https://doi.org/10.1140/epja/i2015-15142-8
Regular Article - Experimental Physics
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
SUPA School of Physics and Astronomy, University of Glasgow, G12 8QQ, Glasgow, 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
SUPA 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, E4L1E6, New Brunswick, Canada
15
University of Regina, SK S4S-0A2, Regina, 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:
12
September
2015
Revised:
6
October
2015
Accepted:
12
October
2015
Published online:
4
November
2015
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.
© SIF, Springer-Verlag Berlin Heidelberg, 2015
