The ATLAS luminosity monitor

The main luminosity monitor of the ATLAS experiment consists of an array of Cherenkov counters. The intrinsically fast response of the detector and its readout electronics makes it ideal for measuring the number of interactions per LHC bunch crossing, and for providing an interaction trigger to the ATLAS experiment. The detector already took some data during the first beam interactions produced by the LHC. An original method to derive the detector response with an analytical approach is presented here.