A novel technique for the reconstruction and simulation of hits in the CMS pixel detector

New techniques for the reconstruction of hits in the pixel detectors of the Compact Muon Solenoid (CMS) experiment are described. The techniques are based upon the use of pre-computed projected cluster shapes or templates. A detailed simulation called Pixelav that has successfully described the profiles of clusters measured in beam tests of radiation-damaged sensors is used to generate the templates. Although the reconstruction technique was originally developed to optimally estimate the coordinates of hits after the detector became radiation damaged, it also has superior performance before irradiation. The technique requires a priori knowledge of the track angle which makes it suitable for the second in a two-pass reconstruction algorithm. However, the same modest angle sensitivity allows the algorithm to determine if the sizes and shapes of the cluster projections are consistent with the input angles. This information is proved to be useful in suppressing spurious hits caused by secondary particles and in validating seeds used in track finding. In addition, the template technique improves the resolution on the track impact parameter and light quark background rejection in b-tagged jets. Finally, a new procedure that uses the templates to reweigh clusters generated by the CMS offline simulation is described.