THRESHOLD NEUTRAL PION ELECTROPRODUCTION IN HEAVY BARYON CHIRAL PERTURBATION-THEORY

We investigate neutral pion electroproduction off protons in the frame work of heavy baryon chiral perturbation theory. The chiral expansion of the S-wave multipoles E(0+) and L(0+) = is carried out to three ord ers. There appear several undetermined low-energy constants. Three are taken from a recent study of the new TAPS threshold pi(0) photoproduc tion data, one is fixed from the proton Dirac form factor and the nove l S-wave constants appearing at orders q(4) and q(5) are determined fr om a best fit (constrained by resonance exchange) to the recent NIKHEF and MAMI data at photon momentum transfer squared k(2) = -0.1 GeV2. T he inclusion of a particular dimension-five operator is forced upon th e fit by a soft-pion theorem which severely constrains the momentum de pendence of the longitudinal S-wave multipole L(0+) at order (q)4. We give predictions for lower photon virtualities for the various multipo les and differential cross sections. Further improvements are briefly touched upon.