Date of Award

12-15-2014

Document Type

Open Access Dissertation

Department

Physics and Astronomy

First Advisor

Steffen Strauch

Abstract

Meson production from excited nucleons is important in the study of baryon resonances and pion photoproduction is attracting much attention. To date a rather large amount of unpolarized cross-section measurements have been reported for both single- and double-pion photoproduction. However, polarization observables provide complementary information as they probe different combinations of transition amplitudes. The database for polarization observables remains quite sparse. Double-pion photoproduction have been studied in Hall B at Jefferson Lab with linearly polarized tagged photon beams incident on longitudinally polarized protons. The experiment covered center-of-mass energies between 1.4 GeV and 2.3 GeV. The target was a FROzen Spin butanol Target (FROST) and the final-state particles were detected by the CEBAF Large Acceptance Spectrometer (CLAS). In various polarization configurations, asymmetries of the experimental yields are constructed to extract polarization observables. In order to evaluate the background from unpolarized bound nucleons in the butanol target, the data collected from an additional unpolarized carbon target is used. A set of single- and double-polarization observables, Pz, Ps z and Pc z were extracted; the double-polarization observables for the first time. The double-pion observables show even or odd symmetries, as expected by parity conservation, and are compared with results of an effective Lagrangian model by A. Fix. The model predictions have the same symmetry behavior as the data and resemble main features of the data in most kinematic bins. In the comparison with models, the data test our understanding of the nucleon structure, establishes nucleon excitation and non-resonant reaction amplitudes, and possibly help to identify new baryon resonances.

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Physics Commons

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