Pion Photo Research Articles

Generalized polarizabilities and electroexcitation of the nucleon

Generalized nucleon polarizabilities for virtual photons can be defined in terms of electroproduction cross sections as function of the 4-momentum transfer Q 2. In particular, the sum of the generalized electric and magnetic polarizabilities Σ= α+ β and the spin polarizability γ can be expressed by virtual photon absorption cross sections integrated over the excitation energy. These quantities have been calculated within the framework of the recently developed unitary isobar model for pion photo- and electroproduction on the proton, which describes the available experimental data up to an excitation energy of about 1 GeV. Our results have been compared to the predictions of chiral perturbation theory.

A unitary isobar model for pion photo- and electroproduction on the proton up to 1 GeV

A new operator for pion photo- and electroproduction has been developed for nuclear applications at photon equivalent energies up to 1 GeV. The model contains Born terms, vector mesons and nucleon resonances ( P 33(1232), P 11(1440), D 13(1520), S 11(1535), F 15(1680), and D 33(1700)). The resonance contributions are included taking into account unitarity to provide the correct phases of the pion photoproduction multipoles. The Q 2 dependence of electromagnetic resonance vertices is described with appropriate form factors in the electromagnetic helicity amplitudes. Within this model we have obtained good agreement with the experimental data for pion photo- and electroproduction on the nucleon for both differential cross sections and polarization observables. The model can be used as a starting point to predict and analyze forthcoming data.

Gauge-invariant nuclear pion-photoproduction amplitude with binding-induced contributions

The amplitude for pion photo- and electroproduction from finite nuclei is constructed in a model that ignores rescattering of the pion. A consistent expansion in inverse powers of the nucleon mass is performed, which generates effective one-body and two-body nuclear operators. The former agree with the Blomqvist-Laget effective operators, while the latter are new and arise from binding-induced and binding-modified emission. Current conservation is applied to this set of operators and they are cast separately into manifestly gaugeinvariant forms. A simple approximation to the complicated two-body terms is constructed, which shares many of the features of the latter.

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

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.

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.

Threshold pion photoproduction on nucleons

Pion photo- and electroproduction of the nucleon at threshold is reviewed in theory and experiment. In this region the leading S-wave amplitudes (E0+) are predicted by low-energy theorems (LET) based on current conservation and chiral symmetry. While these predictions are in agreement with the data for charged pion production, LET cannot explain the reaction p( gamma , pi 0)p over the energy range of the first 10 MeV above threshold. Present calculations involving final state interactions are difficult to reconcile with the requirements of LET and suffer from a strong model dependence due to off-shell formfactors. With the completion of the new electron accelerators with high duty factor and high intensity, new types of coincidence experiments with polarization degrees of freedom will be performed. The authors review the formalism for such reactions and present the multipole decomposition of the various response functions. The new experimental capabilities allow some of the most challenging problems of intermediate energy physics to be attacked, such as the puzzle of neutral pion production at threshold, the electric quadrupole amplitude in the region of the Delta resonance (bag deformation), the Coulomb monopole amplitude near the Roper resonance ('breathing mode'), the production of eta -mesons via the N*(1535) and the production of two pions and more to test consequences of chiral symmetry and the interaction of pions at low energies.

Pion photo- and electroproduction in the Δ region

A nonrelativistic operator appropriate for the description of electromagnetic pion production in the Δ region is presented. Feynman diagrams are used as driving terms upon which rescattering takes place so that unitarity is automatically satisfied. The amplitude compares well as regards multipoles and differential cross-section in the nucleon case, and is constructed in a frame-independent and gauge-invariant way. Since the operator stems from a microscopical theory, it can handle off-shell effects. It is therefore suitable for nuclear applications, incorporating in a parameter-free way π-nucleus dynamics.

Some comments on the π-3He-3H coupling constant

Some comments are made on recent determinations of the π-3He-3H coupling constant. It is pointed out how the impulse approximation and the elementary particle method treat differently two step processes like radiative pion or muon capture and photo pion production.

Low-energy photo- and electroproduction for physical pions. I. Ward-identity and chiral-symmetry-breaking structure

The Ward identities of current algebra are combined with gauge-invariance constraints, on-shell partial conservation of the axial-vector current, and the Bjorken limit to obtain the low-energy expressions of the pion photo- and electroproduction invariant amplitudes for physical pions.

Comparison of exchange mechanisms in πN → ϱN and pion photo- and electroproduction

Pho-, photo- and electroproduction are compared and contrasted from a t-channel exchange point of view. A common exchange mechanism is evident. Systematic differences associated with the variable mass of the vector particle are found - in particular, the mass dependence of the non-pole-like contributions is reminiscent of that seen in higher mass resonance production. Naive vector meson dominance arguments which do not allow for these mass-dependent effects are shown to disagree both qualitatively and quantitatively with the data. We emphasize the implications of this, and similar studies, for an understanding of absorption effects in two-body scattering.

Absorption corrections in a mueller regge analysis of inclusive photo- and electroproduction

We discuss the photon fragmentation region of inclusive pion photo- and electroproduction in terms of a helicity dependent Mueller-Regge model, in which cut contributions are taken into accout. It is shown that the inadequacies of a pure Regge pole expansion can be corrected if certain types of Regge cut contributions are included. The transition from electro-production to the photo-production limit is shown to be non-smooth in the case of charged pions and we point out that normal triple-Regge mechanisms are unlikely to be important in the deep inelastic region.

Low-energy pion photo- and electroproduction in a dual relativistic quark model

A dual relativistic quark model is constructed for pion photo- and electroproduction. It has a satisfactory high-energy behaviour, and a quark model mass spectrum and factorization properties on the leading trajectory. The introduction ofU6 symmetry breaking for the exchanged trajectories is examined in detail and the gauge invariance constraints are also discussed. The low-energy properties of the model are compared with the algebra of currents predictions and experiment.

The isovector asymmetry in ϱ-production, pion photo- and electroproduction

The asymmetry ratio ϱ 1−1 ϱ 11 of the density matrix elements measured in ϱ-production, pion photo- and electroproduction is discussed in the limit s → ∞. We stress the sensitivity of this asymmetry to the ratio R = F 1(k 2) F π(k 2) of the nucleon to pion form factor, which may be different in the three reactions. The breakdown of vector meson dominance found in ϱ 1−1 ϱ 11 is very likely also due to a change of R in going from k 2 = 0 to k 2 = m ϱ 2 and it will therefore persist even in the limit s → ∞.

Inelastic photopion production with excitation of giant resonance analogs: application to 16O

Using a generalized Goldhaber-Teller model of the giant multipole resonances in 16O, we have evaluated the differential cross section for the photoproduction of positive pions near threshold leading to giant resonance spin-flip state of various multipolarities in 16N, and have obtained the corresponding pion spectra assuming monochromatic incident photons. A possible subsequent neutron emission was also considered We find that at forward angles of the emitted pion the spectra are dominated by the dipole resonances, but that at larger angles the quadrupole resonances appear strongly, making it possible to experimentally identify the two types of resonances Owing to the variability of its momentum transfer, inelastic nuclear pion photo- production is potentially as powerful for a study of analog excitation, as inelastic electron scattering has become for ΔT 3 = 0 nuclear excitations