H. von der Schmitt 99 , J. von Loeben 99 , H. von Radziewski 48 , E. von Toerne 20 , V. Vorobel 126 , V. Vorwerk 11 , M. Vos 166 , R. Voss 29 , T.T. Voss 173 , J.H. Vossebeld 73 , N. Vranjes 12a , M. Vranjes Milosavljevic 12a , V. Vrba 125 , M. Vreeswijk 105 , T. Abstract The simulation software for the ATLAS Experiment at the Large Hadron Collider is being used for large-scale production of events on the LHC Computing Grid. This simulation requires many components, from the generators that simulate particle collisions, through packages simulating the response of the various detectors and triggers. All of these components come together under the ATLAS simulation infrastructure. In this paper, that infrastructure is discussed, including that supporting the detector description , interfacing the event generation, and combining the GEANT4 simulation of the response of the individual detectors. Also described are the tools allowing the software validation, performance testing, and the validation of the simulated output against known physics processes.

The simulation software for the ATLAS Experiment at the Large Hadron Collider is being used for largescale production of events on the LHC Computing Grid. This simulation requires many components, from the generators that simulate particle collisions, through packages simulating the response of the various detectors and triggers. All of these components come together under the AT-LAS simulation infrastructure. In this paper, that infrastructure is discussed, including that supporting the detector description, interfacing the event generation, and combining the GEANT4 simulation of the response of the individual detectors. Also described are the tools allowing the software validation, performance testing, and the validation of the simulated output against known physics processes.

A detailed analysis is presented of the diffractive deep-inelastic scattering process ep → eXY , where Y is a proton or a low mass proton excitation carrying a fraction 1−x I P > 0.95 of the incident proton longitudinal momentum and the squared four-momentum transfer at the proton vertex satisfies |t| < 1 GeV 2 . Using data taken by the H1 experiment, the cross section is measured for photon virtualities in the range 3.5 ≤ Q 2 ≤ 1600 GeV 2 , triple differentially in x I P , Q 2 and β = x/x I P , where x is the Bjorken scaling variable. At low x I P , the data are consistent with a factorisable x I P dependence, which can be described by the exchange of an effective pomeron trajectory with intercept α IP (0) = 1.118 ± 0.008 (exp.) +0.029 −0.010 (model). Diffractive parton distribution functions and their uncertainties are determined from a next-to-leading order DGLAP QCD analysis of the Q 2 and β dependences of the cross section. The resulting gluon distribution carries an integrated fraction of around 70% of the exchanged momentum in the Q 2 range studied. Total and differential cross sections are also measured for the diffractive charged current process e + p →ν e XY and are found to be well described by predictions based on the diffractive parton distributions. The ratio of the diffractive to the inclusive neutral current ep cross sections is studied. Over most of the kinematic range, this ratio shows no significant dependence on Q 2 at fixed x I P and x or on x at fixed Q 2 and β.

A measurement of charm and beauty dijet photoproduction cross sections at the ep collider HERA is presented. Events are selected with two or more jets of transverse momentum p jet 1(2) t > 11(8) GeV in the central range of pseudo-rapidity −0.9 < η jet 1(2) < 1.3. The fractions of events containing charm and beauty quarks are determined using a method based on the impact parameter, in the transverse plane, of tracks to the primary vertex, as measured by the H1 central vertex detector. Differential dijet cross sections for charm and beauty, and their relative contributions to the flavour inclusive dijet photoproduction cross section, are measured as a function of the transverse momentum of the leading jet, the average pseudo-rapidity of the two jets and the observable x obs γ . Taking into account the theoretical uncertainties, the charm cross sections are consistent with a QCD calculation in next-to-leading order, while the predicted cross sections for beauty production are somewhat lower than the measurement.

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