Diffraction and rapidity gap measurements in ATLAS

The early data recorded by the ATLAS detector during 2010 presents a great opportunity to study diffraction cross section in proton-proton collision. The differential cross section of diffractive dissociation is studied as a function of the maximum rapidity gap, up to 8 in rapidity units. Data are compared to different models of diffractive dynamics in standard event generators. A rise at large rapidity gaps is interpreted with a triple pomeron based approach, using Pythia 8 prediction (with a Donnachie-Landshoff model). A pomeron intercept of 1.058 ± 0.003(stat) +0.034-0.039 (syst) is found. A measurement of the dijet production with a jet veto on additional central activity using 2010 data is also presented. The use of a veto scale at 20 GeV allows to measure the jet activity in dijet events. As the veto scale is much larger than Lambda_s different QCD phenomena can be studied. Moreover, ATLAS data explores regions of the phase space for the first time. The main observable in this analysis is the fraction of events with no additional jet with a transverse momentum greater than the veto scale. The fraction of gap events is studied as a function of the rapidity interval and average transverse momentum of the dijet system. Data is compared to standard MC generators (Pythia, Herwig, Alpgen) and two theoretical predictions: HEJ and Powheg Box. In overall Powheg, linked to Herwig Parton Shower and hadronisation provides the best agreement with data.