Measurement of polarisation observables for a circularly polarised photon beam and a transversally polarised target in the photoproduction of neutral pion pairs off the proton

The inner structure of nucleons and thus the fundamental interaction between the constituents, the quarks, can be investigated by means of the hadron spectroscopy. On the basis of the hadron spectroscopy, the different excitation states of the proton, the so-called resonances, can be determined, and thus also the possible degrees of freedom. Since the degrees of freedom determine the number of excited states, conclusions can then be done about the structure of the nucleons. However, the previously measured excitation spectrum of the proton cannot yet be fully described with the theoretical predictions of the quark models or the lattice gauge theories. Significantly more resonances are predicted at higher energies which so far could not be found in the experiments. This phenomenon is known as the problem of the missing resonances. But so far it is unclear, if these discrepancies are caused by assumed degrees of freedom in the theoretical models which are not realisable in nature or from the experimental bias. Due to their low lifetime, the different resonances of the excitation spectrum overlap and are difficult to identify. Through the polarisation observable measurements, the individual resonance contributions can be identified more easily because they are more sensitive to interference terms and therefore also to the weak resonance contributions. For this purpose the polarisation observable for the double π0 photoproduction off the free proton were analysed. The double π0 photoproduction is one of the most interesting reaction for the measurement of these observables. It allows to search for excited nucleon states which decay preferentially via cascades involving intermediate excited states. Furthermore, the background of non-resonant terms is small since the photon does not couple directly to neutral pions. The measurement of the double polarisation observable F and the single polarisation observable T were performed at the MAMI tagged photon facility in Mainz, Germany, using circularly polarised bremsstrahlung photons with incident energies from 450 up to 1450 MeV and a transversally polarised butanol target. The double π0 reaction was identified using a combined setup of the Crystal Ball calorimeter and a TAPS forward wall and additionally the PID and the MWPCs for the charged particle tracking, which results in an almost 4π acceptance. The polarisation observables were extracted from the data sets by the use of two different methods, the carbon subtraction method and the hydrogen normalisation method. It could be shown that the polarisation observable results for both methods are consistent. In order to check possible systematic uncertainties from the measurement with butanol targets also the absolutely normalised unpolarised cross sections were extracted and composed to results from measurements with liquid hydrogen targets. The experimental results were measured for further studies of the partial wave content of the double pion photoproduction in the second and third resonance regions. Therefore, the measured polarisation observables are compared to two different predictions of partial waves analysis, the isobar MAID model and the Bonn-Gatchina partial wave analysis. The measured results show distinct differences to the predictions and deliver new information for partial wave analysis.

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