Alignment of the CMS silicon tracker

Abstract

The CMS all-silicon tracker consists of 16 588 modules, embedded in a solenoidal magnet providing a field of B = 3.8 T. The targeted performance requires that the software alignment tools determine the module positions with a precision of a few micrometers. Ultimate local precision is reached by the determination of sensor curvatures, challenging the algorithms to determine about 200 000 parameters simultaneously. The main remaining challenge for alignment are the global distortions that systematically bias the track parameters and thus physics measurements. They are controlled by adding further information into the alignment work-flow, e.g. the mass of decaying resonances or track data taken with B = 0T. To make use of the latter and also to integrate the determination of the Lorentz angle into the alignment procedure, the alignment framework has been extended to treat position sensitive calibration parameters. This is relevant since due to the increased LHC luminosity in 2012, the Lorentz angle exhibits time dependence. Cooling failures and ramping of the magnet can induce movements of large detector sub-structures. These movements are now detected in the CMS prompt calibration loop to make the corrections available for the reconstruction of the data for physics analysis. The geometries are finally carefully validated. The monitored quantities include the basic track quantities for tracks from both collisions and cosmic ray muons and physics observables.