Physics – High Energy Physics – High Energy Physics - Phenomenology
Scientific paper
1996-01-15
Nucl.Phys. A607 (1996) 379-401; Erratum-ibid. A633 (1998) 695-697
Physics
High Energy Physics
High Energy Physics - Phenomenology
25 pp, LaTeX, uses epsf and elsart12.sty, 8 figures in 10 PS files
Scientific paper
10.1016/0375-9474(96)00184-4
We investigate neutral pion electroproduction off protons in the framework of heavy baryon chiral perturbation theory. The chiral expansion of the S--wave multipoles $E_{0+}$ and $L_{0+}$ 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 $\pi^0$ photoproduction data, one is fixed from the proton Dirac form factor and the novel S--wave constants appearing at orders $q^4$ and $q^5$ are determined from a best fit (constrained by resonance exchange) to the recent NIKHEF and MAMI data at photon momentum transfer squared $k^2 = -0.1 \, \, {\rm GeV}^2$. The inclusion of a particular set of dimension five operators is forced upon the fit by a soft--pion theorem which severely constrains the momentum dependence of the longitudinal S--wave multipole $L_{0+}$ at order $q^4$. We give predictions for lower photon virtualities for the various multipoles and differential cross sections. Further improvements are briefly touched upon.
Bernard Véronique
Kaiser Nicholas
Meißner Ulf-G.
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