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.

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Proton-proton collisions at √ s = 7 TeV and heavy ion collisions at √ s NN = 2.76 TeV were produced by the LHC and recorded using the ATLAS experiment's trigger system in 2010. The LHC is designed with a maximum bunch crossing rate of 40 MHz and the ATLAS trigger system is designed to record approximately 200 of these per second. The trigger system selects events by rapidly identifying signatures of muon, electron, photon, tau lepton, jet, and B meson candidates, as well as using global event signatures, such as missing transverse energy. An overview of the ATLAS trigger system, the evolution of the system during 2010 and the performance of the trigger system components and selections based on the 2010 collision data are shown. A brief outline of plans for the trigger system in 2011 is presented.

The measurement of missing transverse momentum in the ATLAS detector, described in this paper, makes use of the full event reconstruction and a calibration based on reconstructed physics objects. The performance of the missing transverse momentum reconstruction is evaluated using data collected in pp collisions at a centre-of-mass energy of 7 TeV in 2010. Minimum bias events and events with jets of hadrons are used from data samples corresponding to an integrated luminosity of about 0.3 nb −1 and 600 nb −1 respectively, together with events containing a Z boson decaying to two leptons (electrons or muons) or a W boson decaying to a lepton (electron or muon) and a neutrino, from a data sample corresponding to an integrated luminosity of about 36 pb −1 . An estimate of the systematic uncertainty on the missing transverse momentum scale is presented. IntroductionIn a collider event the missing transverse momentum is defined as the momentum imbalance in the plane transverse to the beam axis, where momentum conservation is expected. Such an imbalance may signal the presence of unseen particles, such as neutrinos or stable, weakly-interacting supersymmetric (SUSY) particles. The vector momentum imbalance in the transverse plane is obtained from the negative vector sum of the momenta of all particles detected in a pp collision and is denoted as missing transverse momentum, E miss T . The symbol E miss T is used for its magnitude. A precise measurement of the missing transverse momentum, E miss T , is essential for physics at the LHC. A large E miss T is a key signature for searches for new physics processes such as SUSY and extra dimensions. The measurement of E miss T also has a direct impact on the quality of a number of measurements of Standard Model (SM) physics, such as the reconstruction of the top-quark mass in tt events. This paper describes an optimized reconstruction and calibration of E miss T developed by the ATLAS Collaboration. The performance achieved represents a significant improvement compared to earlier results [2] presented by ATLAS. The optimal reconstruction of E miss T in the ATLAS detector is complex and validation with data, in terms of resolution, scale and tails, is essential. A number of data samples encompassing a variety of event topologies are used. Specifically, the event samples used to assess the quality of the E miss T reconstruction are: minimum bias events, events where jets at high transverse momentum are produced via strong interactions described by Quantum Chromodynamics (QCD) and events with leptonically decaying W and Z bosons. This allows the full exploitation of the detector capability in the reconstruction and calibration of different physics objects and optimization of the E miss T calculation. Moreover, in events with W → ν , where is an electron or muon, the E miss T performance can be studied in events where genuine E miss T is present due to the neutrino, thus allowing a validation of the E miss T scale. In simulated events, the genuine E miss T , E miss,True T ...