The CMS tracker alignment in pp collisions

Abstract

The CMS all-silicon tracker consists of 16,588 modules. Aligning these with the desired precision of a few micrometers is only feasible using track based alignment procedures. Ultimate local precision is now achieved by the determination of sensor curvatures. This faces the algorithms with about 200,000 parameters to be calculated simultaneously. The Millipede II program interfaced with CMS software is optimized to provide solution in one step. The main remaining challenges are systematic distortions in the achieved geometry that are systematically biasing the track parameters like the track momenta. These distortions are controlled by adding further information into the alignment workflow, e.g. the mass of decaying resonances. In addition, the orientation of the tracker with respect to the magnetic field of CMS is determined with a stand-alone chi-square minimization procedure. The geometries are finally carefully validated. The monitored quantities include the basic track quantities for tracks from both collisions and cosmic muons and physics observables like resonance masses.