energy and select events with 0 selected jet. The major backgrounds, mainly including the $W$+jets, top, $Z$+jets, are estimated by data-driven technique. These background estimations are cross-checked by independent methods. The measured cross section is \WZtotXsec$^{+\WZtotXsecStatErrUp}_{\WZtotXsecStatErrDw}$(stat)$^{+\WZtotXsecSysErrUp}_{\WZtotXsecSysErrDw}$(syst)$^{+\WZtotXsecLumiErrUp}_{\WZtotXsecLumiErrDw}$(lumi) pb, which is consistent with SM Next-Leading Order prediction of $63.2^{+2.0}_{-1.8}$ pb. The normalized differential $W^+W^-$ cross section $\frac{1}{\sigma}\frac{d\sigma}{dX}$ is determined as a function of 6 kinematic variables. The unfolding distribution for these variables are given. The possible deviation from the SM can be parameterized with operators of higher order dimensions. The operators of lowest dimension introduce anomalous triple gauge couplings (ATGC). The distribution of leading lepton \pt is used to constrain the anomalous coupling constants. The measurement of the coupling constants provides an indirect search for new physics at mass scales not accessible by the LHC. In our study, no evidence for anomalous $WWZ$ and $WW\gamma$ triple gauge-boson couplings is found, and limits on their magnitudes are set.
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J. Gao. Measurement of the WW production cross-section in Proton-Proton Collisions at √s = 8 TeV with the ATLAS Detector. High Energy Physics - Experiment [hep-ex]. Aix Marseille Université, 2015. English. ⟨tel-01233634⟩

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