Measurement of Polarization Observables in the Reaction γp → pπ0π0 with the CBELSA/TAPS Experiment
Measurement of Polarization Observables in the Reaction γp → pπ0π0 with the CBELSA/TAPS Experiment
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DissertationDate of Exam
10.12.2025Date of Publication
20.03.2026Advisor
Thoma, UlrikeCo-Referee
Beck, ReinhardMetadata
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Abstract
Quantum Chromodynamics (QCD), the theory of the strong interaction, presents significant challenges as perturbation theory cannot be applied in the energy region where hadrons form. A promising approach to probe QCD in this region is the study of the excitation spectrum of baryons. However, classical (unpolarized) cross section measurements alone are insufficient to isolate the contributions of individual excited states. To disentangle overlapping resonance contributions, measurements of polarized cross sections are performed, which further constrain the spin degrees of freedom. Extracted polarization observables are then used in partial wave analysis to gain insight into resonance parameters and their properties. The CBELSA/TAPS experiment at the ELSA accelerator facility in Bonn makes an important contribution to the field of baryon spectroscopy by studying meson photoproduction off protons or neutrons.
In this work, the reaction γp → pπ0π0 has been analyzed using a linearly polarized photon beam and a transversely polarized target. The cross section for these multi-meson final states remains large at energies where many predicted resonances remain unobserved. They are sensitive to higher-mass resonances with significant branching ratios to other excited states. Despite these advantages, the existing database for multi-meson photoproduction still leaves room for improvement.
Four different periods of data taking were analyzed. In total, 8.17 × 105 events on quasi-free protons with a pπ0π0 final state were selected in a beam photon energy range of 650 − 3100 MeV and an average reconstruction efficiency of 22 %. At the same time, the average background was reduced to < 2 %. Events on bound protons were successfully subtracted using reference measurements with carbon foam as a target material. Using an event-based likelihood fit, polarization observables were extracted for different combinations of five kinematic variables. The single polarization observables Px and Py were extracted in a beam photon energy range of 650 − 3100 MeV with high statistics. The double polarization observables Pxs, Pys, Pxc, and Pyc were extracted in a beam photon range of 850 − 1700 MeV. This is the first measurement of these double polarization observables at such high energies.
In addition, a recent upgrade of the CBELSA/TAPS experiment has improved both the readout electronics and the trigger system. Therefore, this work also includes an investigation of the newly available time information for the Crystal Barrel detector and its incorporation into the reconstruction process of particles. The time information was further used to validate the pile-up detection and correction of the newly installed sampling-analog-to-digital converters (SADCs), revealing significant shortcomings. These shortcomings were subsequently resolved in a separate work.
In this work, the reaction γp → pπ0π0 has been analyzed using a linearly polarized photon beam and a transversely polarized target. The cross section for these multi-meson final states remains large at energies where many predicted resonances remain unobserved. They are sensitive to higher-mass resonances with significant branching ratios to other excited states. Despite these advantages, the existing database for multi-meson photoproduction still leaves room for improvement.
Four different periods of data taking were analyzed. In total, 8.17 × 105 events on quasi-free protons with a pπ0π0 final state were selected in a beam photon energy range of 650 − 3100 MeV and an average reconstruction efficiency of 22 %. At the same time, the average background was reduced to < 2 %. Events on bound protons were successfully subtracted using reference measurements with carbon foam as a target material. Using an event-based likelihood fit, polarization observables were extracted for different combinations of five kinematic variables. The single polarization observables Px and Py were extracted in a beam photon energy range of 650 − 3100 MeV with high statistics. The double polarization observables Pxs, Pys, Pxc, and Pyc were extracted in a beam photon range of 850 − 1700 MeV. This is the first measurement of these double polarization observables at such high energies.
In addition, a recent upgrade of the CBELSA/TAPS experiment has improved both the readout electronics and the trigger system. Therefore, this work also includes an investigation of the newly available time information for the Crystal Barrel detector and its incorporation into the reconstruction process of particles. The time information was further used to validate the pile-up detection and correction of the newly installed sampling-analog-to-digital converters (SADCs), revealing significant shortcomings. These shortcomings were subsequently resolved in a separate work.
Classification (DDC)
530 PhysikStausberg, Nils: Measurement of Polarization Observables in the Reaction γp → pπ0π0 with the CBELSA/TAPS Experiment. - Bonn, 2026. - Dissertation, Rheinische Friedrich-Wilhelms-Universität Bonn.
Online-Ausgabe in bonndoc: https://nbn-resolving.org/urn:nbn:de:hbz:5-88840
Online-Ausgabe in bonndoc: https://nbn-resolving.org/urn:nbn:de:hbz:5-88840
@phdthesis{handle:20.500.11811/13989,
urn: https://nbn-resolving.org/urn:nbn:de:hbz:5-88840,
author = {{Nils Stausberg}},
title = {Measurement of Polarization Observables in the Reaction γp → pπ0π0 with the CBELSA/TAPS Experiment},
school = {Rheinische Friedrich-Wilhelms-Universität Bonn},
year = 2026,
month = mar,
note = {Quantum Chromodynamics (QCD), the theory of the strong interaction, presents significant challenges as perturbation theory cannot be applied in the energy region where hadrons form. A promising approach to probe QCD in this region is the study of the excitation spectrum of baryons. However, classical (unpolarized) cross section measurements alone are insufficient to isolate the contributions of individual excited states. To disentangle overlapping resonance contributions, measurements of polarized cross sections are performed, which further constrain the spin degrees of freedom. Extracted polarization observables are then used in partial wave analysis to gain insight into resonance parameters and their properties. The CBELSA/TAPS experiment at the ELSA accelerator facility in Bonn makes an important contribution to the field of baryon spectroscopy by studying meson photoproduction off protons or neutrons.
In this work, the reaction γp → pπ0π0 has been analyzed using a linearly polarized photon beam and a transversely polarized target. The cross section for these multi-meson final states remains large at energies where many predicted resonances remain unobserved. They are sensitive to higher-mass resonances with significant branching ratios to other excited states. Despite these advantages, the existing database for multi-meson photoproduction still leaves room for improvement.
Four different periods of data taking were analyzed. In total, 8.17 × 105 events on quasi-free protons with a pπ0π0 final state were selected in a beam photon energy range of 650 − 3100 MeV and an average reconstruction efficiency of 22 %. At the same time, the average background was reduced to < 2 %. Events on bound protons were successfully subtracted using reference measurements with carbon foam as a target material. Using an event-based likelihood fit, polarization observables were extracted for different combinations of five kinematic variables. The single polarization observables Px and Py were extracted in a beam photon energy range of 650 − 3100 MeV with high statistics. The double polarization observables Pxs, Pys, Pxc, and Pyc were extracted in a beam photon range of 850 − 1700 MeV. This is the first measurement of these double polarization observables at such high energies.
In addition, a recent upgrade of the CBELSA/TAPS experiment has improved both the readout electronics and the trigger system. Therefore, this work also includes an investigation of the newly available time information for the Crystal Barrel detector and its incorporation into the reconstruction process of particles. The time information was further used to validate the pile-up detection and correction of the newly installed sampling-analog-to-digital converters (SADCs), revealing significant shortcomings. These shortcomings were subsequently resolved in a separate work.},
url = {https://hdl.handle.net/20.500.11811/13989}
}
urn: https://nbn-resolving.org/urn:nbn:de:hbz:5-88840,
author = {{Nils Stausberg}},
title = {Measurement of Polarization Observables in the Reaction γp → pπ0π0 with the CBELSA/TAPS Experiment},
school = {Rheinische Friedrich-Wilhelms-Universität Bonn},
year = 2026,
month = mar,
note = {Quantum Chromodynamics (QCD), the theory of the strong interaction, presents significant challenges as perturbation theory cannot be applied in the energy region where hadrons form. A promising approach to probe QCD in this region is the study of the excitation spectrum of baryons. However, classical (unpolarized) cross section measurements alone are insufficient to isolate the contributions of individual excited states. To disentangle overlapping resonance contributions, measurements of polarized cross sections are performed, which further constrain the spin degrees of freedom. Extracted polarization observables are then used in partial wave analysis to gain insight into resonance parameters and their properties. The CBELSA/TAPS experiment at the ELSA accelerator facility in Bonn makes an important contribution to the field of baryon spectroscopy by studying meson photoproduction off protons or neutrons.
In this work, the reaction γp → pπ0π0 has been analyzed using a linearly polarized photon beam and a transversely polarized target. The cross section for these multi-meson final states remains large at energies where many predicted resonances remain unobserved. They are sensitive to higher-mass resonances with significant branching ratios to other excited states. Despite these advantages, the existing database for multi-meson photoproduction still leaves room for improvement.
Four different periods of data taking were analyzed. In total, 8.17 × 105 events on quasi-free protons with a pπ0π0 final state were selected in a beam photon energy range of 650 − 3100 MeV and an average reconstruction efficiency of 22 %. At the same time, the average background was reduced to < 2 %. Events on bound protons were successfully subtracted using reference measurements with carbon foam as a target material. Using an event-based likelihood fit, polarization observables were extracted for different combinations of five kinematic variables. The single polarization observables Px and Py were extracted in a beam photon energy range of 650 − 3100 MeV with high statistics. The double polarization observables Pxs, Pys, Pxc, and Pyc were extracted in a beam photon range of 850 − 1700 MeV. This is the first measurement of these double polarization observables at such high energies.
In addition, a recent upgrade of the CBELSA/TAPS experiment has improved both the readout electronics and the trigger system. Therefore, this work also includes an investigation of the newly available time information for the Crystal Barrel detector and its incorporation into the reconstruction process of particles. The time information was further used to validate the pile-up detection and correction of the newly installed sampling-analog-to-digital converters (SADCs), revealing significant shortcomings. These shortcomings were subsequently resolved in a separate work.},
url = {https://hdl.handle.net/20.500.11811/13989}
}
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