The Dimuon Trigger of the ALICE - LHC Experiment

We describe the trigger system of the ALICE dimuon spectrometer, based on RPC detectors. The background rate is at least one order of magnitude less in ALICE than in LHC p-p experiments due to the lower luminosity in ion running. Hence we study the possibility to operate the RPCs either in streamer or proportional mode. We report a study done with cosmic rays concerning gas mixtures for streamer mode. The aim is to minimise the streamer charge for a better flux capability of the RPC and a smaller cluster size, as needed for ALICE. We also give the results of a beam test at SPS where the RPC was placed in the vicinity of a hadron absorber, simulating the operation conditions in ALICE.