Weak Pion and Photon Production off Nucleons in a Chiral Effective Field Theory
Neutrino-induced pion and photon production from nucleons and nuclei are important for the interpretation of neutrino-oscillation experiments, and these processes are potential backgrounds in the MiniBooNE experiment (A. A. Aquilar-Arevalo et al. (MiniBooNE Collaboration), Phys. Rev. Lett. 100, 032301 (2008)). Pion and photon production are investigated at intermediate energies, where theresonance becomes important. The Lore ntz-covariant e୥ctiveeld theory contains nucleons, pions, Deltas, isoscalar scalar (�) and vector (!) �elds, and isovector vector (�) �elds. The lagrangian exhibits a nonlinear realization of (approximate) SU(2)L SU(2)R chiral symmetry and incorporates vector meson dominance. Power counting for vertices and Feynman diagrams involving theis explained. Because of the built-in symmetries, the vector currents are automatically conserved, and the axial-vector currents satisfy PCAC. The irrelevance of so-called oଭshellcouplings and the structure of the dressedpropag ator, which has a pole only in the spin-3/2 channel, are discussed. To calibrate the axial-vector transition current (N $ �), pion production from the nucleon is used as a benchmark and compared to bubble-chamber data from Argonne and Brookhaven National Laboratories. At low energies, the convergence of our power- counting scheme is investigated, and next-to-leading-order tree-level corrections are found to be very small.