Mainz 2022 – scientific programme
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HK: Fachverband Physik der Hadronen und Kerne
HK 20: Hadron Structure and Spectroscopy IV
HK 20.1: Group Report
Monday, March 28, 2022, 16:00–16:30, HK-H9
Polarisation observables Σ, T, P, and H in π0 and η photoproduction off quasifree nucleons — •Nicolas Jermann for the CBELSA/TAPS collaboration — University of Basel, Switzerland
The excitation spectrum of the nucleon is an important testing ground for quantum chromodynamics in the regime where it cannot be treated perturbatively. During the last two decades much progress has been made on the theory side, e.g. lattice gauge methods, and in experiments, particularly using energy tagged photon beams at electron accelerators, which has now reached a state where not only differential cross sections but also asymmetries measured with polarised photons and polarised targets allow for detailed partial wave analyses. This provides much more stringent information about the involved reaction multipoles and thus the contributing nucleon resonances.
The present experiment was done at the ELSA accelerator in Bonn with the CBELSA/TAPS detector setup. The incident electron beam of 3.2 GeV impinged on a diamond radiator where it produced coherent bremsstrahlung photons with linear polarisation, which again impinged on a transversely polarised, deuterated butanol target. This allows the simultaneous measurement of the polarisation observables Σ, T, P, and H. Analysed were the final states Nπ0 and Nη with the almost 4π covering electromagnetic calorimeter CBELSA/TAPS.
One of the main motivations of this experiment was a more detailed investigation of the not yet understood narrow structure in the excitation function of the γ n → n η reaction at approximately 1 GeV. Preliminary results will be discussed.