Measurement of double polarization observables in the reactions γp→pπ⁰ and γp→pη with the crystal barrel/TAPS experiment at ELSA

One of the remaining challenges within the standard model of particle physics is to gain a good understanding of QCD in the non-perturbative regime. A key step toward this aim is to obtain further insight into the structure of baryons and their excitation spectrum. To gain access to resonances with small πN partial width, photoproduction experiments provide essential information. However, the measurement of (unpolarized) cross sections is insufficient to unambiguously determine the contribution amplitudes in a partial wave analysis. The spin degrees of freedom need to be constrained further, approaching the complete experiment requiring single and double polarization measurements with polarized beams, polarized targets and recoil polarimetry. This work presents such a double polarization measurement performed with the Crystal Barrel/TAPS experiment at the accelerator facility ELSA in Bonn. The photoproduction of single π⁰ and η mesons with a linearly polarized photon beam impinging on a transversely polarized proton target was investigated.The reactions were identified nearly background-free by fully reconstructing the pγγ final state in an incoming photon energy range from 640 MeV to 3000 MeV. A new method was developed to simultaneously extract all accessible polarization observables from the data using an event-based maximum-likelihood fit. Using this method, the target asymmetry T was determined in the full energy range as a function of the beam energy and the scattering angle. In addition, the recoil polarization P and the double polarization observable H were determined in the incoming photon energy range below 975 MeV, where a high degree of linear polarization of the photon beam has been achieved. The results are more precise compared to earlier data from other experiments, and extend the covered range in energy and angle substantially. It is the first measurement of the observable H in the given energy range. Systematic errors were investigated in detail and found to be significantly smaller compared to the statistical uncertainty of the results. Within the Bonn-Gatchina partial wave analysis, the new results lead to a significant narrowing of the error band for the π⁰ photoproduction multipoles, a more precise determination of resonance parameters, in particular N^*→Nη branching ratios, further evidence for the poorly known baryon resonance Δ(2200)7/2^-, and indications for a new resonance N(2200)5/2^-.