Performances of a prototype for the ALICE Muon trigger at LHC

ALICE, the experiment dedicated to the study of heavy ion collisions at LHC, will be equipped with a forward spectrometer to identify heavy quarkonium states from their decay into muons. The trigger system of the spectrometer consists of four planes of resistive plate chambers, front-end and fast-decision electronics. It is designed to reconstruct muon tracks in a large background environment and to provide a fast trigger signal to the spectrometer. We present results from the test of a trigger prototype that have been carried out with the moon beam of the CERN/SPS at the Gamma Irradiation Facility (GIF). The track finding efficiency as well as the robustness of the system to the uncorrelated background induced on the detector by the GIF source are discussed. Special emphasis is put on timing aspects which are crucial at LHC. A self-consistent method for timing optimization and monitoring is proposed. The findings, although specific to ALICE, are relevant to any muon trigger system operating at LHC.