Afzal, Farah Noreen: Measurement of the beam and helicity asymmetries in the reactions γp → pπ0 and γp → pη. - Bonn, 2019. - Dissertation, Rheinische Friedrich-Wilhelms-Universität Bonn.
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author = {{Farah Noreen Afzal}},
title = {Measurement of the beam and helicity asymmetries in the reactions γp → pπ0 and γp → pη},
school = {Rheinische Friedrich-Wilhelms-Universität Bonn},
year = 2019,
month = sep,

note = {Nucleons are bound states composed of quarks and gluons. The study of the nucleon excitation spectra gives insight into the underlying dynamics between quarks and gluons that are governed by the strong interactions and described by Quantum Chromodynamics (QCD). However, the non-perturbative regime of QCD is not well understood, e.g. discrepancies exist between theoretically predicted and experimentally observed excited states. To solve this issue, the study of the photoproduction of mesons offers an important experimental tool to increase the sensitivity to resonances that couple only weakly to the πN channel. However, precise data regarding photoproduction reactions exist mostly for the unpolarized cross section, which can not determine all complex amplitudes needed to describe the photoproduction process unambiguously on its own. In order to achieve this goal in partial wave analyses (PWAs), a so-called complete experiment is needed. Here, single and double polarization observables have to be measured additionally, utilizing polarized photon beams, polarized targets and by measuring the polarization degree of the recoil nucleon.
For this work, data were taken with the CBELSA/TAPS experiment at the accelerator facility ELSA in Bonn and with the A2 experiment at the accelerator facility MAMI in Mainz. Both experimental facilities probe nucleons within a fixed target electromagnetically, using a real photon beam. The CBELSA/TAPS data were taken with a linearly polarized photon beam and an unpolarized liquid hydrogen target in order to access the beam asymmetry Σ for the reactions γp → pπ0 and γp → pη at beam photon energies ranging from 1130 MeV to 1790 MeV.
An elliptically polarized photon beam in combination with a polarized butanol target was used at the A2 experiment, which enabled the measurement of two double polarization observables, namely G and the helicity asymmetry E, at the same time for both the pπ0 and pη final states. The simultaneous measurement of G and E has not been attempted before and offers the opportunity to significantly reduce the required beamtime to measure all single and double polarization observables involving beam and target polarization measurements.
After calibrating both data sets, the pπ0 and pη final states were selected with low background contributions. Subsequently, the beam asymmetry Σ was determined using two different fit methods, firstly a binned X2-fit and secondly an unbinned maximum likelihood fit. The helicity asymmetry E was determined utilizing the carbon subtraction method, which required an intensive study of background contributions stemming from bound nuclei within the employed butanol target.
The new CBELSA/TAPS and A2 data provide one of the most precise data sets of measured polarization observables until now. The high statistical precision allows the observation of the pη and pη’ cusp-effects in the data. Additionally, the extracted data for the beam asymmetry Σ have such a high statistical precision that sensitivity up to partial waves corresponding to an angular momentum quantum number l = 4 (G-waves) could be determined. Moreover, the data for the helicity asymmetry E close the existing gap of the database down to the Δ(1232)3/2+ (P33) resonance in the pπ0 final state and show with high precision a good agreement to existing data, whereby the latter were taken using only circularly polarized photons. Thus, first experimental evidence is given in this work for the fact that the double polarization observable E can be measured with an elliptically polarized photon beam.},

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