Mechanical stability of the CMS Tracker

The CMS Tracker consists of about 210 m$^2$ of silicon strip sensors assembled on various carbon fiber composite structures. The engineering design of the Tracker provides the mounting accuracy of tracker components of about 50$\mu$m but allows some constrained movements. The mechanical stability of Tracker components during Tracker operation is monitored with a dedicated Laser Alignment System and using particle tracks from cosmics and collisions. The laser system monitors a relative movement of large substructures with an accuracy of a few microns in a few minutes interval while the alignment with tracks reconstructs the absolute position of individual detector modules with a similar accuracy but after days of data taking. During the long term operation at fixed temperature of +4$^o$C in years 2011--2013 the alignment of tracker components was stable within 10 microns. Temperature variations in the Tracker volume are found to cause the displacements of tracker structures of about 20 microns/10$^o$C that are recovered with the temperature restoration. The sufficient temperature measurements and the appropriate modelling of the temperature effects are important for the future upgrade of LHC detectors.