Photoproduction of $ \pi^{0}$ -pairs off protons and off neutrons

M Dieterle,B Demissie,Th Strub,M Thiel,J C Mcgeorge,D Werthmüller,D Branford,N K Walford,A Starostin,A Braghieri,L Witthauer,S Schumann,A Nikolaev,S Prakhov,D Hamilton,B Oussena,S Cherepnya,V Bekrenev,S Garni,V Lisin,B Krusche,D Hornidge,J R M Annand,T C Jude,E J Downie,A Mushkarenkov,D I Glazier,R Kondratiev,Y Maghrbi,O Jahn,M Korolija,J Mancell,F Pheron,A Polonski,E Heid,S Costanza,J Robinson,A Käser,P Drexler,I J D Macgregor,H J Arends,P Pedroni,J Ahrens,I Keshelashvili,K Livingston,R Beck,M Unverzagt,H Berghäuser,D I Sober,T Rostomyan,D M Manley,V L Kashevarov,D P Watts,A Thomas,I Supek,G M Huber,M H Sikora,J Brudvik,D G Middleton,R Novotny,V Metag,K Bantawa,M Oberle,M Ostrick,D Howdle,W J Briscoe,Z Marinides,A Fix,P B Otte ,P Aguar Bartolomé ,L.v Filʼkov ,Saša Mićanović ,D Mekterović ,E F Mcnicoll

doi:10.1140/epja/i2015-15142-8

Abstract

Total cross sections, angular distributions, and invariant-mass distributions have been measured for the photoproduction of pairs off free protons and off nucleons bound in the deuteron. The experiments were performed at the MAMI accelerator facility in Mainz using the Glasgow photon tagging spectrometer and the Crystal Ball/TAPS detector. The accelerator delivered electron beams of 1508 and 1557MeV, which produced bremsstrahlung in thin radiator foils. The tagged photon beam covered energies up to 1400MeV. The data from the free proton target are in good agreement with previous measurements and were only used to test the analysis procedures. The results for differential cross sections (angular distributions and invariant-mass distributions) for free and quasi-free protons are almost identical in shape, but differ in absolute magnitude up to 15%. Thus, moderate final-state interaction effects are present. The data for quasi-free neutrons are similar to the proton data in the second resonance region (final-state invariant masses up to MeV), where both reactions are dominated by the decay. At higher energies, angular and invariant-mass distributions are different. A simple analysis of the shapes of the invariant-mass distributions in the third resonance region is consistent with strong contributions of an decay for the proton, while the reaction is dominated by a sequential decay via a intermediate state for the neutron. The data are compared to predictions from the Two-Pion-MAID model and the Bonn-Gatchina coupled-channel analysis.

Highlights

The properties of the nucleon and its excited states are a key for the investigation of the strong interaction in the non-perturbative regime
We report the results of a detailed study of the total cross section, the invariant-mass distributions, and the angular distributions of quasi-free photoproduction of π0 pairs from nucleons bound in the deuteron compared to the same observables for this final state measured off free protons
The main findings are the following: The comparison of free and quasi-free data for the proton shows that nuclear final-state interactions (FSI) effects are significant

Summary

Introduction

The properties of the nucleon and its excited states are a key for the investigation of the strong interaction in the non-perturbative regime. The application of the DysonSchwinger approach to QCD has led to promising results (see, e.g., [2,3,4]) and the advances in lattice gauge calculations allowed first predictions of the excitation spectrum based on unquenched lattice simulations [5]. These results are still in early stages, using pion masses around 400 MeV (lattice predictions for ground-state properties are nowadays possible for physical quark masses). They are interesting because they “re-discovered” the SU(6) ⊗ O(3) excitation structure of the nucleon with a level counting consistent with the standard nonrelativistic quark model

Methods

Results

Conclusion