Performance of the HADES spectrometer for dilepton identification in the reaction C+C at 1-2 AGeV

In the framework of the present thesis a detailed study of the performance of the HADES trigger is presented. To investigate and characterize the functionality of this trigger, all hardware components have been emulated with software. The most selective algorithm of the second level trigger is the identification of Cherenkov rings from electrons traversing the RICH, which alone can provide an event reduction factor higher than 10 for C+C collisions. The behavior of the online ring recognition has been studied with the help of simulations, where an efficiency of around 85% has been measured, and by comparing it to the o.ine analysis algorithm. Both online and offline ring recognition algorithms are dominated by misidentified rings, which have to be rejected in order to make a proper comparison. The rejection is done with the selection of a lepton candidate sample with an analysis, developed and performed in the framework of the present thesis, which makes used of the information collected by the full detector. Depending upon the conditions, the online ring finder is capable to recognize up to 80% of the lepton candidates reconstructed with the offline analysis. The second level trigger was fully operational during the beamtime of November 2002 with an event reduction by a factor 12. It has been estimated that higher reduction, up to a factor 20 are achievable without further loss of efficiency. Due to a more restrictive implementation of the ring recognition algorithm, a lepton efficiency of 62% was measured, while 84% efficiency was estimated for dileptons with opening angle larger than 4., and 92% for opening angle larger than 8. However no physical bias was introduced in the data, as the agreement between leptons and dileptons spectra for triggered and untriggered events shows. In the triggered events an enhancement by a factor 7.5 is found in the lepton yield, and by a factor 11 in the open pairs dilepton yield with respect to the untriggered ones. (orig.)