Study of the Internal Alignment of the CMS Muon Barrel Drift Chambers Using Cosmic Ray Tracks

This note describes the alignment studies performed on 21 Muon Barrel Drift Tube chambers for the CMS experiment, assembled in the INFN production center at the Legnaro National Laboratories. Data were collected using the cosmic ray test facility which was setup in Legnaro to test the chamber behaviour before being moved to CERN. An alignment procedure using cosmic ray tracks has been developed, allowing a measurement of the internal misalignment of the wire layers inside a chamber "superlayer" (SL) and of the relative misalignment of the 2 SL's in the r-phi bending plane. The analysis shows that the wire layers are positioned with an rms precision of about 45 micron; the relative positioning of the 2 SL's in the r-phi bending plane is meausured to have a distribution with ~ 50 micron average value and 200 micron rms.