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

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$\,$%.