Eur. Phys. J. A (2012) 48: 98
https://doi.org/10.1140/epja/i2012-12098-1

Regular Article - Experimental Physics

Photoproduction of π⁰π⁰ - and π⁰π⁺ -pairs off the proton from threshold to the second resonance region

¹ Department of Physics, University of Basel, Ch-4056, Basel, Switzerland
² Institut für Kernphysik, University of Mainz, D-55099, Mainz, Germany
³ SUPA, School of Physics and Astronomy, University of Glasgow, G12 8QQ, Glasgow, UK
⁴ Helmholtz-Institut für Strahlen- und Kernphysik, University Bonn, D-53115, Bonn, Germany
⁵ Petersburg Nuclear Physics Institute, RU-188300, Gatchina, Russia
⁶ INFN, Sezione di Pavia, I-27100 Pavia, Pavia, Italy
⁷ School of Physics, University of Edinburgh, EH9 3JZ, Edinburgh, UK
⁸ Center for Nuclear Studies, The George Washington University, DC 20052, Washington, USA
⁹ University of California Los Angeles, 90095-1547, Los Angeles, California, USA
¹⁰ Lebedev Physical Institute, RU-119991, Moscow, Russia
¹¹ Laboratory of Mathematical Physics, Tomsk Polytechnic University, Tomsk, Russia
¹² II. Physikalisches Institut, University of Giessen, D-35392, Giessen, Germany
¹³ Mount Allison University, Sackville, E4L3B5, New Brunswick, Canada
¹⁴ Rudjer Boskovic Institute, HR-10000, Zagreb, Croatia
¹⁵ Institute for Nuclear Research, RU-125047, Moscow, Russia
¹⁶ Kent State University, 44242, Kent, Ohio, USA
¹⁷ The Catholic University of America, DC 20064, Washington, USA

^* e-mail: Bernd.Krusche@unibas.ch

Received: 8 April 2012
Accepted: 10 July 2012
Published online: 26 July 2012

Abstract

Precise total cross-sections and invariant-mass distributions have been measured for photoproduction of pion pairs off the proton producing and final states from the threshold region up to 800MeV incident photon energy. Additionally, beam helicity asymmetries have been measured in the second resonance region (550MeV-820MeV). The experiment was performed at the tagged photon beam of the Mainz MAMI accelerator with the Crystal Ball and TAPS detectors combined to give an almost 4 solid-angle electromagnetic calorimeter. The results are much more precise than any previous measurements and confirm the chiral perturbation theory predictions for the threshold behavior of these reactions. In the second resonance region, the invariant-mass distributions of meson-meson and meson-nucleon pairs are in reasonable agreement with model predictions, but none of the models reproduce the asymmetries for the mixed-charge channel.