A simple quark model for resonance electroproduction

A quark model for resonance electroproduction is developed which is based on the successful photoproduction model of Kubata and Ohta. A detailed comparison is made between the predictions of the model and the results of recent phenomenological analyses of electroproduction data.     

Electroproduction of strangeness

Formalism for the electroproduction of strange particles based on the one photon exchange approximation and the Feynman diagrams representation of hadron current is reviewed. Different photoproduction models are compared with available photo and electroproduction data. New experiments are proposed that may resolve existing uncertainties and inconsistencies. The formation of hypernuclei via kaon electroproduction is investigated theoretically and compared with other reactions involving purely hadronic processes.     

A polarization relation and the measurement of the longitudinal response in pseudoscalar meson electroproduction off the nucleon

For pseudoscalar meson electroproduction off the nucleon in parallel kinematics a relation between three polarization observables is derived. It is shown that, without Rosenbluth separation, a measurement of the longitudinal strength can be achieved through three different ways. They are discussed with preliminary MAMI data for the reaction in the energy range of the resonance. Received: 30 March 2000 / Accepted: 8 May 2000     

Electroproduction of neutral pions and test of the quark-parton model

We present the first data on the inclusive structure function for π⁰-electroproduction. The data are compared to charged pion electroproduction and charged pion production from electron-positron annihilation taking the quark-parton model as a guide.     

Medium effects in coherent pion photo- and electroproduction on He and C

Coherent π⁰ photo- and electroproduction on ⁴He and ¹²C nuclei is investigated in the framework of a distorted wave impulse approximation in momentum space. The elementary process is described by the recently developed unitary isobar model. Medium effects are considered by introducing a phenomenological Δ self-energy. Recent experimental data for ⁴He and ¹²C can be well described over a wide range of energies and emission angles by the assumption that the Δ–nuclear interaction saturates.     

Partial-wave analysis and baryon spectroscopy<Superscript>⋆</Superscript>

We continue to update our PWA of fundamental reactions as new data become available. Of primary interest to the N^* program are πN elastic scattering and the photo- and electroproduction of pions. Other photo-reactions are more model dependent, but may reveal resonances coupling weakly to πN final states. We also mention recent results from nucleon-nucleon elastic scattering, which have a less well-known connection to experimental programs proposing to extract N ^* properties from complete experiments.     

Photo- and electroproduction of strange mesons: From elementary process to hypernuclei

The general formalism for electromagnetic production of pseudoscalar mesons off nucleons is reviewed. An effective Lagrangian model for low and medium energy photo- and electroproduction of K⁺ mesons is introduced and compared with available experimental data. Finally, the electroproduction of K⁺ mesons on complex nuclear targets is investigated and it is demonstrated that planned experiments at the CEBAF electron accelerator may shed new light on the problems of the spectroscopy of hypernuclei.     

Pion electroproduction from nuclei near threshold

The electroproduction of low energy pions is calculated for experimental situations corresponding to the detection of only one of the outgoing particles. The final nuclear state is a superposition of particle-hole states, corresponding to the giant resonances. A comparison with the predictions of the Fermi gas model is made. The effect of final state interactions was included by using a distorted wave impulse approximation to describe electroproduction from nuclei. Distortion effects near threshold have a significant effect on the magnitude of the electroproduction cross section.     

Measurement of the generalized polarizabilities of the proton in virtual Compton scattering at MAMI

Virtual Compton scattering off the proton has been studied at Q ² = 0.33 (GeV/c)² at the MAMI accelerator in Mainz (Germany). The goal of the experiment is to measure the generalized polarizabilities (GPs) of the proton using the double polarized e p→e' p'γ reaction. This paper reports the measurement of the unpolarized photon electroproduction cross-section for the new data and the extraction of two linear combinations of GPs, which are essential for the analysis of the double spin asymmetry.     

Parity violation in pion electroproduction and photoproduction with a proton target

Parity violation in pion electroproduction and photoproduction on a proton target is investigated as a possible means of elucidating the structure of the electronic and non-leptonic neutral weak currents. We calculate both parity-violating effects caused directly by Z-boson exchange, which contributes only to electron scattering, and those due to parity-violating electromagnetic and nuclear interactions. The asymmetry in electroproduction is of order 20^?5, whereas that in photoproduction is ～10^?7. The sensitivity to the parameter sin²θ_w is investigated; it is a maximum at backward electron scattering angles. In addition to the Born terms, contributions from the Δ(1232) resonance and the light vector mesons [?(776) and ω(782)] are included in the model. Fits to the observed π⁰ and π⁺ electroproduction and photoproduction cross sections indicate that the model accounts for the production data in the medium energy region.     

Baryon resonance analysis from MAID

The unitary isobar model MAID2007 has been used to analyze the recent data of pion electroproduction. The model contains all four-star resonances in the region below W = 2 GeV and both single-Q~2 and Q~2 dependent transition form factors could be obtained for the Delta, Roper, D_(13)(1520), S_(11) (1535), S_(31) (1620), S_(11)(1650), D_(15)(1675), F_(15)(1680) and P_(13)(1720). From the complete world data base, including also π~- data on the neutron, also Q~2 dependent neutron form factors are obtained. For all transition form factors we also give convenient numerical parameterizations that can be used in other reactions. Furthermore, we show how the transition form factors can be used to obtain empirical transverse charge densities and our first results are given for the Roper, the S_(11) and D_(13) resonances.     

Baryon Structure in AdS/QCD

We discuss baryon structure in a soft-wall anti-de Sitter/quantum chromodynamics model. From unified point of view we describe nucleon form factors and the electroproduction of the N(1440) Roper resonance. The Roper resonance is identified as the first radially excited state of the nucleon. The obtained results for helicity amplitudes of the Roper electroproduction are in good agreement with the recent results of the CLAS Collaboration at JLab.     

Charged pion electroproduction on light nuclei near the threshold

The experimental data for charged pion electroproduction on ¹²C and ¹⁶O measured at Mainz and Sendai are analyzed in a modified equivalent photon approximation. For the Sendai experiment we use distorted waves for the pion, while for the Mainz experiment we adopt a closure sum with a finite Fermi-gas model. We can roughly reproduce both sets of experimental data.     

Onset of quark-hadron duality in pion electroproduction

A large data set of charged-pion (pi+/-) electroproduction from both hydrogen and deuterium targets has been obtained spanning the low-energy residual-mass region. These data conclusively show the onset of the quark-hadron duality phenomenon, as predicted for high-energy hadron electroproduction. We construct several ratios from these data to exhibit the relation of this phenomenon to the high-energy factorization ansatz of electron-quark scattering and subsequent quark-->pion production mechanisms.     

Double-spin asymmetries in the cross section of \rho^0 and \phi production at intermediate energies

Double-spin asymmetries in the cross section of electroproduction of and mesons on the proton and deuteron are measured at the HERMES experiment. The photoabsorption asymmetry in exclusive electroproduction on the proton exhibits a positive tendency. This is consistent with theoretical predictions that the exchange of an object with unnatural parity contributes to exclusive electroproduction by transverse photons. The photoabsorption asymmetry on the deuteron is found to be consistent with zero. Double-spin asymmetries in and meson electroproduction by quasi-real photons were also found to be consistent with zero; the asymmetry in the case of the meson is compatible with a theoretical prediction which involves knockout from the nucleon. Received: 11 February 2003 / Published online: 11 June 2003     

Inclusive photo- and electroproduction of pions in the triple Regge region

We develop a Regge pole expansion including spin for inclusive photo- and electroproduction of pions in the photon fragmentation region and see to what extent the available data at intermediate energies can be explained in terms of Regge poles alone. It is shown that a pure Regge pole model is not adequate, although some qualitative features are reproduced.