Commissioning of the ALICE muon spectrometer trigger at LHC

ALICE (a large ion collider experiment) is the LHC experiment dedicated to the study of ultra-relativistic heavy ion collisions. The ALICE muon spectrometer covers a large range in pseudo-rapidity and is designed to study quarkonia and heavy flavours decaying into (di-)muons. The high particle multiplicities environment in such collisions require a specific, fast and efficient trigger system, the muon trigger. It consists of four planes of RPC detectors, covering an area of 36 m2 each, 21k front-end channels and a fast-decision electronics. The muon trigger is designed to reconstruct (muon) tracks and deliver a trigger signal 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. The performance measured with the first p-p collisions at √(()s)=900 GeV carried out in December 2009 is reported.