More than half a million minimum-bias events of LHC collision data were collected by the ATLAS experiment in December 2009 at centre-of-mass energies of 0.9 TeV and 2.36 TeV. This paper reports on studies of the initial performance of the ATLAS detector from these data. Comparisons between data and Monte Carlo predictions are shown for distributions of several track-and calorimeter-based quantities. The good performance of the ATLAS detector in these first data gives confidence for successful running at higher energies.

On Behalf of the ATLAS LIQUID ARGON Collaboration Complete calorimetric hermeticity is important for many physics studies at the LHC. In order to extend the pseudorapidity coverage up to 4.9 units, ATLAS uses a liquid argon forward calorimeter integrated into the endcap cryostat. In this region only a modest stochastic term is required in the energy resolution. The main challenge is survivability and reliability in this hostile environment. We discuss the development of the forward calorimeter from construction to its utility in physics studies.

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