H. Haberzettl

Gauge-invariant theory of pion photoproduction with dressed hadrons

Based on an effective field theory of hadrons in which quantum chromodynamics is assumed to provide the necessary bare cutoff functions, a gauge-invariant theory of pion photoproduction with fully dressed nucleons is developed. The formalism provides consistent dynamical descriptions of {pi}N{r_arrow}{pi}N scattering and {gamma}N{r_arrow}{pi}N production mechanisms in terms of nonlinear integral equations for fully dressed hadrons. Defining electromagnetic currents via the gauging of hadronic n-point Green{close_quote}s functions, dynamically detailed currents for dressed nucleons are introduced. The dressed hadron currents and the pion photoproduction current are explicitly shown to satisfy gauge invariance in a self-consistent manner. Approximations are discussed that make the nonlinear formalism manageable in practice and yet preserve gauge invariance. This is achieved by recasting the gauge conditions for all contributing interaction currents as continuity equations with {open_quotes}surface{close_quotes} terms for the individual particle legs coming into or going out of the hadronic interaction region. General procedures are given that approximate any type of (global) interaction current in a gauge-invariance-preserving manner as a sum of single-particle {open_quotes}surface{close_quotes} currents. It is argued that these prescriptions carry over to other reactions, irrespective of the number or type of contributing hadrons or hadronic systems. {copyright} {ital 1997} {ital The American Physical Society}

Quasifree kaon photoproduction on nuclei

Abstract Investigations of the quasifree reaction A( γ , KY)B are presented in the distorted wave impulse approximation (DWIA). For this purpose, we present a revised tree-level model of elementary kaon photoproduction that incorporates hadronic form factors consistent with gauge invariance, uses SU (3) values for the Born couplings and uses resonances consistent with multichannel analyses. The potential of exclusive quasifree kaon photoproduction on nuclei to reveal details of the hyperon–nucleus interaction is examined. Detailed predictions for the coincidence cross section, the photon asymmetry, and the hyperon polarization and their sensitivities to the ingredients of the model are obtained for all six production channels. Under selected kinematics these observables are found to be sensitive to the hyperon–nucleus final-state interaction. Some polarization observables are found to be insensitive to distortion effects, making them ideal tools to search for possible medium modifications of the elementary amplitude.

Gauge-invariant tree-level photoproduction amplitudes with form factors

We show how the gauge-invariance formulation given by Haberzettl is implemented in practice for photoproduction amplitudes at tree level with form factors describing composite nucleons. We demonstrate that, in contrast to Ohta{close_quote}s gauge-invariance prescription, this formalism allows electric current contributions to be multiplied by a form factor, i.e., it does not require that they be treated like bare currents. While different in detail, this nevertheless lends support to previous {ital ad hoc} approaches which multiply the Born amplitudes by an overall form factor. Numerical results for kaon photoproduction off the nucleon are given. They show that the gauge procedure by Haberzettl leads to much improved {chi}{sup 2} values compared to Ohta{close_quote}s prescription. thinsp {copyright} {ital 1998} {ital The American Physical Society}

Coupled-channel dynamics in the reactions πN → πN, ηN, KΛ, KΣ

Elastic πN scattering and the world data of the family of reactions π−p → ηn, K0Λ, K0Σ0, K+Σ−, and π+p → K+Σ+ are described simultaneously in an analytic, unitary, coupled-channel approach. SU(3) flavor symmetry is used to relate both the t - and the u - channel exchanges that drive the meson-baryon interaction in the different channels. Angular distributions, polarizations, and spin-rotation parameters are compared with available experimental data. Partial-wave amplitudes are determined and the resonance content is extracted from the analytic continuation, including resonance positions and branching ratios, and possible sources of uncertainties are discussed. The results provide the final-state interactions for the ongoing analysis of photo- and electroproduction data.

Possible solution of the J/psi production puzzle

We argue that the s-channel cut contribution to J/psi hadroproduction can be significantly larger than the usual cut contribution of the color-singlet mechanism, which is known to underestimate the experimental measurements. A scenario accounting for intermediate cc[over ] interactions is proposed that reproduces the data at low and midrange transverse momenta P_(T) from the Fermilab Tevatron and BNL Relativistic Heavy Ion Collider. The J/psi produced in this manner are polarized predominantly longitudinally.

Photocouplings at the pole from pion photoproduction

Abstract.The reactions

Eta photoproduction in a combined analysis of pion- and photon-induced reactions

\gamma p\rightarrow\pi^{0}p

Gauge-invariant approach to meson photoproduction including the final-state interaction

and

Photoproduction of Xi off nucleons

\gamma p\rightarrow\pi^{+}n

Combined analysis of eta meson hadro- and photo-production off nucleons

are analyzed in a semi-phenomenological approach up to