Quasifree pion photoproduction on the deuteron in the Δ region

Photo production of pions on the deuteron is studied in the spectator nucleon model. The Born terms of the elementary production amplitude are determined in pseudovectorπN coupling and supplied with a form factor. TheΔ resonance is considered both in thes and theu channel. The parameters of theΔ resonance and the cutoff of the form factors are fixed on the leading photoproduction multipoles. Results for total and differential cross sections are compared with experimental data. Particular attention is paid to the role of Pauli correlations of the final state nucleons in the quasifree case. The results are compared with those for pion photoproduction on the nucleon.     

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 (P₃₃(1232), P₁₁(1440), D₁₃(1520), S₁₁(1535), F₁₅(1680), and D₃₃(1700)). The resonance contributions are included taking into account unitarity to provide the correct phases of the pion photoproduction multipoles. The Q² 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.     

The reaction πN → ππN in a meson-exchange approach

A resonance model for two-pion production in the pion-nucleon reaction is developed that includes information obtained in the analysis of pion-nucleon scattering in a meson-exchange model. The baryonic resonances Δ(1232), N^*(1440), N^*(1520), N^*(1535), and N^*(1650) are included. The model reproduces the total cross-sections up to kinetic energies of the incident pion of 400MeV and obtains the shapes of the differential cross-sections in reasonable agreement with the data.     

Quasifree pion photoproduction on the deuteron in the Δ region

Photo production of pions on the deuteron is studied in the spectator nucleon model. The Born terms of the elementary production amplitude are determined in pseu-dovector πN coupling and supplied with a form factor. The Δ resonance is considered both in the s and the u channel. The parameters of the Δ resonance and the cutoff of the form factors are fixed on the leading photoproduction multipoles. Results for total and differential cross sections are compared with experimental data. Particular attention is paid to the role of Pauli correlations of the final state nucleons in the quasifree case. The results are compared with those for pion photoproduction on the nucleon.     

Neutral pion photoproduction off nucleons revisited

We investigate threshold pion photoproduction in the framework of heavy baryon chiral perturbation theory. We give the expansion of the electric dipole amplitude E ₀₊ to three orders in μ the ratio of the pion to nucleon mass, and show that it is slowly converging. We argue that this observable is not a good testing ground for the chiral dynamics of QCD. In contrast, we exhibit new and fastly converging low-energy theorems in the P-waves which should be used to constrain the data analysis. We also discuss the importance of polarization observables to accurately pin down certain multipoles and give predictions for the reaction γ n → π⁰ n.     

Pion-production in heavy-ion collisions at SIS energies

We investigate the production of pions in heavyion collisions in the energy range of 1–2 GeV/A. The dynamics of the nucleus-nucleus collisions is described by a set of coupled transport equations of the Boltzmann-Uehling-Uhlenbeck type for baryons and mesons. Besides theN(938) and theΔ(1232) we also take into account nucleon resonances up to masses of 1.95 GeV/c² as well asπ-,η- andρ-mesons. We study in detail the influence of the higher baryonic resonances and the 2π-production channels (NN→NNππ) on the pion spectra in comparison toπ ^? data fromAr+KCl collisions at 1.8 GeV/A andπ ⁰-data forAu+Au at 1.0GeV/A. We, furthermore, present a detailed comparison of differential pion angular distributions with the BEVALAC data forAr+KCl at 1.8 GeV/A. The general agreement obtained indicates that the overall reaction dynamics is well described by our novel transport approach.     

Parity violating pion electroproduction off the nucleon

Parity violating (PV) contributions due to interference betweenγ and Z⁰ exchange are calculated for pion electroproduction off the nucleon. A phenomenological model with effective Lagrangians is used to determine the resulting asymmetry for the energy region between threshold andΔ(1232) resonance. TheΔ resonance is treated as a Rarita-Schwinger field with phenomenological NΔ transition currents. The background contributions are given by the usual Born terms using the pseudovectorπN Lagrangian. Numerical results for the asymmetry are presented.     

A non-relativistic operator convenient for analysis of pion photoproduction on nuclei in the Δ(1236) region

In view of its application to the analysis of medium energy photonuclear reactions we have obtained an operator valid, to order $\text{(}\text{p}\text{m}\text{)}{}^2$, in any frame of reference. We have started from a model where the pion photoproduction on a nucleon is described by the Born terms and the s-channel Δ(1236) formation amplitude and we have deduced the non-relativistic limit of the corresponding matrix elements. Both pseudo-scalar and pseudo-vector pion-nucleon couplings are compared. The Δ(1236) parameters are chosen so as to lead to a good account of pion-nucleon reaction cross sections. The agreement with the charged photoproduction data is very good from threshold through the Δ(1236) energy region, whereas the addition of ω⁰ exchange in the t-channel is necessary to reproduce the neutral pion photoproduction cross sections. As an example this operator is used to derive the cross section of the γD → ppπ^? reaction in the spectator nucleon model when the undiscernability of the two outgoing protons is taken into account.     

Pion and eta photoproduction in nuclei

We calculate the quasifree photoproduction of pions and eta mesons on nuclei from¹²C to²⁰⁸Pb. Assuming that for the quasifree process the cross sections forπ/η photoproduction on a nucleon in medium are — besides a lowering due to Pauli blocking — identical to those on a free nucleon, we use these as an input. The produced mesons or resonances then propagate through the nucleus where they can undergo several scattering and absorption processes. This final state interaction, which leads to a lowering of the initial meson yield up to 70%, we simulate in a coupled channel Boltzmann-Uehling-Uhlenbeck transport model. It allows us to study in detail the influence of Fermi motion, Pauli blocking and of the additional decay channels for nucleon resonances in medium on the photoproduction cross section. The calculated total and differential cross sections are compared to experimental data and the influence of the different medium effects on the cross sections is discussed in detail.     

Measurement of coherent π0 photoproduction on3He and3H in the resonance region

Neutral pion photoproduction has been measured on³He and³H nuclei in theΔ(1232) resonance region. Resonance averaged cross-sections are presented as a function of momentum transfer and compared to theoretical calculations.     

Exclusive electroproduction of K+Λ and K+Σ0 final states at Q2 = 0.030–0.055 (GeV/c)2

Cross-section measurements of the exclusive p(e, e??K ⁺)??, ?? ⁰electroproduction reactions have been performed at the Mainz Microtron MAMI in the A1 spectrometer facility using for the first time the KAOS spectrometer for kaon detection. These processes were studied in a kinematical region not covered by any previous experiment. The nucleon was probed in its third-resonance region with virtual photons of low four-momenta, Q ² = 0.030?C0.055 (GeV/c)². The MAMI data indicate a smooth transition in Q² from photoproduction to electroproduction cross-sections. Comparison with predictions of effective Lagrangian models based on the isobar approach reveal that strong longitudinal couplings of the virtual photon to the N^* resonances can be excluded from these models. Modern isobar and Regge-plus-resonance models are in agreement with the data.     

Determination of Breit-Wigner features of the S 11(1535), S 11(1650), and P 11(1710) nucleon resonances

Basic Breit-Wigner features of the S ₁₁(1535), S ₁₁(1650), and P ₁₁(1710) nucleon resonances are evaluated in a model-independent way on the basis of the results obtained previously from a partial-wave analysis of eta-meson photoproduction on protons.     

Regge approach to charged pion photoproduction at invariant energies above 2 GeV

A Regge model with absorptive corrections is employed in a global analysis of the world data on positive and negative pion photoproduction for photon energies from 3 to 8GeV. In this region resonance contributions are expected to be negligible so that the available experimental information on differential cross-sections and single polarization observables at - t ⩽ 2 GeV^2 allows us to determine the non-resonant part of the reaction amplitude reliably. The model amplitude is then used to predict observables for photon energies below 3GeV. Differences between our predictions and data in this energy region are systematically examined as possible signals for the presence of excited baryons. We find that the data available for the polarized photon asymmetry show promising resonance signatures at invariant energies around 2GeV. With regard to differential cross-sections the analysis of negative pion photoproduction data, obtained recently at JLab, indicates likewise the presence of resonance structures around 2GeV.     

The role of the pion cloud in the interpretation of the valence light-cone wavefunction of the nucleon

The pion cloud renormalises the light-cone wavefunction of the nucleon which is measured in hard, exclusive photon-nucleon reactions. We discuss the leading twist contributions to high-energy exclusive reactions taking into account both the pion cloud and perturbative QCD physics. The nucleon’s electromagnetic form-factor at high Q² is proportional to the bare nucleon probability Z and the cross-sections for hard (real at large angle or deeply virtual) Compton scattering are proportional to Z². Our present knowledge of the pion-nucleon system is consistent with Z = 0.7 ± 0.2. If we apply just perturbative QCD to extract a light-cone wavefunction directly from these hard exclusive cross-sections, then the light-cone wavefunction that we extract measures the three valence quarks partially screened by the pion cloud of the nucleon. We discuss how this pion cloud renormalisation effect might be understood at the quark level in terms of the (in-)stability of the perturbative Dirac vacuum in low energy QCD.     

A partial-wave analysis for proton compton scattering in the Δ(1232) energy region

Based on the nowadays available phenomenological multipoles for pion photoproduction, a systematic dispersion theoretic calculation of Compton partial amplitudes has been performed. By comparing with the new experimental data in the Δ(1232) region it is found that the hitherto existing discrepancies between the data and theory remain. Our results allow us to isolate the basis of this problem. It is due to the fact that the imaginary parts of the Compton amplitudes determined already by unitarity, give too large contributions to the cross section thus leaving no space for real parts required by the dispersion integrals.Therefore a simultaneous partial-wave analysis for Compton scattering and pion photoproduction was carried through employing only the unitarity connections between both. The resulting amplitudes give a good fit to the Compton cross section and all photoproduction data; moreover the photoproduction multipoles agree essentially with those of the previous analyses. On the other hand for Compton scattering, phenomenological amplitudes are obtained for the first time and a model independent test of dispersion theoretic amplitudes can be carried out. For the resonating f_MM¹⁺ amplitude which describes the M1 excitation and deexcitation to the Δ-isobar, large discrepancies have been found. Possible consequences especially for the forward scattering amplitude and the validity of the Kramers-Kronig relation are discussed.     

On narrow nucleon excitation <Emphasis Type="Italic">N</Emphasis>*(1685)

We perform estimates on putative narrow nucleon N* (1685), which is a candidate for the non-strange member of the exotic anti-decuplet of baryons. In particular, we consider the recent high precision data on η photoproduction off free proton obtained by the Crystal Ball Collaboration at MAMI. We show that it is difficult to describe peculiarities of these new data in the invariant energy interval of W ∼ 1650–1750 MeV in terms of known wide resonances. Using very simple estimates, we show that the data may indicate an existence of a narrow N* (1685) with small photocoupling to the proton.     

Photoexcitation of a quasibound delta-nucleus state of a nucleus

The photoproduction of negatively charged pions on a carbon nucleus in the (γ, π ^?, p) and (γ, π ^?, pp) reactions is studied experimentally. The results of the respective measurements are explained on the basis of a model that assumes the formation of an intermediate quasibound delta-nucleus state (delta nucleus) that decays via the emission of a pion and a nucleon. The delta-nucleus mass and width are estimated for the example of ¹¹B_Δ.