Experiences with the Muon Alignment Systems of the Compact Muon Solenoid Detector

After briefly explaining the need for a precise muon chamber alignment, the different muon alignment systems implemented at CMS are described. Due to the tight spatial confinement and challenging large radiation and high magnetic field environment, unique alignment systems had to be developed that handle separately the Barrel and the Endcap regions. A third subsystem, called Link, connects these two together and to the Tracker in a common reference frame. The aligned chamber geometry obtained from the Hardware-based muon alignment is validated by comparisons with photogrammetry information and by studies of residuals of muon tracks extrapolated between chambers. Stability studies, for which the hardware systems are particularly well suited, are also discussed.Alignment methods based on tracks are also described. Muons from cosmic rays and from collisions are used to align the chambers relative to the inner tracker. In addition, beam halo muon tracks traversing overlapping endcap chambers are used for internal endcap alignment. A comparison between the track-based and hardware-based results is given, together with an explanation of the advantages and disadvantages of the different alignment strategies.