Measurements of inelastic, elastic and total pp cross-sections at the LHC with ATLAS

First, a recent measurement of the inelastic cross section using the ATLAS detector with 63 $\mu b^{−1}$ of proton-proton collisions at $\sqrt{s}=13$ TeV is presented. The measurement is performed using scintillators mounted in front of the forward calorimeters. A cross section of $65.2\pm0.8$ (exp.) $\pm5.9$ (lum.) mb is measured in the fiducial region $M_X>13$ GeV, where $M_X$ is the larger of the dissociation masses of the two proton systems in diffractive events. The experimental uncertainty is indicated by (exp.) and the luminosity uncertainty by (lum.). The full inelastic cross section is determined to be $73.1\pm0.9$ (exp.) $\pm6.6$ (lum.) $\pm3.8$ (extr.) mb, where (extr.) indicates model-dependent uncertainties on the extrapolation from the fiducial region. The measured value is about one standard deviation below most current theoretical predictions. Second, a measurement of the total $pp$ cross section at the LHC at $\sqrt{s}=7$ TeV is presented. In a special run with high-$\beta^*$ beam optics, an integrated luminosity of 80 $\mu b^{−1}$ was accumulated in order to measure the differential elastic cross section as a function of the Mandelstam momentum transfer variable t. The measurement is performed with the ALFA sub-detector of ATLAS. The total cross section, $\sigma_\text{tot}(pp\rightarrow X)$, is measured via the optical theorem to be $95.35\pm0.38$ (stat.) $\pm1.25$ (exp.) $\pm0.37$ (extr.) mb, where the first error is statistical, the second accounts for all experimental systematic uncertainties and the last is related to uncertainties in the extrapolation to $|t|\rightarrow0$. In addition, the slope of the elastic cross section at small $|t|$ is determined to be $B=19.73\pm0.14$ (stat.) $\pm0.26$ (syst.) GeV$^{-2}$.