TRACKING IMPROVEMENT · Improvement of the Global Tracking Trigger algorithm for the upgrades of the ZEUS detector at HERA

Final Activity Report Summary - TRACKING IMPROVEMENT (Improvement of the Global Tracking Trigger algorithm for the upgrades of the ZEUS detector at HERA)

ZEUS, with dimensions of 12 x 10 x 19 m and a weight of 3 600 tons, was a detector situated on the Hadron electron ring accelerator (HERA) collider at Deutsches Elektronen-Synchrotron (DESY) in Hamburg. It was dedicated to identifying the nature and properties of particles produced by the high energy electron-proton collisions. The momentum and charge of the particles could be calculated through measuring and fitting their trajectories in a magnetic field, using various 'tracking' techniques. Two kinds of tracking algorithms were implemented, i.e. the online and offline algorithms. The online tracking algorithm, for the Global tracking trigger (GTT), was dedicated to reconstruct tracks within approximately 9 ms and provided quantities used by the second level of the ZEUS trigger to improve the selection of physics events. The offline algorithm reconstructed tracks with more precision within approximately 0.1 s and was used for almost all physics' analysis. During 2000 and 2001, HERA was upgraded, and a Micro-vertex detector (MVD) was introduced in the ZEUS detector and added to the tracking system. This sub-detector measured track trajectories with impact parameter resolutions of about 100 microns for tracks with a momentum of 1 GeV/c, increasing to about 50 microns for high momentum tracks. This precision allowed for the decays of charm and bottom hadrons to be tagged. For example, charged and neutral D mesons which contained a charm quark, had decay lengths of 315 and 123 microns respectively. Precision tracking measurements could resolve the vertex where a D meson decayed from the primary vertex; thereby they could identify such particles. Nevertheless, the ZEUS tracking algorithms had to be enhanced to include the additional measurements coming from the MVD. In this context, the project was divided in two parts. The first objective was the inclusion of the MVD in the GTT, which performed tracks and vertex reconstructions at the second level trigger of the ZEUS detector. This was completed successfully in the first year of the fellowship. The vertex position at the second level trigger was improved from 2 to 0.15 cm. The new code was tuned and tested in the online environment and was used for all physics' runs that started in February 2006. The second objective was the improvement of the offline tracking algorithms involving the MVD. At the end of the study, an improvement in the momentum resolution by a factor of around 1.4 was reached without affecting the resolutions of the other track parameters. The estimated errors on the track parameters were corrected by a factor of 1.2 and the computing time was reduced by a factor of eight. The new Kalman filter fit (KFFIT) algorithm was tested on data stored in 2005 using mass reconstruction of the K_s, lambda and J/psi particles, and improvements of up to a factor of 45 % on the mass resolution were observed. The new KFFIT algorithm was also used for the latest reprocessing of all data stored in 2004, which started in December 2006. The online and offline tracking algorithms were considerably improved, allowing for better selection, at trigger or analyse level, of interesting physics' events, such as heavy quarks production amongst many background events. The impact of those improvements on physics' quantities was anticipated to be seen as soon as the new reprocessed data was analysed.

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