Measurements of elastic $pp$ interactions and exclusive production with the ATLAS detector

The total proton--proton($pp$) cross section is a fundamental observable at the LHC. It can be derived from the measurement of the elastic cross section, using the optical theorem. Measurements of the elastic $pp$ cross section were performed at a center-of-mass energy of 8 TeV at various settings of the beam optics using the ALFA detector. Using a fit to the differential elastic cross section in the $-t$ range from 0.014 GeV$^2$ to 0.1 GeV$^2$ to extrapolate $t\rightarrow 0$, the total cross section, $\sigma_{\rm tot}(pp \rightarrow X)$, is measured via the optical theorem. n addition, the slope of the exponential function describing the elastic cross section at small $t$ is determined. The production of exclusive $\gamma\gamma \rightarrow \mu^+\mu^-$ events in $pp$ collisions at a centre- of-mass energy of 13 TeV is measured with the ATLAS detector, using data corresponding to an integrated luminosity of 3.2 fb$^{-1}$. The measurement is performed for a dimuon invariant mass of 12 GeV $< m_{\mu^+\mu^-} <$ 70 GeV. The integrated cross section is determined within a fiducial acceptance region of the ATLAS detector and differential cross sections are measured as a function of the dimuon invariant mass. The results are compared to theoretical predictions both with and without corrections for absorptive effects.