Exclusive pi0 production at HERMES: detection - simulation - analysis
The Hermes experiment was built to determine the spin structure of the proton. It is already known since a long time that the proton is built out of three quarks. However, experiments have shown that the quarks itself are responsible for only about 30 % of the proton spin. The remaining parts are expected to be coming from orbital angular momenta and gluon spin. The framework of Generalized Parton Distributions (GPDs) provides access to the hitherto unknown quark orbital angular momentum. These GPDs can be probed via the measurement of so-called exclusive reactions. In these reactions the initial and final state are fully known. In the Hermes experiment 27.5 GeV beam particles are scattered off an internal gas target. To improve the measurement of exclusive reactions a recoil detector surrounding the target cell was designed and built to deremine exclusivity at the event level and to measure the kinematics of the recoiling proton. This work reports on the R&D for the readout of the silicon recoil detector. A dedicate charge injection setup, a laser test-stand and a testbeam setup are described. In modelling the GPDs exclusive pion production plays an important role. By comparing the exclusive pi^0 production rate with the exclusive pi^+ production rate, constraints can be imposed on the relative importance of the pion pole to the GPDs. Moreover, by studying pion production information about the polarized quark distribution functions can be extracted. In Hermes, the pi^0 is measured via the detection of its two decay photons in the electromagnetic calorimeter. To determine the detection capabilities of the latter device, a dedicated Monte Carlo simulation was mandatory. The simulation is based on a combination of the GEANT package and a 6-dimensional look-up table. The analysis of the Hermes data shows a small pi^0 cross section. Additionally, when examining the production ratio on the proton and neutron, a clear neutron suppression is observed for the pi^0 (and the eta) in the exclusive regime. This suppression is absent in the semi-inclusive region.
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