The luminosity monitor of the ATLAS experiment

In the ATLAS experiment, the online luminosity will be monitored mainly by a dedicated detector called LUCID (LUminosity Cherenkov Integrating Detector). This apparatus is composed by two modules having 20 Cherenkov tubes which are mainly sensitive to particles coming from the ATLAS interaction point. The Cherenkov light is collected by photomultiplier tubes (PMT) directly placed at the tubes end. The intrinsically fast response of the detector and its readout electronics makes it ideal to follow the number of interactions per LHC bunch crossing therefore providing also an interaction trigger to the ATLAS experiment. A dedicated electronic board performs the on-the-fly evaluation of the LHC luminosity with different algorithms which have different trade-offs between accuracy, dependence on background levels, systematic uncertainties and measurement ranges. The detector took already some data during the first beam circulation in LHC. In this contribution, we report on the relevant details of the detector design, its associated electronics and on real data taken on beam tests and on the short LHC running period of 2008.