Single Pion Photoproduction Research Articles

Single-pion contribution to the Gerasimov-Drell-Hearn sum rule and related integrals

Phenomenological amplitudes obtained in partial-wave analyses (PWA) of single-pion photoproduction are used to evaluate the contribution of this process to the Gerasimov-Drell-Hearn (GDH), Baldin, and Gell-Mann--Goldberger--Thirring (GGT) sum rules, by integrating up to $2\phantom{\rule{4pt}{0ex}}\mathrm{GeV}$ in photon energy. Our study confirms that the single-pion contribution to all these sum rules converges even before the highest considered photon energy, but the levels of saturation are very different in the three cases. Single-pion production almost saturates the GDH sum rule for the proton, while a large fraction is missing in the neutron case. The Baldin integrals for the proton and the neutron are both saturated to about four fifths of the predicted total strength. For the GGT sum rule, the wide variability in predictions precludes any definitive statement.

Polarization Transfer in Wide-Angle Compton Scattering and Single-Pion Photoproduction from the Proton.

Wide-angle exclusive Compton scattering and single-pion photoproduction from the proton have been investigated via measurement of the polarization transfer from a circularly polarized photon beam to the recoil proton. The wide-angle Compton scattering polarization transfer was analyzed at an incident photon energy of 3.7 GeV at a proton scattering angle of θ_{cm}^{p}=70°. The longitudinal transfer K_{LL}, measured to be 0.645±0.059±0.048, where the first error is statistical and the second is systematic, has the same sign as predicted for the reaction mechanism in which the photon interacts with a single quark carrying the spin of the proton. However, the observed value is ~3 times larger than predicted by the generalized-parton-distribution-based calculations, which indicates a significant unknown contribution to the scattering amplitude.

Meson photoproduction and baryon resonances at MAMBO experiment

Photoproduction of mesons within the framework of the MAMBO experiment (BGO-OD at Bonn plus MAMI at Mainz) was studied. The results on the operative work of the cryogenic H2/D2 target system during the last commissioning beam times at the March and June 2012 are shown. Investigation of the single charged pion photoproduction was provided using a polarized 3He target at the tagged photon facility of the MAMI accelerator. Unpolarized and helicity dependent cross sections are presented for channels γN → π±X in the Δ(1232) baryon resonance region.

Multichannel parametrization ofπNscattering amplitudes and extraction of resonance parameters

We present results of a new multichannel partial-wave analysis for $\ensuremath{\pi}N$ scattering in the c.m. energy range 1080 to 2100 MeV. This work explicitly includes $\ensuremath{\eta}N$ and $K\ensuremath{\Lambda}$ channels and the single pion photoproduction channel. Resonance parameters were extracted by fitting partial-wave amplitudes from all considered channels using a multichannel parametrization that is consistent with $S$-matrix unitarity. The resonance parameters so obtained are compared to predictions of quark models.

Photoexcitation of the quasi-bound isobar nuclear state of a nucleus in the (γ,πN) reaction

The data from three experiments performed on the Saclay linac, at the Tomsk synchrotron, and at the MAMI accelerator in Maintz, in which single pion photoproduction on light nuclei was accompanied by the emission of nucleons, are analyzed. The behavior of the experimental cross sections, which cannot be explained in the impulse approximation, is interpreted in a model where an intermediate quasi-bound isobar nuclear state is formed and decays with the emission of a pion and a nucleon.

Double and single pion photoproduction within a dynamical coupled-channels model

Within a dynamical coupled-channels model that has already been fixed by analyzing the data of the $\ensuremath{\pi}N\ensuremath{\rightarrow}\ensuremath{\pi}N$ and $\ensuremath{\gamma}N\ensuremath{\rightarrow}\ensuremath{\pi}N$ reactions, we present the predicted double pion photoproduction cross sections up to the second resonance region, $Wl1.7$ GeV. The roles played by the different mechanisms within our model in determining both the single and double pion photoproduction reactions are analyzed, focusing on the effects attributable to the direct $\ensuremath{\gamma}N\ensuremath{\rightarrow}\ensuremath{\pi}\ensuremath{\pi}N$ mechanism, the interplay between the resonant and nonresonant amplitudes, and the coupled-channels effects. The model parameters that can be determined most effectively in the combined studies of both the single and double pion photoproduction data are identified for future studies.

Photoproduction of pion-nucleon pairs on nuclei in the Δ(1232)-resonance region

Data from three experiments performed at the linear accelerator in Saclay, the Tomsk synchrotron, and the MAMI accelerator in Mainz and devoted to studying single pion photoproduction on light nuclei that is accompanied by nucleon emission are analyzed. The behavior of respective experimental cross sections, which is inexplicable in the impulse approximation, is interpreted within a model where the formation of an intermediate quasibound isobar-nucleus state is followed by its decay to a final state involving a pion and a nucleon.

Monte Carlo study for γ + N → π + N at a new compound target**Supported by National Natural Science Foundation of China (10675138)

An inbuilt compound target composed of carbon and tungsten is designed, and optimized by realistic GEANT4 Monte Carlo simulation. Also, we do a Monte Carlo study for single-pion photoproduction at the target. The results are presented from the simulation of pion yield, angular distribution and spectrum (at θlab, θcm = 41°). These efforts are important to the coming measurement of the differential cross section for γ + N → π + N.

Helicity dependence of the reaction channels above π-threshold

Based on a prior effective Lagrangian model on single pion photoproduction from the nucleon, successfully tested in total and differential cross-sections, we study theoretically the helicity dependence from the interaction of circularly polarized photons and a longitudinally polarized nucleon target in the energy range from π-threshold through the Δ(1232)-resonance region. The doubly polarized total and differential cross-section differences for parallel and antiparallel helicity states are predicted and compared with the experiment. We show that the results are sensitive to interferences among different contributions to the process and, thus, represent a complementary test of the theoretical model. A quite satisfactory agreement with recent experimental data from the GDH-collaboration is obtained.

Double-pion photoproduction on nucleon and deuteron

Photoproduction of two pions on nucleon and deuteron is studied for photon energies from threshold up to $E_\gamma = 1.5$ GeV. For the elementary operator an effective Lagrangean approach is used with resonance and Born contributions. The model parameters are fixed by resonance decay widths and multipole analyses of single pion photoproduction. A satisfactory description of total cross sections of two-pion production on the proton for various charge channels is achieved, except for $\pi^0\pi^0$ production for which a significant underestimation is found near threshold. The operator then serves for the evaluation of this reaction on the deuteron in the impulse approximation. In addition, $NN$ rescattering in the final state is taken into account, but $\pi N$ and $\pi\pi$ rescatterings are neglected. Results are presented for total cross sections and target asymmetries.

Transparency of nuclei and mean free path of protons and pions in nuclear matter at intermediate energies

In the second resonance region of photon energies, the differential yields of neutral and charged pions are measured in the (γ, πp) reactions on Li, C, and Al nuclei versus the proton energy and the azimuthal angle of pion emission. These experimental data are analyzed on the basis of a model that takes into account both single and double quasifree pion photoproduction. This results in deriving the mean free paths of protons and neutral and charged pions and in estimating the transparency of the participant nuclei.

Production of neutral and charged pions in the reaction 12C(γ, πp) in the second resonance region

The results are presented that were obtained by investigating the production of pion-proton pairs in the reaction 12C(γ, πp). The experiment in question was performed in the second resonance region of photon energy. The results of this measurement are analyzed on the basis of a model that takes into account the processes of single and double quasifree pion photoproduction.

Effect of double-polarization data in fits to single-pion photoproduction

We investigate the influence of new Jefferson Lab polarization-transfer ${(C}_{{x}^{\ensuremath{'}}}$ and ${C}_{{z}^{\ensuremath{'}}})$ and recoil polarization ${(P}_{y})$ measurements in multipole fits to the single-pion photoproduction database. Results are compared to those found with the addition of a double-polarization quantity associated with the Gerasimov--Drell-Hearn sum rule.

Final-state interaction in spin asymmetry and GDH sum rule for incoherent pion production on the deuteron

The contribution of incoherent single pion photoproduction to the spin response of the deuteron, i.e., the asymmetry of the total photoabsorption cross section with respect to parallel and antiparallel spins of photon and deuteron, is calculated over the region of the $\Delta$-resonance with inclusion of final state $NN$- and $\pi N$-rescattering. Sizeable effects, mainly from $NN$-rescattering, are found leading to an appreciable reduction of the spin asymmetry. Furthermore, the contribution to the Gerasimov-Drell-Hearn integral is explicitly evaluated by integration up to a photon energy of 550 MeV. Final state interaction reduces the value of the integral to about half of the value obtained for the pure impulse approximation.

THE SAID PWA PROGRAM

The range of SAID activities is outlined. These include database management, partial-wave analysis, and the determination of resonance properties. As the relationships between amplitude analysis, partial-wave analysis, and resonance extraction are often confused, comments are made in order to clarify the key differences. The multipole analysis of single-pion photoproduction is described in detail in order to show model-dependence appears in our fits to data.

Analysis of pion photoproduction data

A partial-wave analysis of single-pion photoproduction data has been completed. This study extends from threshold to 2 GeV in the laboratory photon energy, focusing mainly on the influence of new measurements and model-dependence in the choice of parameterization employed above the two-pion threshold. Results are used to evaluate sum rules and estimate resonance photo-decay amplitudes. These are compared to values obtained in the MAID analysis.

Measurements of the mean free paths of the proton and pions in the nuclear matter

The results of measuring the yields of neutral and charged pions in the (γ, π p) reaction on Li, C, and Al nuclei are presented as functions of the proton energy and azimuth angle of pion emission. The experiment was performed in the second resonance region of the photon energy. The measurements are analyzed within the model of single and double quasi-free pion photoproduction. The energy dependences of the mean free path of the proton and neutral and charged pions are obtained based on the dependence of the reaction yield on the atomic number of the target nucleus.

Helicity dependence of gammap --> Npi below 450 MeV and contribution to the Gerasimov-Drell-Hearn sum rule.

The helicity dependence of the single pion photoproduction on the proton has been measured in the energy range from 200 to 450 MeV for the first time. The experiment, performed at the Mainz microtron MAMI, used a 4pi-detector system, a circularly polarized, tagged photon beam, and a frozen-spin target. The data obtained provide new information for multipole analyses of pion photoproduction and determine the main contributions to the Gerasimov-Drell-Hearn sum rule and the forward spin polarizability gamma(0).

Soft contributions to hard pion photoproduction

Hard, or high transverse momentum, pion photoproduction can be a tool for probing the parton structure of the beam and target. We estimate the soft contributions to this process, with an eye toward delineating the region where perturbatively calculable processes dominate. Our soft process estimate is based on vector meson dominance and data based parameterizations of semiexclusive hadronic cross sections. We find that soft processes dominate in single pion photoproduction somewhat past 2 GeV transverse momentum at a few times 10 GeV incoming energy. The recent polarization asymmetry data is consistent with the perturbative asymmetry being diluted by polarization insensitive soft processes. Determining the polarized gluon distribution using hard pion photoproduction appears feasible with a few hundred GeV incoming energy (in the target rest frame).

Sensitivity to the pion-nucleon coupling constant in partial-wave analyses of πN → πN, NN → NN, and γN → πN

We summarize results obtained in our studies of the pion-nucleon coupling constant. Several different techniques have been applied to πN and NN elastic-scattering data, and the existing database for single-pion photoproduction. The most reliable determination comes from πN elastic scattering. The sensitivity in this reaction was found to be greater, by at least a factor of 3, when compared with analyses of NN elastic scattering or single-pion photoproduction.