The ATLAS LVL1 Barrel Muon Trigger Commissioning with Cosmic Rays

The ATLAS muon spectrometer, currently in the installation phase, uses dedicated detectors to be able to trigger on high transverse momentum muons in the range 6-20 GeV/c resistive plate chambers (RPC) are equipping the barrel region in the middle and outer station, while precision chambers (monitored drift tubes, MDT) are present also in the inner layer. The RPCs have the required timing and spatial resolution of about 2 ns times 1 cm, to be able to associate the muon to the correct bunch crossing and provide the second coordinate measurements to the MDTs. In order to successfully commission the chambers, cosmic runs are taken to check and validate the readout and trigger chain, and cosmic rates are measured and compared against values obtained with a cosmic ray Monte Carlo generator and full detector simulation. The first part of the detector under commission is the set of horizontal chambers positioned between the feet of the detector. The first results obtained in the ATLAS cavern will be presented. The first cosmic data taking collects signals from chambers arranged in six trigger towers, covering about one quarter of the full detector length. The experience gained on this small part of the detector will be very useful to define the commissioning work for the whole detector.