Figure 1. The tracks of μ+ / μ- pairs will be reconstructed by ten chambers shared among five tracking stations
At the end of March 2009, the ALICE Muon Spectrometer took cosmic rays over a period of two weeks. The ALICE group at Saclay2 was closely involved in the design, development, construction and installation of a part of the chambers of this Spectrometer3. The purpose of the cosmic ray test was to check the performance of the entire system, from acquisition to reconstruction of the data. The acquisition system readout about a million channels and the data was recorded on the computing grid. Almost 15,000 tracks were reconstructed under conditions close to those of the real experiment. The cosmic test was a success, demonstrating the performance and the stability of the spectrometer chambers. It also revealed some weak points of the system. Tests are currently underway to correct the observed defaults.
Another cosmic test is planned for August, a few weeks before the first LHC beam injections, involving all ALICE detectors.