Large-scale performance studies of the Resistive Plate Chamber fast tracker for the ATLAS 1st-level muon trigger

In the ATLAS experiment, Resistive Plate Chambers provide the first-level muon trigger and bunch crossing identification over large area of the barrel region, as well as being used as a very fast 2D tracker. To achieve these goals a system of about ~4000 gas gaps operating in avalanche mode was built (resulting in a total readout surface of about 16000 m2 segmented into 350000 strips) and is now fully operational in the ATLAS pit, where its functionality has been widely tested up to now using cosmic rays. Such a large scale system allows to study the performance of RPCs (both from the point of view of gas gaps and readout electronics) with unprecedented sensitivity to rare effects, as well as providing the means to correlate (in a statistically significant way) characteristics at production sites with performance during operation. Calibrating such a system means fine tuning thousands of parameters (involving both front-end electronics and gap voltage), as well as constantly monitoring performance and environmental quantities such as gap efficiency, average cluster size, temperature, gas flow, gap currents, counting rates, trigge The ATLAS RPC calibration effort will be presented here, which spans many different aspects of RPC operation: from the monitoring (by the detector control system) of the gap status to precise measurement of detector performance using ATLAS' muon tracking detectors (MDTs), to computing-related problems, such as the need to use distributed resources (GRID) in order to obtain the results in a sensible time. An overview of the performance measurements done using cosmic rays will be given as well.