Determination of the total cross section and $\rho$-parameter from elastic scattering in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

A new measurement of elastic $pp$ scattering at $\sqrt{s} = 13$ TeV with the ATLAS-ALFA detector is presented. The measurement was performed using data recorded in a special run of the LHC with $\beta^\star = 2.5$ km. The elastic cross-section was measured differentially in the Mandelstam $t$ variable and from a fit to ${\textrm{d}}\sigma/\textrm{d}t$ the total cross section, the $\rho$-parameter and parameters of the nuclear slope are determined. The results for $\sigma_{\textrm{tot}}$ and $\rho$ are \begin{equation*} \sigma_{\textrm{tot}}(pp\rightarrow X) = \mbox{104.7} \; \pm 1.1 \; \mbox{mb} , \rho = \mbox{0.098} \; \pm 0.011 . \end{equation*} The energy evolution of $\sigma_{\textrm{tot}}$ and $\rho$, connected through dispersion relations, is compared to several models. Furthermore, the total inelastic cross section is determined from the difference of the total and elastic cross section, and the ratio of the elastic to total cross section is calculated.