In mid-July, the ALICE Collaboration reached important milestones with the installation of the trigger and tracking chambers of the muon spectrometer. They are the first detectors to be installed in their final position in the ALICE cavern of the Large Hadron Collider.
The role of the trigger detector is to select events containing a muon pair coming, for instance, from the decay of J/Ψ or Υ resonances. All of the eight half-planes of the resistive plate chambers (RPCs) are now in position behind the muon filter. The company General Tecnica fabricated the internal parts of the RPCs, which are made of bakelite, and groups from INFN Torino and Alessandria are constructing the readout chambers The IN2P3 laboratory in Clermont-Ferrand has developed the front-end electronics and Subatech Nantes has produced the readout electronics.
At the same time, workers at ALICE have installed the first half-station of the tracking system a few metres before the muon wall. The main task of this system is to sample the trajectory of muons with a resolution better than 100 µm. It is composed of cathode-pad/strip chambers, among the first of their kind, made from composite material. Extremely thin but still very rigid, the composite material helps to minimize the scattering of the muons. INFN Cagliari, the Petersburg Nuclear Physics Institute in Gatchina, Subatech Nantes and CEA Saclay constructed the big chambers, while the Institut de Physique Nucléaire at Orsay, and the Saha Laboratory in Kolkata, India, made the smaller ones.
