Precision measurement of the luminosity in the ATLAS experiment.

A precision luminosity measurement is of critical importance for the ATLAS physics program, both for searches for new physics as well as for precision measurements of Standard Model cross-sections. The calibration of the luminosity is based on so-called van der Meer scans. The calibration determines the convolved beam sizes in the vertical and horizontal directions, and together with the precise knowledge of the beam current a luminosity calibration can be determined. Based on this analysis ATLAS has determined the luminosity with a total uncertainty of only 3.7% for the 2011 data at sqrt{s}=7 TeV.