Measurement of the double polarization observables <em>G</em> and <em>E</em> in neutral and positive pion photoproduction off the proton

Abstract

The nucleons, as one of the most fundamental building blocks of visible matter, are built up of quarks and gluons. However, the interactions and dynamics inside the nucleons, which are mediated by the strong force and described by Quantum Chromodynamics (QCD), are still a major challenge to modern physics. In the mass region of the excited states of the nucleons, known as resonances, perturbative calculations are not feasible in QCD, and thus phenomenological approaches and numerical methods are essential for the description of the strong interaction. From the experimental side, complementary probes are available to study the excitation spectrum of the nucleon and different sensitivities to unique observables provide stringent tests of the available theoretical approaches. In the past, most resonances have been found in pion-nucleon scattering experiments. However, the meson photoproduction reactions gained more attention during the last decades since several resonances might only weakly couple to the pion-induced reactions. Although over the years a lot of experimental effort from several collaborations at various accelerator facilities was conducted, discrepancies between theoretical models and experimental results are still present, e.g., regarding the number of resonances. From the experimental point of view, the short lifetimes of the resonances are a possible explanation for these discrepancies. They lead to many broad and overlapping resonances that even contribute selectively to distinct decay channels. The measurement of the total unpolarized cross sections alone is not able to identify all resonances since they are only sensitive to the dominant contributing resonances. For the identification of all resonances and their properties, an unambiguous Partial Wave Analysis (PWA) solution needs to be obtained. This requires precise knowledge of several well-chosen single and double polarization observables in different decay channels. Therefore, current experiments measure these polarization observables that are sensitive to weakly contributing resonances. They can be accessed by either polarizing the initial state, by measuring the polarization of the recoiling nucleon in the final state, or even both.<br /> One experiment, which can measure polarization observables through the meson photoproduction process, is the Crystal Ball experiment within the A2 collaboration. It is located at the electron accelerator MAMI in Mainz, Germany. The <em>G/E</em> measurements made use of elliptically polarized photons that have a linearly and circularly polarized component at the same time. They are produced by the interaction of a longitudinally polarized electron beam with a diamond radiator. In combination with a longitudinally polarized butanol target, the elliptically polarized photons are used to obtain for the first time the double polarization observables <em>G</em> and <em>E</em> simultaneously in one experimental run. Whereas for the determination of <em>E</em> the circular polarization component is needed, the linear polarization component of the photon beam allows to access <em>G</em> with the same experiment.<br /> The double polarization observable <em>G</em> was determined for the reactions <em>γp→pπ⁰</em> and <em>γp→nπ⁺</em> in a beam photon energy range from 230 MeV to 840 MeV (<em>W</em> = 1145 MeV - 1569 MeV) with a full angular coverage. In addition, the double polarization observable <em>E</em> was extracted in the same beam photon energy range for the <em>γp→nπ⁺</em> reaction. Background contamination could be effectively reduced and the desired reaction channels were clearly identified. Asymmetries in the selected azimuthal angular distribution of the final state meson were used to extract the double polarization observable <em>G</em>. For the extraction, two methods were used, namely a <em>χ</em>²-fit to the binned event yield asymmetries and an unbinned maximum likelihood fit. The double polarization observable <em>E</em> was determined via the carbon subtraction method.<br /> The results for the double polarization observable <em>G</em> in both analyzed channels provide the first precise measurement in the first resonance region and parts of the second resonance region. Furthermore, the new A2 data in the <em>pπ⁰</em> channel close the gap between the region of the Δ(1232)3/2⁺(<em>P</em>₃₃) resonance and the already existing data in the second resonance region. Both data sets are in good agreement with each other in the overlapping region. Furthermore, the high sensitivity of the double polarization observable <em>G</em> to weakly contributing resonances in combination with the new precise data reveal even contributions of partial waves with angular momentum up to <em>l</em>=4 (<em>G</em>-waves) in the first resonance region. In the future, the measurement of both channels will allow an isospin separation of the partial waves and might help, in particular, to improve the uncertainties of the partial wave <em>P</em>₁₁ to which the Roper resonance <em>N</em>(1440)1/2⁺(<em>P</em>₁₁) couples.<br />For the double polarization observable <em>E</em> in the <em>nπ⁺</em> channel, a very good agreement between the measurements with elliptically and circularly polarized photons was observed. Therefore, the results of this work give experimental evidence that an elliptically polarized photon beam can be used to extract the double polarization observable <em>E</em>. In comparison to the existing data, the new A2 data provide additional points in the backward direction of the charged pion and for beam photon energies below 350 MeV. In the overlapping regions, the data sets agree very well with each other.<br />Overall, the results of the polarization observables still show notable differences to recent PWA models. Therefore, the new data will help to further constraint the different models.<br />