Double polarisation experiments in meson photoproduction

TL;DR

This paper reviews recent double polarisation experiments in meson photoproduction, highlighting their role in advancing baryon spectroscopy and understanding nucleon resonances through improved partial wave analyses.

Contribution

It presents new experimental data from polarized beam and target experiments, enhancing the precision of resonance parameter determination in baryon spectroscopy.

Findings

Unprecedented quality of experimental results

Improved partial wave analysis accuracy

Enhanced understanding of nucleon excitation spectrum

Abstract

One of the remaining challenges within the standard model is to gain a good understanding of QCD in the non-perturbative regime. A key step towards this aim is baryon spectroscopy, investigating the spectrum and the properties of baryon resonances. To gain access to resonances with small $\pi N$ partial width, photoproduction experiments provide essential information. Partial wave analyses need to be performed to extract the contributing resonances. Here, a complete experiment is required to unambiguously determine the contributing amplitudes. This involves the measurement of carefully chosen single and double polarisation observables. In a joint endeavour by MAMI, ELSA, and Jefferson Laboratory, a new generation of experiments with polarised beams, polarised proton and neutron targets, and $4\pi$ particle detectors have been performed in recent years. Many results of unprecedented quality were recently published by all three experiments, and included by the various partial wave analysis groups in their analyses, leading to substantial improvements, e.g. a more precise determination of resonance parameters. An overview of recent results is given, with an emphasis on results from the CBELSA/TAPS experiment, and their impact on our understanding of the nucleon excitation spectrum is discussed.