CERN Accelerating science

Thesis
Report number	CERN-THESIS-2015-429
Title	Measurement of WW production cross section in proton-proton collisions at $\sqrt s$ = 8 TeV with the ATLAS detector and limits on anomalous triple-gauge-boson couplings
Author(s)	Chen, Karen (Stony Brook U.)
Publication	122.
Thesis note	PhD : Stony Brook U. : 2015
Thesis supervisor(s)	Hobbs, John
Subject category	Particle Physics - Experiment
Accelerator/Facility, Experiment	CERN LHC ; ATLAS
Abstract	WW production serves as an important test of the electroweak sector in the Standard Model. It can be sensitive to gauge boson self interactions as well as Higgs boson interactions. Deviations from the Standard Model prediction could arise from anomalous triple gauge couplings or the production of new particles that decay into electroweak bosons. Searches for new physics phenomena are conducted at high energy scales, but in order to constrain them at the electroweak scale, we need precision measurements of Standard Model processes. In this Dissertation, the WW production cross section is measured with p—p collisions at $\sqrt{s}$ = 8 TeV with 20.3 fb$^{-1}$ of data collected by the ATLAS detector. We only consider WW production in the fully leptonic decay channels. The experimental signature consists of two oppositely charged leptons (e or $\mu$) with additional $E_{T}^{miss}$. The main background contributions are Z+jets, top, W+jets, and other diboson production (WZ/ZZ/W$\gamma^{(*)}$). Data driven methods are used to estimate each background contribution except for the other diboson backgrounds, which are estimated from Monte Carlo simulations. Experimental and theoretical sources of systematic uncertainties are assessed and propagated to the final results. The measured total cross section is $71.0_{-1.1}^{+1.1}$(stat)$_{-3.1}^{+3.2}$(theory)$_{-3.9}^{+4.8}$(exp)$_{-2.0}^{+2.1}$(lumi) pb. An unfolding method is applied on differential cross section measurements to give kinematic distributions that can be compared directly to theoretical predictions. The differential leading lepton p$_{T}$distribution is used to search for anomalous WWZ and WW${\gamma}$ triple gauge couplings. The data is fitted and all coupling parameters are found to be consistent with the Standard Model values. 95% confidence level interval limits on anomalous coupling are derived and the limits are improved with respect. to the previous 7 TeVWW analysis.

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