J.H. Koch

Pion electroproduction at threshold and PCAC

Abstract Threshold pion electroproduction is calculated in a model-independent way based on electromagnetic current conservation, PCAC, and crossing symmetry. The threshold s-wave multipoles E0+ and L0+ are expanded in terms of two dimensionless parameters, the pion-nucleon mass ratio μ = m π M , and ν 2 = k 2 M 2 , where k is the four-momentum of the virtual photon. Model-dependent terms appear in order μν2 and μ2 for the charged production amplitudes and in order μν2 and μ3 in the neutral production amplitudes. We comment on recipes to restore gauge invariance for the pseudovector electroproduction Born terms and their relationship with low-energy theorems. Finally, we discuss the possibility of determining the nucleon axial radius using threshold pion electroproduction.

Electron scattering from a bound nucleon

Abstract We comment on electron scattering from a bound nucleon. The general structure of the electromagnetic vertex of an off-shell nucleon, and constraints due to symmetries and gauge invariance are examined. The approximations necessary to arrive at commonly used recipes for the off-shell interaction are discussed. We put them into perspective by giving order of magnitude estimates for these approximations.

Virtual Compton scattering off the nucleon at low energies

We investigate the low-energy behavior of the four-point Green{close_quote}s function {Gamma}{sup {mu}{nu}} describing virtual Compton scattering off the nucleon. Using Lorentz invariance, gauge invariance, and crossing symmetry, we derive the leading terms of an expansion of the operator in the four-momenta {ital q} and {ital q}{sup {prime}} of the initial and final photon, respectively. The model-independent result is expressed in terms of the electromagnetic form factors of the free nucleon, i.e., on-shell information which one obtains from electron-nucleon scattering experiments. Model-dependent terms appear in the operator at {ital O}({ital q}{sub {alpha}}{ital q}{sub {beta}}{sup {prime}}), whereas the orders {ital O}({ital q}{sub {alpha}}{ital q}{sub {beta}}) and {ital O}({ital q}{sub {alpha}}{sup {prime}}{ital q}{sub {beta}}{sup {prime}}) are contained in the low-energy theorem for {Gamma}{sup {mu}{nu}}, i.e., no new parameters appear. We discuss the leading terms of the matrix element and comment on the use of on-shell equivalent electromagnetic vertices in the calculation of {open_quote}{open_quote}Born terms{close_quote}{close_quote} for virtual Compton scattering. {copyright} {ital 1996 The American Physical Society.}

Hadron structure and gauge invariance in photo- and electroproduction of pions

Abstract We give a detailed account of the constraints on the general operator for pion photo- and electroproduction provided by the Ward-Takahashi identities. We investigate the role of off-shell effects in electromagnetic and strong interaction from factors. Special attention is paid to the “internal insertions”, the coupling of the photon into the dressed πNN vertex. To be able to do this consistently, we use a simple field-theoretical model where the nucleon gets dressed by a neutral scalar meson. In the framework of this model we compare the exact production amplitude with that obtained from ad hoc prescriptions which have recently been proposed. Finally the discussion is extended to exchange currents in electron scattering.

Systematics of low-energy theorems for pion photo- and electroproduction

Abstract We examine the role of kinematical variables and analyticity in the derivation of low-energy theorems for pion photo- and electroproduction. Other expansion schemes are compared to the low-energy theorem. We illustrate this with a recent calculation of these processes in chiral perturbation theory. The possibility to test the low-energy theorem experimentally is discussed.

Pion photoproduction and radiative capture on 13C

Abstract We present an analysis of pion photoproduction and radiative capture at energies below the 3,3 resonance. Reasonable agreement with the data is found for large momentum transfers, while at small momentum transfer the data are consistently lower than the calculation. Various sources for the disagreement are explored and we conclude that the nuclear wavefunction provides the most likely explanation.