This Letter presents a measurement of the inelastic proton-proton cross section using $60{\mu \mathrm{b}}^{-1}$ of $pp$ collisions at a center-of-mass energy $\sqrt{s}$ of 13 TeV with the ATLAS detector at the LHC. Inelastic interactions are selected using rings of plastic scintillators in the forward region ($2.07<|\eta |<3.86$) of the detector. A cross section of $68.1\pm 1.4\mathrm{mb}$ is measured in the fiducial region $\xi =M_X^2/s>{10}^{-6}$, where $M_X$ is the larger invariant mass of the two hadronic systems separated by the largest rapidity gap in the event. In this $\xi$ range the scintillators are highly efficient. For diffractive events this corresponds to cases where at least one proton dissociates to a system with $M_X>13\mathrm{GeV}$. The measured cross section is compared with a range of theoretical predictions. When extrapolated to the full phase space, a cross section of $78.1\pm 2.9\mathrm{mb}$ is measured, consistent with the inelastic cross section increasing with center-of-mass energy.

• Received 9 June 2016

DOI:https://doi.org/10.1103/PhysRevLett.117.182002

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© 2016 CERN, for the ATLAS Collaboration