Development of a transition radiation detector and reconstruction of photon conversions in the CBM experiment

The focus of this thesis is the development of a Transition Radiation Detector (TRD) for the Compressed Baryonic Matter (CBM) experiment at the future Facility for Antiproton and Ion Research (FAIR) in Darmstadt. In addition, the usage of the TRD in the measurement of direct photons is investigated. CBM will be a fixed-target heavy-ion experiment, which investigates collisions in the beam energy range of 5-35 AGeV and aims to investigate the regime of high baryon densities where the phase transition is expected to be of first order. It will be a multipurpose experiment with the ability to measure leptons, hadrons, and photons. Therein, a TRD will provide the electron identification and - together with a Silicon Tracking System (STS) - the tracking of charged particles. In conjunction with a ring imaging Cherenkov (RICH) detector and a time-of-flight (TOF) measurement, the TRD is to provide a sufficient electron identification for the measurements of charmonium and low-mass vector mesons. For the TRD, the required pion suppression is a factor of about 100 at 90% electron efficiency, and the position resolution has to be of the order of 300 to 500 um. Moreover, the material budget in terms of radiation length has to be kept at a minimum in order to minimize multiple scattering and conversions which would limit the precise measurement in following TRD stations and other detectors. The largest and up to now unrivaled challenge for the TRD design is that both (PID and tracking) have to be fulfilled in the context of very high particle rates (event rates of up to 10MHz are envisaged) and at the same time large charged-particle multiplicities of up to 600 per event in the CBM detector acceptance. Small prototypes of the TRD based on multiwire proportional chambers (MWPC) with pad readout were developed and tested. The tracking performance and the electron-pion separation were determined for particle rates of up to 200 kHz/cm2. The TRD layout and the detector responses measured in the test beam were implemented in the existing CBM simulation framework in order to perform realistic physics simulations. A feasibility study for the measurement of photons and neutral pions, decaying into photons, via photon conversions in the target is performed in simulations using the TRD layout developed in hardware. In CBM, the ECAL is designated to provide photon identification. In addition, a complementary measurement of dielectrons from photon conversions can be used to increase the precision of the photon measurement. The e+e- pairs from photon conversions in the target can be measured with the charged particle tracking (STS and TRD) and electron identification detectors (RICH, TRD, and TOF). The measurement of direct photons requires precise knowledge of the photon contribution from decays (mainly π0→γ-γ) to the inclusive photon spectrum. The π0-signal is determined through an invariant mass analysis of reconstructed photon pairs. (orig.)