Titel: Measurement and Interpretation of Higgs Boson Differential Cross Sections in the Diphoton Decay Channel and Measurement of the Photon Identification Efficiency in the ATLAS Experiment Sonstige Titel: Messung und Interpretation von Differentiellen Higgs-Boson-Wirkungsquerschnitten und Messung der Photon-Identifikationseffizienz im ATLAS-Experiment Sprache: Englisch Autor*in: Braren, Früd Schlagwörter: Higgs boson; Cross section measurement; Diphoton decay channel; Photon identification efficiency; ATLAS; Particle physics Erscheinungsdatum: 2020 Tag der mündlichen Prüfung: 2020-08-07 Zusammenfassung: The properties of the scalar boson that was first found in 2012 by the ATLAS and CMS experiments at the LHC are so far consistent with those of the Higgs boson, predicted by the Standard Model of particle physics. Further investigations of the boson's properties with increased accuracy are of immediate interest, as any deviation from Standard Model predictions could lead the way to a more fundamental theory of nature. The comparison of predicted and measured cross sections, both inclusive and differential, allows important tests of the Standard Model. One of the decay channels of the Higgs boson that is particularly well suited for measurements of Higgs boson production cross sections is the decay into a pair of photons, $H\rightarrow\gamma\gamma$. Based on proton-proton collision data collected with the ATLAS experiment in the data-taking periods in 2015 -- 2017 at a center-of-mass energy of $\sqrt{s}=13$\,TeV, corresponding to an integrated luminosity of 79.8/fb, inclusive and differential Higgs boson production cross sections are measured in the $H\rightarrow\gamma\gamma$ decay channel in a fiducial phase-space volume. The observed fiducial inclusive cross section of Higgs production with the Higgs decaying to a pair of photons corresponds to $(60.4 \pm 8.6)$\,fb, which is in good agreement with the Standard Model prediction of $(63.5 \pm 3.3)$\,fb.The differential cross section in the Higgs boson transverse momentum, $p_\mathrm{T}^H$, is sensitive to the Yukawa couplings between the Higgs boson and quarks; therefore, limits for these couplings can be set by performing a fit on the observed $p_\mathrm{T}^H$ distribution. In this work, limits are set on the Yukawa couplings between both bottom and charm quarks and the Higgs boson, using ATLAS $pp$ collision data collected in the years 2015 -- 2018, corresponding to an integrated luminosity of 139.0/fb. The charm quark Yukawa coupling modifier, $\kappa_c = y_c/y_c^{SM}$, has been constrained to $\kappa_c \in [-19,\,25]$ at $95\,%$~CL, while for the bottom quark Yukawa coupling modifier, $\kappa_b = y_b/y_b^{SM}$, the corresponding confidence interval was determined to be $\kappa_b \in [-6,\,16]$.The measurement of photon identification efficiencies and a comparison with efficiencies in simulation are an important input for photon-based analyses of ATLAS data such as the above mentioned measurement of $H\rightarrow\gamma\gamma$ cross sections. Using the data recorded in the years 2015 -- 2017, corresponding to an integrated luminosity of 79.8/fb, photon identification efficiencies are measured using a method that relies on applying transformations to variables parametrizing the shape of the electromagnetic showers of electrons and photons. A pure and unbiased sample of electrons to which these transformations are applied is selected with a tag-and-probe method. The transformed electrons are photon-like objects and as such can be used to measure photon identification efficiencies. Depending on the considered region of pseudorapidity and photon transverse momentum, the uncertainties on these efficiencies range from $0.5\,\%$ to $3\,\%$. URL: https://ediss.sub.uni-hamburg.de/handle/ediss/8622.2 URN: urn:nbn:de:gbv:18-ediss-89856 Dokumenttyp: Dissertation Bemerkung: This thesis has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 678215: HiggspT – Differential Higgs distributions as a unique window to New Physics at the LHC). Betreuer*in: Tackmann, KerstinSchleper, Peter Enthalten in den Sammlungen: Elektronische Dissertationen und Habilitationen

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