https://doi.org/10.1140/epja/i2007-10644-6
Regular Article - Experimental Physics
Separated cross-sections in 
electroproduction at threshold at Q2 = 0.05GeV2/c2
1
Institut für Kernphysik, Johannes Gutenberg-Universität Mainz, D-55099, Mainz, Germany
2
Institut für Physik, Johannes Gutenberg-Universität Mainz, D-55099, Mainz, Germany
3
Laboratory for Nuclear Science, Massachusetts Institute of Technology, 02139, Cambridge, MA, USA
4
Jožef Stefan Institute, SI-1001, Ljubljana, Slovenia
5
Institut für Kernphysik, Technische Universität Darmstadt, D-64289, Darmstadt, Germany
6
Department of Physics, University of Ljubljana, SI-1000, Ljubljana, Slovenia
* e-mail: merkel@kph.uni-mainz.de
Received:
25
May
2007
Accepted:
25
August
2008
Published online:
25
September
2008
The differential cross-sections σ0 = σT + εσL , σLT , and σTT of electroproduction from the proton were measured from threshold up to an additional center-of-mass energy of 40MeV, at a value of the photon four-momentum transfer of Q
2 = 0.05 GeV2/c2 and a center-of-mass angle of θ = 90° . By an additional out-of-plane measurement with polarized electrons σLT' was determined. This showed for the first time the presence of an imaginary part of the s -wave above the 
threshold, which is usually interpreted as a unitary cusp. The predictions of the Heavy Baryon Chiral Perturbation Theory (HBChPT) are in disagreement with these data. On the other hand, the data are somewhat better predicted by the MAID phenomenological model and are in good agreement with the dynamical model DMT.
PACS: 25.30.Rw Electroproduction reactions – / 13.60.Le Meson production –
© SIF, Springer-Verlag Berlin Heidelberg, 2008
