Detector Alignment Studies for the CMS Experiment

This thesis presents studies related to track-based alignment for the future CMS experiment at CERN. Excellent geometric alignment is crucial to fully benefit from the outstanding resolution of individual sensors. The large number of sensors makes it difficult in CMS to utilize computationally demanding alignment algorithms. A computationally light alignment algorithm, called the Hits and Impact Points algorithm (HIP), is developed and studied. It is based on minimization of the hit residuals. It can be applied to individual sensors or to composite objects. All six alignment parameters (three translations and three rotations), or their subgroup can be considered. The algorithm is expected to be particularly suitable for the alignment of the innermost part of CMS, the pixel detector, during its early operation, but can be easily utilized to align other parts of CMS also. The HIP algorithm is applied to simulated CMS data and real data measured with a test-beam setup. The simulation studies demonstrate that the algorithm is a promising candidate for the alignment of the pixel detector. The test-beam study shows that the use of the algorithm significantly improves the data measured with genuine CMS hardware. The positioning uncertainties of different parts of CMS have also been systematically estimated. Ready-made scenarios corresponding to these uncertainties have been implemented in the CMS reconstruction software ORCA. These scenarios have been used in the alignment studies. They have also been widely used for more realistic misalignment simulation in the physics performance studies of the CMS collaboration.