Measurement of the total cross section from elastic scattering in $pp$ collisions at $\sqrt{s}=8$ TeV with the ATLAS detector

A measurement of the total $pp$ cross section at the LHC at $\sqrt{s}=8$ TeV is presented. An integrated luminosity of $500$ $\mu$b${}^{-1}$ was accumulated in a special run with high-${\beta }^{\star }$ beam optics 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. 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 $t\to 0$, the total cross section, ${\sigma }_{\mathrm{t}\mathrm{o}\mathrm{t}}(pp\to X)$, is measured via the optical theorem to be: $\sigma_{$\mathrm{t}\mathrm{o}\mathrm{t}$}(pp\rightarrow X) = {96.07} ; \pm 0.18 ; ({{stat.}}) \pm 0.85 ; ({{exp.}}) \pm 0.31 ; ({extr.}) ; {mb} ;,$ where the first error is statistical, the second accounts for all experimental systematic uncertainties and the last is related to uncertainties in the extrapolation $t\to 0$. In addition, the slope of the exponential function describing the elastic cross section at small $t$ is determined to be $B = 19.74 \pm 0.05 ; ({{stat.}}) \pm 0.23 ; ({{syst.}}) ; {GeV}^{-2}$.