Study of the performances of the Level-1 trigger system for muons in the barrel of ATLAS with cosmic rays

The muon trigger in ATLAS is crucial for all the main physics channels to be studied at the design center of mass energy and luminosity of LHC and also for the initial phase of data taking with beams for the calibration of all the detectors. The level-1 trigger system for muons in ATLAS is performed using dedicated RPC chambers in the vertical bar eta vertical bar < 1 region. The trigger electronics provides both trigger and readout information. This system is being commissioned using cosmic rays with the full trigger and data acquisition chains. Monitoring tools have been developed in order to spot problems during data taking and determine the quality of the data and the trigger performance. Moreover, using the data taken during the commissioning phase, with partial availability of the detector, some studies have been performed to allow the optimization of the trigger system, based on different choises of the configuration parameters, also as a function of the detector working point. In this paper we would like to describe the extensive tests performed on the system and illustrate the possible performances with different conditions.