6533b85afe1ef96bd12b8aa3

RESEARCH PRODUCT

Neutral Pion Threshold Production atQ2=0.05GeV2/c2and Chiral Perturbation Theory

subject

description

New data are presented on the $p({e,e}^{\ensuremath{'}}p){\ensuremath{\pi}}^{0}$ reaction at threshold at a four-momentum transfer of ${Q}^{2}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.05{\mathrm{GeV}}^{2}{/c}^{2}$. 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 $\ensuremath{\epsilon}$, the structure functions ${\ensuremath{\sigma}}_{T}$, ${\ensuremath{\sigma}}_{L}$, ${\ensuremath{\sigma}}_{\mathrm{TT}}$, and ${\ensuremath{\sigma}}_{\mathrm{TL}}$ 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.