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= ATLAS - an Introduction =

{{attachment:ATLAS/atlasdetector.jpg|The ATLAS detector|width=300px,align="left"}} The '''L'''arge '''H'''adron '''Co'''llider (LHC) is currently approaching completion at CERN  in Geneva, Switzerland. This accelerator complex is designed to collide protons in a ring accelerator with a circumference of about 27 km. The protons will collide with centre-of-mass energies of up to 14 TeV. This high energy together with the LHC's enormous luminosity will be exploited to answer some of the most fundamental questions of particle physics, such as the origin of mass and to search for long-anticipated extensions of the Standard Model of particle physics. The experiments at the LHC will play a crucial role in improving our understanding of our universe. First collisions are  scheduled for mid 2008.

{{attachment:cern_surface.jpg|Aerial photo of CERN|width=300px,align="right"}}In order to study the collisions, four experiments are currently under construction: ATLAS, CMS, ALICE and LHCb. ATLAS and CMS are designed as general purpose detectors, while ALICE and LHCb are experiments dedicated to heavy ion physics and b-physics respectively.  ATLAS, '''a''' '''T'''oridial '''L'''HC-'''A'''pperatu'''s''', is located about 100 m underground and has a length of about 40m and a diameter of 22m. The ATLAS-Detector consists of several detector components, each of which is a highly complex system in itself. The main distinguishing feature of the detector is the muon spectrometer which identifies and measures muon trajectories in a toroidal magnet field generated by large super-conducting coils. A more detailed description of the ATLAS-Detector can be found at the ATLAS [[http://atlas.ch/etours_exper/etours_exper08.html|home page]].


A major challenge for experiments at the LHC is the online selection of interesting events. A total interaction rate of 1GHz is expected at nominal luminosity leading to 25 interactions per bunch crossing. In this challenging environment the ATLAS Trigger System  must reduce the rate to below the maximum rate that can be processed by the offline computing facilities, about 200 Hz, while efficiently selecting previously undetected and rare physics processes. For example a Standard Model Higgs boson with a mass of 120 GeV, decaying into two photons, is expected to occur in one out of 1013 interactions. The ATLAS trigger is composed of three levels. Its first level (LVL1) is implemented in electronics and firmware, whereas the higher levels are based on software algorithms running in huge processor farms. One of the main working areas of the DESY-ATLAS group is the construction of the ATLAS trigger system.

Accepted events are recorded at the CERN computer centre. However, the huge amount of data requires the data to be processed by a number of outside computing centres which are connected to each-other via the Grid in a tier-like structure. DESY operates a Tier-2 centre for ATLAS at Hamburg and Zeuthen.