Measurement of the total cross section in proton proton collisions at $\mathbf{\sqrt{s}=7}$ TeV with the ALFA sub-detector of ATLAS

Measurement of the total pp cross section at the LHC at $\sqrt{s} = 7$ TeV is presented. In a special run with high-$\beta^*$ beam optics, an integrated luminosity of 80 $\mu$b$^{−1}$ was accumulated in order 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 0.01 GeV$^2$ to 0.1 GeV$^2$ to extrapolate to $|t|\rightarrow 0$, the total cross section, $\sigma_{tot}(pp \rightarrow X)$, is measured via the optical theorem to be: $\sigma_{tot}(pp \rightarrow X) = 95.35 \pm 0.38$ (stat.) $\pm 1.25 $ (exp.) $\pm 0.37$ (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 to $|t|\rightarrow 0$ . In addition, the slope of the elastic cross section at small $t$ is determined to be $B = 19.73 \pm 0.14$ (stat.) $\pm 0.26 $ (syst.) $GeV^{−2}$.