Higgs cross section measurements at $\sqrt{s} = 13$ TeV using the ATLAS detector

A measurement of the Higgs boson fiducial cross section in the diphoton decay channel is presented using $36.1~\text{fb}^{-1}$ data collected by the ATLAS detector with proton-proton collisions provided by the Large Hadron Collider at a centre-of-mass energy of $\sqrt{s}=13$~TeV. The fiducial volume is defined by kinematic and particle-level isolation requirements applied to the final state photons. A result of $55~\pm~9~\left(\text{stat.}\right)~\pm4~\left(\text{syst.}\right)$~fb is obtained. The statistical precision is approximately double that of the measurement at $\sqrt{s}=8$~TeV and probes production at higher partonic centre-of-mass energies. Several differential cross sections are presented in the fiducial phase space. These characterise the properties of Higgs production and decay in a minimally model dependent way. A measurement of the total Higgs boson cross section is presented using the same dataset and the combination of diphoton and four-lepton decay channels. A result of $57.0~^{+6.0}_{-5.9}~\left(\text{stat.}\right)~^{+4.0}_{-3.3}~\left(\text{syst.}\right)$~pb is obtained. The fiducial acceptances and branching ratios are assumed to follow the Standard Model expectations. Four differential cross sections are presented using the combination of channels. The expected sensitivity of a differential cross section measurement in the diphoton channel is presented assuming the phase 2 upgrade of the electromagnetic calorimeter and a dataset of 3~ab$^{-1}$. This is predicted to significantly improve the measurement of Higgs boson production at high transverse momentum. A luminosity measurement based on the multiplicity of charged particle tracks is presented. This is used to perform corrections to the stability and scale of the ATLAS luminosity measurement in data collected in 2012, 2015 and 2016. Along with other offline measurements it is used to constrain the calibration transfer and long-term stability which are two of the dominant luminosity uncertainties.