The trigger system of the ALICE muon spectrometer at the LHC

The ALICE experiment aims at investigating the properties of strongly interacting matter at the extreme energy density reached in heavy ion collisions at the LHC. The trigger system of the muon spectrometer consists in four planes of Resistive Plate Chamber (RPC) detectors operated in streamer mode, 21 k front-end channels and fast-decision electronics, covering an area of 140 m2. It is designed to reconstruct (muon) tracks, in a large background environment, to provide a fast trigger signal. Indeed, a trigger decision is delivered each 25 ns (40 MHz) with a total latency of 800 ns. The hit position on the RPC is measured in two orthogonal directions with an accuracy of about 1 cm. Selected aspects of the Muon Trigger design will be described. The performances, of both detectors and electronics, measured with dedicated test tools and cosmic rays during the commissioning runs carried out in the first half of year 2008 will be reported.