Eta-meson photoproduction on protons,neutrons, and light nuclei in the nucleon-resonance region

Eta-meson photoproduction on protons, neutrons, and light nuclei is reviewed. New results obtained for photoproduction on protons and neutrons at the GRAAL facility are analyzed in detail. The experiment in question was performed in a gamma-ray beam that was obtained by means of Compton backscattering of laser photons on electrons at the European Synchrotron Radiation Facility (ESRF, Grenoble, France). A comparative analysis of data on the differential cross sections and the asymmetries for eta-meson photoproduction allowed us to reveal some special features and to study the mechanisms of eta-meson photoproduction on nucleons in detail. Data available from the literature on eta-meson photoproduction on light nuclei are discussed in detail. Results that suggest the existence of bound states (etamesic nuclei) are presented. Particular attention is given to the newmethod of “taggedmesons,” which, according to the GRAAL experiment and to simulated results, makes it possible to study thoroughly the interaction of unstable mesons with intranuclear nucleons.     

Photoproduction within the two-component Dual Parton Model: Amplitudes and cross sections

In the framework of the Dual Parton Model an approximation scheme to describe high energy photoproduction processes is presented. Based on the distinction between direct, resolved soft, and resolved hard interaction processes we construct effective impact parameter amplitudes. In order to treat low mass diffraction within the eikonal formalism in a consistent way a phenomenological ansatz is proposed. The free parameters of the model are determined by fits to high energy hadro- and photoproduction cross sections. We calculate the partial photoproduction cross sections and discuss predictions of the model at HERA energies. Using hadro- and photoproduction data together, the uncertainties of the model predictions are strongly reduced.     

Rescattering effects in \phi photoproduction

We calculate the contribution from rescattering of pairs produced by photoproduction of to the photoproduction of mesons. The photoproduction of mesons is presently a hot topic due to the possibility that an intrinsic hidden strangeness contribution in the nucleon flavor wave function may be revealed in this process, and due to the possibility that the structure of scalar mesons could be accessible experimentally via interference in the system. We show that rescattering makes an important contribution to photoproduction which could – if neglected – mislead the interpretation of the forthcoming high precision data. Received: 14 September 2000 / Revised version: 1 February 2001 / Published online: 12 April 2001     

Coulomb coupling, gauge invariance and the Regge theory of photoproduction

The special nature of the Coulomb coupling of a charged particle suggests that non-analytic terms are present in the Regge theory of photoproduction. This provides a natural explanation of the sharp forward peaks observed in charged pion photoproduction off nucleons. A high energy theorem for the forward photoproduction of pseudoscalar mesons follows.     

Isoscalar diffractive photoproduction: A source of gluonium

Data on diffractive photoproduction of isoscalar states appear to be anomalously large in a number of channels. Isoscalar diffractive photoproduction is studied in a model in which the pomeron is approximated by two gluons, and in which gluon intermediate states are allowed in the pomeron-meson interaction. It is shown that the model allows a resolution of the long-standing problem of the small (in vector meson dominance terms) diffractive θ photoproduction cross section. This sets the scale for the calculation, and it is then demonstrated that the model can explain the large isoscalar cross section if it is due primarily to the diffractive photoproduction of a system of gluons.     

pp和PbPb碰撞中大横动量K和π的光生过程(英文)

基于微扰量子色动力学(pQCD),计算了考虑核遮蔽效应和喷注淬火效应的pp和PbPb碰撞中大横动量K和π的光生过程的产额。在光生过程中可以看到K~+/π~+的增强。数值计算结果表明,所考虑的光生过程对K和π产生是一个很好的修正。     

Current-algebra calculation of low-energy pion electro- and photoproduction for physical pions

A hard-pion current-algebra model is employed to derive expressions for the pion electro- and photoproduction amplitudes. The low-energy predictions for the multipole amplitudes and differential cross sections of photoproduction are analysed and compared with the experimental data.     

Photoproduction of mesons off nuclei

Recent results for the photoproduction of mesons off nuclei are reviewed. These experiments have been performed for two major lines of research related to the properties of the strong interaction. The investigation of nucleon resonances requires light nuclei as targets for the extraction of the isospin composition of the electromagnetic excitations. This is done with quasi-free meson photoproduction off the bound neutron and supplemented with the measurement of coherent photoproduction reactions, serving as spin and/or isospin filters. Furthermore, photoproduction from light and heavy nuclei is a very efficient tool for the study of the interactions of mesons with nuclear matter and the in-medium properties of hadrons. Experiments are currently rapidly developing due to the combination of high quality tagged (and polarized) photon beams with state-of-the-art 4π detectors and polarized targets.     

Compton scattering on the pion and radiative pion photoproduction from the proton

The contribution of the pion polarizabilities to radiative pion photoproduction has been investigated. It has been shown how an extrapolation of future experimental data on radiative pion photoproduction to the pion pole can give information on the polarizabilities of pion.     

Interference effect in the photoproduction of electron-positron pairs on a nucleus in the field of two light waves

In the general relativistic case, the nonresonance photoproduction of electron-positron pairs on a nucleus in the field of two circularly polarized light waves propagating in one direction is theoretically investigated. It is shown that there are two essentially different kinematical regions: the noninterference region, where the Bunkin-Fedorov quantum parameters play the role of multiphoton parameters, and the interference region, where interference effects are important and where quantum interference parameters are treated as multiphoton parameters. The interference effect is found in the photoproduction of an electron-positron pair on a nucleus in the field of two light waves. This effect occurs in the interference region and is due to the production of an electron-positron pair in the plane spanned by the light wave vector and the incident-photon momentum and to stimulated correlated emission and absorption of photons of the two waves. The cross sections for pair photoproduction on a nucleus in the above kinematical regions are determined for moderately strong fields. It is shown that, in the interference region, the partial cross sections for the case where there is a correlated emission (absorption) of the same number of photons of the two waves can significantly exceed the corresponding cross sections in any other geometry.     

Double pion photoproduction from3He at low energies

A generalization of the “Gauge-invariant OPE model” to double pion photoproduction on³He is presented. Results for the total and differential cross-sections are shown. The relevance of double pion photoproduction in the nuclear shadowing of photons is remarked.     

Signatures of the handbag mechanism in wide-angle photoproduction of pseudoscalar mesons

Wide-angle photoproduction of pseudoscalar mesons is investigated under the assumption of dominance of the handbag mechanism, considering both quark helicity flip and non-flip. The partonic subprocess, meson photoproduction off quarks, is analysed with the help of a covariant decomposition of the subprocess amplitudes which is independent of a specific meson generation mechanism. As examples of subprocess dynamics, however, the twist-2 as well as two-particle twist-3 contributions are explicitly calculated. Characteristic features of the handbag approach are discussed in dependence upon the relative magnitudes of the invariant functions. Differential cross sections and spin correlations are predicted to show a characteristic behaviour which allows one to test the underlying assumption of handbag dominance.Received: 15 September 2003, Revised: 2 December 2003, Published online: 29 January 2004     

Photoproduction of pions from deuterons in impulse approximation

The differential cross section for photoproduction of pions from deuterons is calculated in the relativistic impulse approximation. In the discussion of the resulting formula special emphasis is given to the following three problems: test of the impulse approximation, test of the photoproduction amplitudes, and measurement of the deuteron wave-function.     

Theoretische Untersuchungen zur Asymmetrie der Pionphotoproduktion mit einem polarisierten Target

The theoretical importance of experiments with polarised targets for pion photoproduction at high energies is discussed. In an introductory chapter the basic formulas and simple Regge pole models are introduced in connection with elastic pion-proton scattering. The relevance of Regge-cut models is considered in connection with pion-charge-exchange scattering. In the main part we write down formulas for the polarisation parameters in terms of the helicity amplitudes for pion photoproduction. Then we concentrate on the target asymmetry A for photoproduction of neutral and charged pions. Several models with Regge poles and Regge cuts generated by absorption are presented and discussed concerning the results forA. In the last part we report a fit to π^± photoproduction data and give predictions of A for γp → π⁺n and γn → π−p.     

A new photoproduction model in field theory (II)

An earlier study of a gauge-invariant field-theory model for pion photoproduction is continued. The complications due to photon and nucleon spin are considered. The infinity of higher-order graphs are summed to the form of a relativistic eikonal model for photoproduction and its static limit is determined.     

High energy pion photoproduction amplitudes from fixed-t dispersion relations and duality

Contributions to the high energy imaginary parts of the charged and neutral pion photoproduction amplitudes from degenerate ? and A₂ exchanges and ω and B exchanges are parametrized similarly to the dual absorptive model. These contributions together with the imaginary parts of the amplitudes found at low energies from partial-wave analyses are then used to evaluate the high energy real parts of the photoproduction amplitudes from fixed-t dispersion relations. A fit is made to data on pion photoproduction at 3.4 and 16 GeV incident photon energies, for momentum transfers up to ?1(GeV/c)². It is shown that much of the data is reproduced both qualitatively and quantitatively by our model. The high energy imaginary parts of the photoproduction amplitudes given by the fit are shown to be in accord with their average low energy behaviour.     

Nuclear effects in photoproduction and leptoproduction where are they? Are they Negligible?

We discuss nuclear effects in hadronic final states in photoproduction and leptoproduction on nuclei. Whereas the final states in photoproduction and leptoproduction on free nucleons are similar, striking differences are predicted for the case of nuclear targets, the differences being largest in multiplicity correlations. Quantitative predictions are given for the cases of practical interest: photoproduction and muonproduction on emulsion nuclei, and neutrino-production in neon and propanefreon bubble chambers. Neutrino-production on nuclei is emphasized as offering the most direct test of the formation length hypotheses.