Speaker
Description
A detailed study of the nucleon resonances and their decays is an essential step towards exploring the low energy region of QCD. By analysing the excitation spectrum of (quasi-) free nucleons we are able to establish a baseline for comparisons with in-medium modifications. Of particular interest are the mixed-charged channels of double-pion photoproduction, as they are sensitive to sequential decays of $\Delta$ resonances, but also to the charged $\rho$ channel. The coupling to a $\rho$ could induce substantial in-medium effects when the $\rho$ spectral function is modified in the medium. Recent results from data taken with the A2 experiment at MAMI are presented and compared to recent MAID model calculations up to beam energies of 1500 MeV. The unpolarized cross sections could be obtained by collecting data with a liquid hydrogen target. The analysis shows a significant contribution of the $\rho$ channel to the second resonance peak.
| speaker affiliation | Universität Basel |
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