The RPC Level-1 muon trigger of the ATLAS experiment at the LHC

The initial interactions rate foreseen at LHC at the designed luminosity of 1034 cm-2 s-1 is 1 GHz. Such an extremely large rate must be reduced by the trigger system to 100 Hz in order to allow permanent data storage preserving the much less probable physics signals against a large background. The ATLAS Level-1 muon trigger will be crucial for the online selection of events with high transverse momentum muons and for its correct association to the bunch-crossing of interest. The overall rejection factor is 104. The muon trigger in the barrel region is provided by three layers of resistive plate chambers (RPC). The logic is based on the search of patterns of hits in the RPC stations consistent with a muon track originated from the interaction vertex. Two pT regimes with different programmable thresholds have been implemented: 3 low-pT trigger thresholds and 3 high-pT one. The associated trigger electronics is based on a custom chip, the coincidence matrix (CM) that performs space coincidences and time gates providing also the readout of the RPCs. A package with very detailed simulation of the algorithm and the logic of the hardware components has been developed in order to optimize the performances of the system. Trigger performances and rates calculation has been evaluated for muons on a wide range of pT and will be presented along with studies on the impact of accidental triggers due to low energy background particles in the experimental area.