Neutral pion threshold production at Q^2=0.05 GeV^2/c^2 and Chiral Perturbation Theory

Abstract

New data are presented on the p(e,e'p)π^0 reaction at threshold at a four-momentum transfer of Q^2=0.05 GeV^2/c^2. The data were taken with the three-spectrometer setup of the A1 Collaboration at the Mainz Microtron MAMI. The complete center of mass solid angle was covered up to a center of mass energy of 4 MeV above threshold. Combined with measurements at three different values of the virtual photon polarization ε, the structure functions σ_T, σ_L, σ_{TT}, and σ_{TL} are determined. The results are compared with calculations in Heavy Baryon Chiral Perturbation Theory and with a phenomenological model. The measured cross section is significantly smaller than both predictions.