[en] The ATLAS experiment has successfully recorded, reconstructed and analyzed millions of collision events delivered by the LHC at an un-precedent centre-of-mass energy of 7 TeV. The involved large-scale data processing operations worked remarkably well, from an early commissioning period that gradually evolved to a stable operation mode aiming at physics. The early commissioning samples were promptly processed at the CERN 'Tier-0' computer farm in all ATLAS data formats: from raw, to reconstructed, to a variety of reduced datasets. From these, the reconstruction software was tuned and state-of-the-art calibration and alignment constants were derived very quickly. These improvements were then used for a series of centralized data reprocessing, using world-wide distributed computing on the Grid. The reprocessing exercises were done coherently for real and simulated data samples, and could provide the high-quality data sets required for the publication of the first ATLAS physics results. By April 2010, the prompt reconstruction was stabilized and pre-calibrated run by run, so that the new data samples produced by the Tier0 are directly usable for physics analysis and can be combined with the earlier reprocessed data, real and simulated. (authors)

[en] It is essential to get quick feedback at all stages of the collection and reconstruction of particle physics data, and to correctly record quality decisions to ensure that only good data are used to obtain physics results. The ATLAS data quality system provides prompt investigation of collected data, initial calibrations, and later reconstruction, and propagates the corresponding quality decisions to analysis users. This talk describes the significant experience we have gained In 2009 and 2010 with collision data operations and analysis. (authors)

[en] The production of single top quarks via the electroweak interaction promises to provide new opportunities to both test the standard model and search for new physics. In particular, electroweak top production provides the only means to directly measure the CKM matrix element V_tb at ATLAS. The s-channel has the lowest rate, but is the best theoretically understood mechanism of electroweak top production. An evaluation of the potential for background suppression and V_tb measurement in this channel is presented. It is found that significant background suppression can be achieved and V_tb can be measured in the s-channel to a statistical precision of 2.8% after taking three years of low luminosity data (3x10⁴ pb^-1) at the large hadron collider. (author)

[en] The measurement of the properties of proton-proton interactions at center-of-mass energies ranging from 900 GeV (injection energy) to 7 TeV in the ATLAS detector are presented. The charged-particle density, its dependence on transverse momentum and pseudo-rapidity, and the relationship between transverse momentum and charged particle multiplicity are measured for events with at least one charged particle in a defined kinematic range. The measurements are compared to Monte Carlo models for inelastic events and to results from other experiments. (authors)

[en] Making use of the excellent calorimetry of the ATLAS experiment, the first measurement of the cross-section for inclusive single-jet and di-jet production in proton-proton interactions at √(s) = 7 TeV will be presented. The corrected and unfolded spectrum for high-pT jets in pp collision at a center-of-mass energy of 7 TeV will be described. Special emphasis will be given to the discussion of the initial understanding of the jet energy scale. For the di-jet system, additional corrected distributions will be shown, including the δ φ distribution between the two leading jets in the transverse plane. Comparisons will be made between jets reconstructed using charged tracks and calorimeter jets to study fragmentation effects. (authors)

[en] It is foreseen to increase the luminosity of the Large Hadron Collider (LHC) at CERN by a factor ten, with the upgraded machine dubbed Super-LHC or sLHC. The ATLAS experiment will require a new tracker for sLHC operation. In order to cope with the increase in pile-up backgrounds at the higher luminosity, an all silicon detector is being designed. The new strip detector will use significantly shorter strips than the current SCT in order to minimise the occupancy. As the increased luminosity will mean a corresponding increase in radiation dose, a new generation of extremely radiation hard silicon detectors is required. Extensive R programmes are underway to develop silicon sensors with sufficient radiation hardness. In parallel, new front-end electronics and readout systems are being designed to cope with the higher data rates. The challenges of powering and cooling a very large strip detector will be discussed. Ideas on possible schemes for the layout and support mechanics will be shown. (authors)

[en] In this paper we summarise ATLAS operations from the STEP09 campaign in June 2009 through to ATLAS taking data in the first 7 TeV collisions at the LHC in 2010. We describe the lessons which were learned from the STEP09 challenge, both in proving which parts of the system were in good shape, but also in highlighting those areas which required improvement. We then describe the experience of ATLAS computing operations during the first LHC data taking era. The ATLAS experiment has successfully recorded, reconstructed, distributed and analysed millions of collision events delivered by the LHC at an un-precedent centre-of-mass energy of 7 TeV. The involved large-scale data processing operations, both the prompt reconstruction at Tier0 and the subsequent reprocessing campaigns in the Tier1 sites in the Grid, worked remarkably well. (author)

[en] Photon identification is important for many physics signatures at the LHC, as well as for detector calibration purposes. Prompt photon identification in ATLAS relies on the fine granularity of the electromagnetic calorimeter, which provides event by event rejection of the dominant background from photons from π⁰ decays, and on the inner detector, which allows us to reconstruct photon conversions to electron-positron pairs. The cross-section of prompt photon production at LHC is large enough that only a small integrated luminosity is required to produce a significant number of signal events. This contribution describes the extraction of the prompt photon signal above the background in the early LHC data with the ATLAS detector collected at a center of mass energy of 7 TeV. (authors)

[en] Studies of the long-lived resonances K_S and Lambda, reconstructed in inelastic collisions at 900 GeV and 7 TeV using a minimum bias trigger, have been performed using the ATLAS inner tracking detector. The spectra for these resonances are measured as a function of their transverse momentum and rapidity, and compared with Monte Carlo models. The ratio of anti-Lambda to Lambda production is evaluated, providing further tests of Monte Carlo models. The results are corrected for all detector effects, to simplify comparisons to models and other experiments. (author)

[en] Studies of the momentum flow in inelastic collisions at 900 GeV and 7 TeV recorded with a minimum bias trigger strategy are reported. A single high pT track is selected, and the distribution of other tracks in the event is evaluated relative to this reference track. The evolution of the charged momentum flow in the rest of the event, as a function of the pT of the reference track, gives important information about the transition from minimum bias event structure to the full underlying event observed in high-pT collision events. Results are presented after correction and unfolding of detector effects to allow simpler comparison to Monte Carlo models. In addition, the PYTHIA Monte Carlo generator has been tuned to ATLAS measurements at 900 GeV and 7 TeV. Standard distributions from Minimum Bias events, as well as the Underlying Event studies are included in the first tunes to ATLAS measurements at the LHC. The tunes aim for one consistent description of the new measurements as well as data from the Tevatron and LEP. (author)