Helicity dependence of pion photoproduction on the proton: new results from Mainz Original Research Article

The helicity dependence of all single- and double-pion photoproduction channels on the proton has been measured for the first time at incident photon energies from 200 to 800 MeV. The measurement at the tagged photon beam facility of the MAMI accelerator in Mainz used the large acceptance detector DAPHNE and a frozen-spin target. This completely new and up to now inaccessible observable represents a very sensitive tool to study baryon resonances since the smaller resonant multipoles can be accessed via the interference with the dominant ones. The most recent results from the Mainz experiment are presented, with a particular emphasis on the second resonance region, where there are clear systematic discrepancies with the existing multipole analyses. In this region, double polarization data are mostly sensitive to the properties of the D13(1520) resonance and can be used to determine its properties much more accurately than in previous measurements.