Double pion photoproduction off the proton at threshold and in the second resonance region

Abstract

The nucleon, as we know it today, is a complex system made of three valence quarks, gluons and quark-antiquark pairs from the sea. This picture has been �rmly established in the framework of deep inelastic scattering. At lower energies, where the perturbative expansion is not valid any more, the behavior of the nucleon and its resonances is however a much more complex issue. An exact treatment of the problem is today out of reach and the main part of our current theoretical knowledge on the resonances is based on constituent quark models which subsume the complex internal structure of the nucleon in three massive constituent quarks. The discrepancies between the predictions of such models and the experimental resonance spectrum, such as the problem of missing resonances (the number of resonances predicted by the models is higher than what is actually observed) or the mass of the lowest lying resonance -the P11(1440) or Roper resonance- which most models overestimate, calls for a more detailed investigation of this �eld. On the experimental side, meson photoproduction o� nucleons is the ideal complement to pion induced reactions which have been studied for more than 40 years and forms our main source of knowledge on nucleon resonances. The reaction studied in this work, double pion photoproduction N ! ��N, is especially worthwhile since it gives access to decay properties that can't be observed with single meson photoproduction: the sequential decay of the resonance via an intermediate � resonance p ! N� ! �� ! N�� or the decay via emission of � or � mesons which decay in two pions. In this work, we measured the p ! p�0�0 and p ! n�+�0 reactions with a special emphasis on the energies close to threshold and on recently available polarization observables. The experiment was performed at MAMI in Mainz in the years 2004 and 2005 using a beam of linearly and circularly polarized tagged photons produced by bremsstrahlung of electrons on a thin diamond radiator with a maximal energy of 820 MeV. The reaction happened in a liquid hydrogen target. The decay particles were detected by the Crystal Ball detector in conjunction with the TAPS detector as forward wall, covering almost 100% of the total 4� solid angle and thus ensuring a very high detecting e�ciency. This state-of-the art facility allowed the determination of total cross sections for the double �0 and �0 �+ channels with a very good precision. Especially interesting is the p ! p�0�0 cross section close to threshold since it provides a stringent test for chiral perturbation theory. The calculation of ChPT, which predicts a dominant contribution of pion loops, are very well reproduced by our data. In addition, Dalitz plots and invariant mass distributions have also been produced. Their good precision con�rms the important features of the double pion mechanisms such as the dominance of the D13(1520) sequencial decay, the small contribution of the P11(1440) ! p(�0�0)I=0 S