N ∗ and ∆ ∗ decays into Nπ 0 π 0

Decays of baryon resonances in the second and the third resonance region into Nπ0π0 are studied by photoproduction of two neutral pions off protons. Partial decay widths of N and ∆ resonances decaying into ∆(1232)π, N(ππ)S ,N(1440)P11π, and N(1520)D13π are determined in a partial wave analysis of this data, and data from other reactions. Several partial decay widths were not known before. Interesting decay patterns are observed which are not even qualitatively reproduced by quark model calculations. In the second resonance region, decays into ∆(1232)π dominate clearly. The N(ππ)S-wave provides a significant contribution to the cross section, especially in the third resonance region. The P13(1720) properties found here are at clear variance to PDG values. Preprint submitted to Elsevier 9 April 2008 PACS: 11.80.Et, 11.80.Gw, 13.30.-a, 13.30.Ce, 13.30.Eg, 13.60.Le 14.20.Gk The structure of baryons and their excitation spectrum is one of the unsolved issues of strong interaction physics. The ground states and the low–mass excitations evidence the decisive role of SU(3) symmetry and suggest an interpretation of the spectrum in constituent quark models [1,2,3]. Baryon decays can be calculated in quark models using harmonic–oscillator wave functions and assuming a qq̄ pair creation operator for meson production. A collective string-like model gives a description of the mass spectrum of similar quality [4] and predicts partial decay widths of resonances [5]. A comprehensive review of predictions of baryon masses and decays can be found in [6]. An alternative description of the baryon spectrum may be developed in effective field theories in which baryon resonances are generated dynamically from their decays [7]. At present, the approach is restricted to resonances coupling to octet baryons and pseudoscalar mesons, yet it can possibly be extended to include vector mesons and decuplet baryons [8]. To test the different approaches, detailed information on the spectrum and decays of resonances is needed, including more complex decay modes such as ∆π or N(ππ)S, where (ππ)S stands for the (ππ)-S-wave. The analysis of complex final states requires the use of eventbased likelihood fits to fully exploit the sensitivity of the data. In baryon spectroscopy such fits have, to our knowledge, never been performed so far. In this letter we report on a study of ∆π and other p2π decay modes of baryon resonances belonging to the second and third resonance region. The results are obtained from data on the reaction