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 ATLAS CollaborationResults of a search for new phenomena in final states with an energetic jet and large missing transverse momentum are reported. The search uses proton-proton collision data corresponding to an integrated luminosity of 36.1 fb −1 at a centre-of-mass energy of 13 TeV collected in 2015 and 2016 with the ATLAS detector at the Large Hadron Collider. Events are required to have at least one jet with a transverse momentum above 250 GeV and no leptons (e or µ). Several signal regions are considered with increasing requirements on the missing transverse momentum above 250 GeV. Good agreement is observed between the number of events in data and Standard Model predictions. The results are translated into exclusion limits in models with pair-produced weakly interacting dark-matter candidates, large extra spatial dimensions, and supersymmetric particles in several compressed scenarios.

Next-to-leading-order QCD analyses of the ZEUS data on deep inelastic scattering together with fixed-target data have been performed, from which the gluon and quark densities of the proton and the value of the strong coupling constant ␣ s (M Z ) were extracted. The study includes a full treatment of the experimental systematic uncertainties including point-to-point correlations. The resulting uncertainties in the parton density functions are presented. A combined fit for ␣ s (M Z ) and the gluon and quark densities yields a value for ␣ s (M Z ) in agreement with the world average. The parton density functions derived from ZEUS data alone indicate the importance of HERA data in determining the sea quark and gluon distributions at low x. The limits of applicability of the theoretical formalism have been explored by comparing the fit predictions to ZEUS data at very low Q 2 .

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Search for new phenomena in high-mass diphoton final states using 37 fb −1 of proton-proton collisions collected at √ s = 13 TeV with the ATLAS detectorThe ATLAS Collaboration Searches for new phenomena in high-mass diphoton final states with the ATLAS experiment at the LHC are presented. The analysis is based on pp collision data corresponding to an integrated luminosity of 36.7 fb −1 at a centre-of-mass energy √ s = 13 TeV recorded in 2015 and 2016. Searches are performed for resonances with spin 0, as predicted by theories with an extended Higgs sector, and for resonances with spin 2, using a warped extra-dimension model as a benchmark model, as well as for non-resonant signals, assuming a large extradimension scenario. No significant deviation from the Standard Model is observed. Upper limits are placed on the production cross section times branching ratio to two photons as a function of the resonance mass. In addition, lower limits are set on the ultraviolet cutoff scale in the large extra-dimensions model. c 2017 CERN for the benefit of the ATLAS Collaboration. 1 The ATLAS experiment uses a right-handed coordinate system with its origin at the nominal interaction point (IP) in the centre of the detector and the z-axis along the beam pipe. The x-axis points from the IP to the centre of the LHC ring, and the y-axis points upward. Cylindrical coordinates (r, φ) are used in the transverse plane, φ being the azimuthal angle around the z-axis. The pseudorapidity is defined in terms of the polar angle θ as η = − ln tan(θ/2). The transverse energy is defined as3 Simulated Monte Carlo (MC) events are used for optimizing the search strategy [23], and for the signal and background modelling studies detailed in Sections 5 and 6, respectively. Interference effects between the resonant signal and the background processes are neglected.The spin-0 signal MC samples were generated using the effective-field-theory approach implemented in MadGraph5_aMC@NLO [24] version 2.3.3 at next-to-leading order (NLO) in quantum chromodynamics (QCD). From the Higgs characterization framework [25], CP-even dimension-five operators coupling the new resonance to gluons and photons were included. Samples were generated with the NNPDF3.0 NLO parton distribution functions (PDFs) [26], using the A14 set of tuned parameters (tune) of Pythia 8.186 [27,28] for the parton-shower and hadronization simulation. Simulated samples were produced for fixed values of the mass and width of the assumed resonance, spanning the range 200-2400 GeV for the mass, and the range from 4 MeV to 10% of the mass for the decay width. Choosing an improved signal model with an event generator different from the one used in Ref.[1] provides a description of the signal which is less sensitive to modelling effects from the off-shell region. The impact of this change is only visible in scenarios with a large signal decay width, with mass values at the TeV scale.Spin-2 signal samples for the RS1 model were generated using Pythia 8.186, with the NNPDF23LO PDF set [29] and the A1...