https://doi.org/10.1140/epja/i2014-14054-5
Regular Article - Experimental Physics
Measurement of the beam-helicity asymmetry I⊙ in the photoproduction of 
pairs off protons and off neutrons
1
Department of Physics, University of Basel, CH-4056, Basel, Switzerland
2
Institut für Kernphysik, University of Mainz, D-55099, Mainz, Germany
3
School of Physics and Astronomy, University of Glasgow, Glasgow, G12 8QQ, UK
4
Kent State University, 44242, Kent, Ohio, USA
5
Helmholtz-Institut für Strahlen- und Kernphysik, University of Bonn, D-53115, Bonn, Germany
6
Petersburg Nuclear Physics Institute, RU-188300, Gatchina, Russia
7
II. Physikalisches Institut, University of Giessen, D-35392, Giessen, Germany
8
INFN, Sezione di Pavia, I-27100 Pavia, Pavia, Italy
9
School of Physics, University of Edinburgh, EH9 3JZ, Edinburgh, UK
10
Center for Nuclear Studies, The George Washington University, 20052, Washington, DC, USA
11
University of California Los Angeles, 90095-1547, Los Angeles, California, USA
12
Lebedev Physical Institute, RU-119991, Moscow, Russia
13
Laboratory of Mathematical Physics, Tomsk Polytechnic University, Tomsk, Russia
14
Mount Allison University, Sackville, E4L1E6, New Brunswick, Canada
15
University of Regina, S4S-0A2, Regina, SK, Canada
16
Institute for Nuclear Research, RU-125047, Moscow, Russia
17
Rudjer Boskovic Institute, HR-10000, Zagreb, Croatia
18
The Catholic University of America, 20064, Washington, DC, USA
* e-mail: Bernd.Krusche@unibas.ch
Received:
17
May
2013
Revised:
28
January
2014
Accepted:
5
February
2014
Published online:
10
March
2014
Beam-helicity asymmetries have been measured at the MAMI accelerator in Mainz for the photoproduction of mixed-charge pion pairs in the reactions 
off free protons and 
and 
off quasi-free nucleons bound in the deuteron for incident photon energies up to 1.4GeV. Circularly polarized photons were produced from bremsstrahlung of longitudinally polarized electrons and tagged with the Glasgow-Mainz magnetic spectrometer. The charged pions, recoil protons, recoil neutrons, and decay photons from mesons were detected in the 
electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors. Using a complete kinematic reconstruction of the final state, excellent agreement was found between the results for free and quasi-free protons, suggesting that the quasi-free neutron results are also a close approximation of the free-neutron asymmetries. A comparison of the results to the predictions of the Two-Pion-MAID reaction model shows that the reaction mechanisms are still not well understood, in particular at low incident photon energies in the second nucleon-resonance region.
© SIF, Springer-Verlag Berlin Heidelberg, 2014
