Nucleon Resonances in Completely Neutral Final States with the CBELSA/TAPS Experiment

• Applicants: Dr. Farah Noreen Afzal; Professorin Dr. Annika Thiel
• Subject Area: Nuclear and Elementary Particle Physics, Quantum Mechanics, Relativity, Fields
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 505387544

Matter is composed of quarks, which are bound inside the nucleons by the strong interaction. To shed light on this interaction, which is described by quantum chromodynamics (QCD), the excitation spectrum of the nucleons needs to be measured and compared to theoretical models. Since QCD is not solvable in the energy regime of the stable hadrons, calculations from constituent quark models or LatticeQCD need to be exploited. Comparisons with the experimental data show strong, so far insufficiently understood, differences. Experimentally, real photons can be used to determine the excitation spectrum of the nucleons. However, the measurement of unpolarized cross sections alone is not sufficient, as these are only sensitive to the squares of the amplitudes, and thus to the dominantly contributing resonances. Here, the measurement of (double) polarization observables offer a useful tool, using linear or circularly polarized photons in combination with a transversely or longitudinally polarized target. These observables are sensitive to the interference terms between the amplitudes and thus also to small resonance contributions. The CBELSA/TAPS experiment in Bonn is ideally suited to detect final states comprising photons and is therefore complementary to other international experiments. In the previous decades, extensive data sets were extracted for measurements off proton targets. Since the main calorimeter, the Crystal Barrel detector, provided only a low efficiency for a trigger on completely neutral final states, only a sparse data base exists there. To tackle this issue, the detector was upgraded to a new readout utilizing APDs. With this improvement, high statistics measurements off (polarized) neutron targets are within reach.In the last years, several discussions emerged about measurements off neutrons. For the final state off neutrons containing a single eta meson, a narrow, yet unidentified structure was observed, which does not appear for protons. Additionally, an analysis of the reaction nπ0π0 revealed major differences in the contributing partial waves compared to proton data. For a clarification, detailed measurements are needed.This project focuses on the measurement of double polarization observables for the completely neutral final states nπ0, nπ0π0 and nπ0η. This proposal aims to analyze data sets, utilizing linearly polarized photons impinging on a transversely polarized deuteron target in order to determine the polarization observables T, P and H. This enables the detailed investigation of the observed differences between neutrons and protons as well as for a search of narrow structures.

• DFG Programme: Research Grants