Measurement of the Luminosity by the ATLAS Experiment

The luminosity is measured in ATLAS by several detectors and methods, both online and offline. This poster will present briefly the luminosity detectors in ATLAS and explain in some detail how they were used to extract and publish luminosity during the 2010 data taking. One of the challenges in measuring the luminosity at the LHC comes from the fact that many interactions typically occur in every bunch crossing. Comparisons of the number of interactions as measured by different detectors and methods will be presented. The very successful collaboration with the LHC machine has recently lead to a calibration, via beam separation scans, which is now estimated to have a systematic error of 3.4% for 2010 and a similar value for 2011 is expected. Since the error in the luminosity analysis was the limiting factor in the precision of many cross section measurements in ATLAS during 2010, this error analysis will also be discussed in some detail.