CMS muon alignment: System description and first results

The CMS detector has been instrumented with a precise and complex opto-mechanical alignment subsystem that provides a common reference frame between tracker and muon detection systems by means of a net of laser beams. The system allows a continuous and accurate monitoring of the muon chambers positions with respect to the tracker body. Preliminary results of operation during the test of the CMS 4 T solenoid magnet, performed in 2006, are presented. These measurements complement the information provided by the use of survey techniques and the results of alignment algorithms based on muon tracks crossing the detector.