Measurement of the total proton-proton cross section with ATLAS at LHC

It is now nearly fifty years since total proton-proton (pp) cross sections have been found to grow with energy after it was believed for long time that they would become asymptotically constant . The uncertainties of the cosmic ray data, at high energy, do not allow to determine the exact growth with energy of the total cross section .The Large Hadron Collider (LHC) at CERN in Geneva has already delivered collisions with an energy never reached in a particle accelerator. The energy in the center of mass was 7 TeV (2010 - 2011) or 8 TeV (2012) and will ultimately reached 14 TeV in the near future. Thus, this will provide a good environment for a new precise measurement of the total pp cross section at this energy.The ATLAS detector installed in one of the four LHC interaction points is used to collect the result of the pp collisions. Its sub-detector ALFA located 240 m from the interaction point, is used to track protons resulting from elastic collisions.Therefore, within special beam optics conditions, ALFA is able to measure the total cross section and the nuclear slope. During this PhD the analysis performed on the first data led to σtot = 94.88 ± 0.12 stat ± 1.56syst mb and b = 19.45 ± 0.05stat ± 0.31syst GeV-2 at 7 TeV.