Measurement of the total cross section from elastic scattering in $pp$ collisions at $\sqrt{s}=7$TeV with the ATLAS detector

In this contribution a measurement of the total $pp$ cross section with the ATLAS detector at the LHC at $\sqrt{s}=7$TeV is presented. In a special run with high-$\beta^\star$, beam optics, the differential elastic cross section is measured 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 $0.01$ GeV$^2$ to $0.1$ GeV$^2$ to extrapolate to $|t|\rightarrow 0$, the total cross section, $\sigma_\mathrm{tot}(pp\rightarrow X)$, is measured via the optical theorem to be: \begin{equation*} \sigma_\mathrm{tot}(pp\rightarrow X) = \mbox{95.35} \; \pm 0.38 \; ({\mbox{stat.}}) \pm 1.25 \; ({\mbox{exp.}}) \pm 0.37 \; (\mbox{extr.}) \; \mbox{mb} \; \; , \end{equation*} 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 $|t|\rightarrow 0$. In addition, the slope of the elastic cross section at small $|t|$ is determined to be $B = 19.73 \pm 0.14 \; ({\mbox{stat.}}) \pm 0.26 \; ({\mbox{syst.}}) \; \mbox{GeV}^{-2}$.