Alignment of the CMS silicon tracker – and how to improve detectors in the future

The complex system of the CMS all-silicon tracker detector, with 15 148 silicon strip and 1440 silicon pixel modules, requires sophisticated alignment procedures. In order to achieve an optimal track-parameter resolution, the position and orientation of its modules need to be determined with a precision of few micrometers. The alignment of pixel modules is crucial for the analyses requiring a precise vertex reconstruction. The aligned geometry is based on the analysis of several million reconstructed tracks recorded during the commissioning of the CMS experiment, both with cosmic rays and with the first proton–proton collisions. Statistical precision of the alignment of the modules with respect to the particle trajectories to less than 10μm has been achieved. The results have been validated by several data-driven studies and compared with predictions obtained from a detailed detector simulation.Recent developments include the determination of sensor bow and displacements between sensors of composite modules and are presented here for the first time.Ideas on improving future detectors with respect to alignment are given.