Measurement of the Inelastic Proton-Proton Cross Section at $\sqrt{s} = 13$ TeV with the ATLAS Detector at the LHC

A measurement of the inelastic cross section is performed using the ATLAS detector with 63~$\mu$b$^{-1}$ of proton-proton collisions at $\sqrt{s}=13$~\TeV. The measurement is performed using scintillators mounted in front of the forward calorimeters. A cross section of $65.2\pm0.8\ \mbox{(exp.)}\ \pm5.9\ \mbox{(lum.)\ mb}$ 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 $73.1\pm0.9\ \mbox{(exp.)}\ \pm6.6\ \mbox{(lum.)}\ \pm3.8\ \mbox{(extr.)\ mb}$, 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.