Inclusive e+e- pair production in pp collisions at 2.2 GeV

The work presented reports on the inclusive measurement of e+e- pairs in pp collisions at a beam energy of Ekin=2.2 GeV using the HADES spectrometer. HADES focuses on the measurement of e+e- pairs which are an appropriate probe to study properties of hadronic matter in heavy ion collisions. Utilizing well-known sources, the inclusive analysis of the e+e- pair signal from pp collisions gives deeper insight in the understanding of the properties of the detector system. On the other hand vaguely known additional sources are for the first time accessible with a higher precision, which is important for the later understanding of heavy ion data. One main part of the work was to perform large Monte Carlo simulations that provided a reference data sample for comparison in each analysis step. The first step was the calculation of production cross sections for resonance states which are needed as an input for the event generator Pluto++. The events produced have then been propagated through a full-scale GEANT-based detector simulation to create a realistic detector response. This work puts effort on the correct treatment of particularities of the spectrometer set-up in the pp experiment. The single particle background was strongly reduced by applying special cuts to remove electrons and positrons that originate from γ-conversion. This resulted in an overall signal-to-background-ratio (S)/(B)∼4.7, while the invariant mass region above the π0 mass - which is dominated by the η Dalitz decay (η→e+e-) still has (S)/(B)∼2.0. The remaining pair background has been calculated using a statistical analysis of like-sign pair distributions (e+e+ and e-e-) as well as uncorrelated e+e- pairs from event mixing. The remaining signal contains 2020±53 e+e- pairs and is dominated by the two dalitz decays π0→γe+e- and η→γe+e-, respectively. Comparisons with the resulting simulation data then allowed to study the decay η→e+e- that has not yet been measured. From this a preliminary new upper limit for its branching ratio was calculated as (Γe+e-)/(Γtot)<4.5 x 10-5. Finally, the resulting spectra have been efficiency corrected and compared with data from CC collisions at Ekin=2.0 AGeV (also measured with the HADES spectrometer). It was observed that the strict trigger condition used during the pp experiment does not allow to apply a simple scaling of e+e- yields with the number of participants without deploying additional corrections. (orig.)