Spin observables in pion photoproduction from a unitary and causal effective field theory

Abstract

Pion photoproduction is analyzed with the chiral Lagrangian. Partial-wave amplitudes are obtained by an analytic extrapolation of subthreshold reaction amplitudes computed in chiral perturbation theory, where the constraints set by electromagnetic-gauge invariance, causality and unitarity are used to stabilize the extrapolation. The experimental data set is reproduced up to energies $\sqrt{s}\simeq 1300$ MeV in terms of the parameters relevant at order $Q^3$. We present and discuss predictions for various spin observables.

Comments: 10 pages, 5 figures, writeup of two talks presented at the NAPP 2010 conference and the Crystal-Ball Collaboration meeting in Dubrovnik, October 2010, revised version - corrected an error in Fig. 2 and Fig. 4