ATLAS Data Challenge 1

The ATLAS Collaboration at CERN is preparing for the data taking and analysis at LHC that will start in 2007. Therefore, in 2002 a series of Data Challenges (DC's) was started whose goals are the validation of the Computing Model, of the complete software suite, of the data model, and to ensure the correctness of the technical choices to be made. A major feature of the first Data Challenge (DC1) was the preparation and the deployment of the software required for the production of large event samples for the High Level Trigger and Physics communities, and the production of those large data samples as a worldwide distributed activity. It should be noted that it was not an option to "run everything at CERN" even if we had wanted to; the resources were not available at CERN to carry out the production on a reasonable time-scale. We were therefore faced with the great challenge of organising and then carrying out this large-scale production at a significant number of sites around the world. However, the benefits of this are manifold: apart from realising the required computing resources, this exercise builds worldwide momentum for ATLAS computing as a whole. The first phase (event generation, simulation) of DC1 was run during Summer 2002, and involved 40 institutes in 19 countries. In the second phase (October 2002-March 2003) the next processing step ("pile-up") was performed with the participation of 56 institutes in 21 countries. Distributed reconstruction of the most demanding high-statistics samples was carried out at the 9 largest sites (April-June 2003) Much has been learned from DC1, and much more will doubtless be learned over the next months. However, we can already be rather confident that ATLAS will be able to marshal world-wide resources in an effective way; let us hope that the Grid will make it all rather easy. This report describes in detail the main steps carried out in DC1 and what has been learned from them.