Threshold neutral pion electroproduction in heavy baryon chiral perturbation theory

We investigate neutral pion electroproduction off protons in the framework of heavy baryon chiral perturbation theory. The chiral expansion of the S-wave multipoles E₀₊ and L₀₊ is carried out to three orders. There appear several undetermined low-energy constants. Three are taken from a recent study of the new TAPS threshold π⁰ photoproduction data, one is fixed from the proton Dirac form factor and the novel S-wave constants appearing at orders q⁴ and q⁵ are determined from a best fit (constrained by resonance exchange) to the recent NIKHEF and MAMI data at photon momentum transfer squared k² = -0.1 GeV². The inclusion of a particular dimension-five operator is forced upon the fit by a soft-pion theorem which severely constrains the momentum dependence of the longitudinal S-wave multipole L₀₊ at order q⁴. We give predictions for lower photon virtualities for the various multipoles and differential cross sections. Further improvements are briefly touched upon.