Photoproduction of pion pairs off nucleons

In the absence of a solution for Quantum Chromodynamics (QCD) in the low energy regime, so called effective models are being used to describe the nucleon and
its excited states. These models include the basic symmetries from QCD, but on the other hand, compared to quarks and gluons, use higher lying degrees of freedom.
Experimental contributions are mandatory to validate these models and fix free parameters. Today still most of the world-data in this field was obtained by
meson-induced excitation of the nucleon. Even though numerous excited states of the nucleon could be identified, the number of model predicted states is much
higher. This is known as the missing resonance problem. One explanation could be that some excited states just couple weakly to pion-N (kaon-N) and hence the
excitation via photons was proposed to further test the model predictions.
During the last 15 years, much experimental effort was made at various photoproduction facilities like MAMI, ELSA, JLab or ESRF and a large number of states
could be confirmed, but the missing resonance problem could not be solved. Higher lying resonances (M > 1.6 GeV) decay preferably via sequential decays with
many meson final states, and especially double pion decay channels are assumed to dominate in this region. The reactions investigated in this work, namely
g+p(n)->pion^0+pion^0+p(n), g+n(p)->pion^0+pion^0+n(p), g+p->pion^0+pion^0+p, g+p(n)->pion^++pion^0+n(n), g+n(p)->pion^-+pion^0+p(p), g+p->pion^++pion^0+n
thus form the primary source of information on photocouplings of higher lying resonances.
This work explores neutral and mixed-charge double pion production channels up to invariant masses of the final state center-of-mass system of about 1.9 GeV
and presents unpolarized as well as single-polarized observables. All results have high precision, are compared to different model predictions, and will
considerably constrain future model analyses in the field of double pion photoproduction and beyond.
The data of this work were taken at MAMI in four different experiments with liquid hydrogen and deuterium targets in 2007 and 2009 and include over 600 hours
of beam time. A longitudinally polarized electron beam was used to produce circularly polarized bremsstrahlung photons with energies up 1.4 GeV. The reaction
products, charged pions, photons and nucleons, were detected in the combined calorimeter consisting of Crystal Ball and TAPS.
Total and differential cross sections, invariant mass distributions of N-pion and pion-pion and beam helicity asymmetries were computed in the fully reconstructed
final state center-of-mass system. Effects from Fermi motion in the deuteron target could be reliably defolded leading to a good agreement between free and
quasifree proton data, and thus the neutron results can be interpreted as a good approximation of free neutron data.
Beam-helicity asymmetries for g+n(p)->pion^0+pion^0+n(p) and g+n(p)->pion^-+pion^0+p(p) have been measured for the first time and published together with the
results from the proton data. Especially for the mixed-charge results, the available model calculations fail to reproduce the data, and for the neutral channel
data, an unexpected similarity for proton and neutron results was observed.
Total and differential cross sections as well as invariant mass distributions of N-pion and pion-pion for g+n(p)->pion^-+pion^0+p(p) have also been measured for
the first time and previous results for g+p(n)->pion^++pion^0+n(n) could be reproduced and extended into the third resonance region.