arXiv2003
Experiments that study the hadronic and electromagnetic production of the pseudoscalar mesons -- pions, etas and kaons, contribute to our knowledge of the properties of baryon and hyperon resonances. Fixed-target programs at hadronic facilities such as BNL-AGS have been phased out. However, the availability of modern experimental facilities with pseudo-monochromatic or tagged medium-energy photon beams at GRAAL, SPring-8, Bonn, Mainz, and Jefferson Lab, together with LEGS, Max-Lab, and HIGS at lower energies, are beginning to produce high-quality results. These new data have smaller statistical uncertainties and better understood systematic uncertainties, than those obtained at the older bremsstrahlung facilities, for measurements of differential and integrated cross sections, as well as polarization and asymmetry. Experimental results are compared with the predictions of QCD-based approaches, such as the lattice-gauge calculations of baryon properties, and Chiral Perturbation Theory applied to threshold photoproduction, and are essential to the performance of Partial-Wave Analyses (PWA). These PWA studies are less model dependent than in the past, and are used in coupled-channels calculations that incorporate unitarity dynamically, and combine results from hadronic reaction channels with electromagnetic processes. This approach is necessary to extract resonance properties and may lead to the discovery of the "missing resonances" predicted by a number of different QCD-inspired calculations. We discuss recent experimental and phenomenological results for single and double pseudoscalar meson hadronic and photoproduction channels with emphasis on the JLab Hall B and the BNL/AGS Crystal Ball programs.