Electromagnetic Interactions in Nuclear and Hadron Physics

The following sections are included:

• COMMITTEES

• PICTURE

• PREFACE

• CONTENTS

A review of data on the nucleon electromagnetic form factors in the space-like region is presented. Recent results from experiments using polarized beams and either polarized targets or nucleon recoil polarimeters have yielded a significant improvement on the precision of the data obtained with the traditional Rosenbluth separation. Future plans for extended measurements are outlined.

Within a chiral constituent quark formalism, allowing the inclusion of all known resonances, a comprehensive study of the recent η photoproduction data on the proton up to is performed. This study shows evidence for a new S₁₁ resonances and indicates the presence of an additional missing P₁₃ resonance.

The GeV photon beam at SPring-8 is produced by backward-Compton scattering of laser photons from 8 GeV electrons. Polarization of the photon beam will be ∼100% at the maximum energy with fully polarized laser photons. We report the status of the new facility and the prospect of hadron physics study with this high quality beam. Preliminary results from the first physics run are presented.

Experiments at JLab, Mainz, and MIT-Bates are probing weak nucleon form factors by measuring the parity-violating asymmetry in the scattering of polarized electrons from nucleons. The goal of this work is the extraction of the contribution of strange quarks to nucleon form factors.

High energy electron beams can act as sensitive probes of the structure of nuclear matter at very small scales. Electron scattering experiments have given information on the distribution of electric charge and magnetization inside the proton and neutron, represented by the electric and magnetic form factors, the so called Sachs form factors. These are functions of the four-momentum transferred by the virtual photon, which mediates the electromagnetic scattering process. But the weak interaction also contributes to electron scattering through the exchange of the Z⁰ boson, and one can probe entirely different aspects of the proton and neutron structure. With the Z⁰ contribution so small one has to make use of the fact that the weak interaction exhibits parity violation. At the present relatively little is known about the analogous weak form factors of the proton and neutron. Of particular interest is the possibility of disentangling the strange quark contributions to these form factors, which can be accomplished by combining measurements of the parity violating asymmetry (longitudinal analyzing power) with measurements of the electric and magnetic form factors. This presents the objective of the G0 experiment at Jefferson Laboratory. A status report is given in light of recent results of the kinematically more restrictive SAMPLE (MIT-Bates), HAPPEX (Jefferson Laboratory), and PV-A4 (Mainz) experiments. The first one of these showed the importance of the nucleon anapole moment contribution to the axial form factor. The anapole moment is discussed with reference to the weak-meson nucleon couplings determined in hadronic parity violation experiments.

Measurement of asymmetry in polarized electron-nucleon elastic scattering provides information on the weak charge and current distributions in the nucleon through the interference between photon and Z⁰ exchange graphs. This asymmetry is due to the parity violation in the weak interaction. The expansion of weak form factors in term of flavor (u,d,s) form factors leads to information on quark structure of the nucleon, especially on the strangeness contribution to the vector form factors. The measurement of the parity violating asymmetry in elastic scattering of polarized electrons on protons at MAMI facility in Mainz is presented. At the MAMI current maximum energy of 0.855 GeV, the electrons scattered out a 10 cm liquid hydrogen target are detected between 30° and 40°. Under these kinematic conditions, the value of the transfer momentum Q², weighted by the elastic cross section in this angular range, is Q² = 0.233 (GeV/c)². This experiment measures the , combination of the electric and magnetic strange form factors. The detection is done using a calorimeter built of 511 Čerenkov crystals in the phase I of the experiment. The experimental set-up with its main characteristics are shown. The origin and magnitude of possible systematic and helicity correlated false asymmetries are discussed.

Parity violating electron scattering can measure the neutron density of a heavy nucleus accurately and model independently. This is because the weak charge of the neutron is much larger then that of the proton. The Parity Radius Experiment (PREX) at Jefferson Laboratory aims to measure the root mean square neutron radius of ²⁰⁸Pb with an absolute accuracy of 1% (±0.05 Fm). This is more accurate then past measurements with hadronic probes, which all suffer from controversial strong interaction uncertainties. PREX should clearly resolve the neutron-rich skin. Furthermore, this benchmark value for ²⁰⁸Pb will provide a calibration for hadronic probes, such as proton scattering, which can then be used to measure neutron densities of many exotic nuclei. The PREX result will also have many implications for neutron stars. The neutron radius of Pb depends on the pressure of neutron-rich matter: the greater the pressure, the larger the radius as neutrons are pushed out against surface tension. The same pressure supports a neutron star against gravity. The Pb radius is sensitive to the equation of state at normal densities while the radius of a 1.4 solar mass neutron star also depends on the equation of state at higher densities. Measurements of the radii of a number of isolated neutron stars such as Geminga and RX J185635-3754 should soon improve significantly. By comparing the equation of state information from the radii of both Pb and neutron stars one can search for a softening of the high density equation of state from a phase transition to an exotic state. Possibilities include kaon condensates, strange quark matter or color superconductors.

Studies in nuclear astrophysics have long been associated with long runs at small accelerators, measuring ever-decreasing cross sections as one approached (but rarely reached) the energy of reactions in stars. But in recent years pioneering studies have shown that studies at high-energy accelerators can often yield the same information, and in some important cases, provide information not otherwise available. This is particularly so for studies of the properties and reactions of the short-lived radioactive nuclei that play a crucial role in explosive phenomena such as novae, supernovae, and neutron stars. I'll give an overview of some of the possibilities, and then concentrate on two extended examples: measurements of the rates of radioactive capture reactions using Coulomb breakup reactions, and the relationship of charge exchange cross sections and beta-decay strength for L = 1 transitions.

Photonuclear reactions have direct impact on the nucleosynthesis of the p-process nuclei and play a role of probing radiative capture processes. Laser-electron MeV photon beams (LEMPBs) developed at AIST have been used for nuclear astrophysics experiments. We discuss two topics of the supernova nucleosynthesis of ⁹Be and the threshold behavior of ¹⁸¹Ta(γ,n) cross sections in conjunction with the origin of the p-process nuclei.

The outcome of helium burning is the formation of the two elements, carbon and oxygen. The ratio of carbon to oxygen at the end of helium burning is crucial for understanding the final fate of a progenitor star and the nucleosynthesis of heavy elements in Type II supernova. While an oxygen rich star is predicted to end up as a black hole, a carbon rich star leads to a neutron star. Type Ia supernovae (SNIa) are used as standard candles for measuring cosmological distances with the use of an empirical light curve-luminosity stretching factor. It is essential to understand helium burning that creates the carbon/oxygen white dwarf and thus the initial stage of SNIa. Since the triple alpha-particle capture reaction, ⁸Be(α,γ)¹²C, the first burning stage in helium burning, is well understood, one must extract the cross section of the ¹²C(α,γ)¹⁶O reaction at the Gamow peak (300 keV) with high accuracy of approximately 10% or better. This goal has not been achieved, despite repeated strong statements that appeared in the literature. Constraints from the beta-delayed alpha-particle emission of ¹⁶N were shown to not sufficiently restrict the p-wave cross section factor; e.g. low values can not be rulled out. Measurements at low energies, are thus mandatory for measuring the ellusive cross section factor for the 12C(α,γ)¹⁶O reaction. We are constructing a Time projection Chamber (TPC) for use with high intensity photon beams extracted from the HIγS-TUNL facility at Duke University to study the ¹⁶O(γ,α)¹²C reaction, and thus the direct reaction at low energies, as low as 0.7 MeV. This work is in progress.

Recent observation of the power spectrum of Cosmic Microwave Background Radiation has exhibited that the flat cosmology is most likely. This suggests too large universal baryon-density parameter Ω_bh² ≈ 0.022 ~ 0.030 to accept a theoretical prediction, Ω_bh² ≤ 0.017, in the homogeneous Big-Bang model for primordial nucleosynthesis. Theoretical upper limit arises from the sever constraints on the primordial ⁷Li abundance. We propose two cosmological models in order to resolve the descrepancy; lepton asymmetric Big-Bang nucleosynthesis model, and baryon inhomogeneous Big-Bang nucleosynthesis model. In these cosmological models the nuclear processes are similar to those of the r-process nucleosynthesis in gravitational collapse supernova explosions. Massive stars ≥ 10M_⊙ culminate their evolution by supernova explosions which are presumed to be the most viable candidate site for the r-process nucleosynthesis. Even in the nucleosynthesis of heavy elements, initial entropy and density at the surface of proto-neutron stars are so high that nuclear statistical equilibrium favors production of abundant light nuclei. In such explosive circumstances many neutron-rich radioactive nuclei of light-to-intermediate mass as well as heavy mass nuclei play the significant roles.

The γn → π⁻p and γp → π⁺n reactions are essential probes of the transition from meson-nucleon degrees of freedom to quark-gluon degrees of freedom in exclusive processes. The cross sections of these processes are also advantageous, for investigation of the oscillatory behavior around the quark counting prediction, since they decrease relatively slower with energy compared with other photon-induced processes. Moreover, these photoreactions in nuclei can probe the QCD nuclear filtering effects. In this talk, I discuss the preliminary results on the γn → π⁻p process at a center-of-mass angle of 90° from Jefferson Lab experiment E94-104. I also discuss a new proposal in which singles γp → π⁺n measurement from hydrogen, and coincidence γn → π⁻p measurements at the quasifree kinematics from deuterium and ¹²C for photon energies between 2.25 GeV to 5.8 GeV in fine steps at a center-of-mass angle of 90° are planned. The proposed measurement will allow the detailed investigation of the oscillatory scaling behavior in the photopion production differential cross-section and the study of the nuclear dependence of rather mysterious oscillations with energy that previous experiments have indicated. The various nuclear and perturbative QCD approaches, ranging from Glauber theory, to quark-counting, to Sudakov-corrected independent scattering, make dramatically different predictions for the experimental outcomes.

The Crystal Ball Spectrometer is being used at Brookhaven National Laboratory in a series of experiments which study all neutral final states of π⁻p and K⁻p induced reactions. We report about the experimental set up and progress in obtaining new results for the radiative capture reactions π⁻p → γn and K⁻p → γΛ,charge exchange π⁻p → π°n, two π° production π⁻p → π°π°n, and η production π⁻p → η reactions. Data have also been obtained on the decays of N*, Δ, λ, and Σ resonances. Threshold η production has been studied in detail for both π⁻p and K⁻p. Sequential resonance decays have been studied by studying the 2π° production mechanism both in the fundamental interaction and in nuclei. In addition, we have used the ηs produced near threshold to make precision measurements searching in particular for rare and forbidden η decays.

A solid, polarized HD target has been developed for the measurement of double-polarization observables in the Δ resonance region. We report here the inaugural data obtained with this target. This new polarized target technology, combined with the high quality LEGS photon beam and the large acceptance spectrometer, SASY, provides a unique facility for studying the spin structure of the nucleon below 500 MeV. Pion production data collected on a longitudinally polarized target with six γ-ray polarization states provides the first simultaneous measurement of as well as the Σ, G and E asymmetries. With the future addition of magnetic analysis to SASY, a complete set of pion production observables on the proton and the deuteron (neutron) will be obtained.

Neutral pion photoproduction from a liquid deuterium target was measured in the energy region near 300 MeV at the LEGS facility of Brookhaven National Laboratory. The inclusive cross sections from deuterium are in agreement with measurments from Mainz, yet the exclusive cross sections and spin asymmetries for neutral pion production in coincidence with a detected nucleon are much smaller than expected from a quasi-free approximation. This may indicate that substantial final state interactions play a significant role, which will complicate the extraction of the desired amplitudes that would be measured if a free neutron target could be used.

Pion cloud effect on nucleon structure and on nucleon resonance transition properties is studied based on a relativistic quark model approach. We display our calculated results for the electromagnetic form factors of nucleon, for the electro-production amplitudes of the Δ(1232) and P₁₁(1440) resonances and moreover, for the strong decay width of the Δ(1232) resonance. Our results indicate the important effect of the pion cloud on those physical observables in the low energy region.

The real- and virtual-photon emission during interactions between few-nucleon systems have been investigated at KVI with a 190 MeV proton beam. Here I will concentrate the discussion on the results of the virtual-photon emission for the proton-proton system and proton-deuteron capture. Predictions of a fully-relativistic microscopic-model of the proton-proton interaction are discussed. For the proton-deuteron capture process the data is compared with predictions of a relativistic gauge-invariant impulse approximation and a Faddeev calculation. For the virtual photon processes, the nucleonic electromagnetic response functions were obtained for the first time and are compared to model predictions.

The differential coincident pion electroproduction cross section of the ³He(e,e′π⁺)³H reaction in the excitation region of the Δ resonance has been measured with the high resolution three-spectrometer facility at the Mainz Microtron MAMI. It was the aim of the experiment to study the influence of the nuclear medium on the properties of the pion and the Δ(1232) resonance. Two experimental methods have been applied. For fixed four-momentum transfers Q² = 0.045 [0.100] (GeV/c)² with the pions detected in parallel kinematics, the incident energy was varied between 555 and 855 MeV in order to separate the longitudinal (L) and transverse (T) structure functions. In the second case the emitted pions with respect to the momentum transfer direction were detected over a large angular range at fixed incident energy E₀ = 855 MeV and the two fixed four–momentum transfers. From the angular distributions the LT interference term has been extracted. The experimental data are compared to model calculations which are based on the elementary pion production amplitude that contains besides the Born terms also the excitation of the Δ and higher resonances. Moreover, three-body Faddeev wave functions are used and the final state interaction of the outgoing pion is taken into account. The experimental cross sections are reproduced only after additional medium modifications of the pion and the Δ isobar have been considered in terms of self energies. In the framework of Chiral Perturbation Theory the pion self energy is related to a reduction of the π⁺ mass of in the neutron-rich nuclear medium at a density of . This result is fully consistent with the one obtained within a two-loop approximation of ChPT. It is also interesting to compare the determined negative mass shift Δm_π⁺ with a positive mass shift Δm_π− of 23 to 27 MeV/c² derived recently from deeply bound pionic states in ²⁰⁷Pb and ²⁰⁵Pb. Both pion mass shifts observed in complementary approaches may be understood within the Tomozawa-Weinberg scheme of isovector dominance of the πN interaction, which provides a strong guidance for the understanding of the pion modifications in the nuclear medium. The Δ self energy as extracted from previous data of π⁰ photoproduction from ⁴He is compatible with the results of the present experiment. Subsequently, the mass and the decay width of the Δ resonance in the nuclear medium are increased by ΔM_Δ = (40–50) MeV/c² and ΔΓ_Δ = (60–70) MeV in the considered kinematical region.

A review of the deuteron electromagnetic form factors, as measured in unpolarized and polarized elastic electron-deuteron scattering experiments is presented along with a brief description of theoretical models. Particular emphasis is given to measurements at large momentum transfers. The experimental data are compared to theoretical calculations based on Propagator and Hamiltonian Dynamics and predictions of perturbative Quantum Chromodynamics.

The deuteron has been studied in numerous elastic scattering and photodisintegration experiments over a wide range of momentum transfers. A primary motivation is investigating the validity of theories based on hadronic vs. quark degrees of freedom. It is often thought that hadronic approaches will break down at large momentum transfers, and it will be necessary to use quark degrees of freedom. I will very briefly review the status of our understanding of the underlying theoretical issues, the world data sets for elastic scattering and photodisintegration, and the ability of hadronic and quark based theories to explain the data. Hadronic theories appear to provide good explanations for elastic scattering, but photodisintegration is very difficult to understand.

Photon-induced reactions are the key reactions for the nucleosynthesis of neutron-deficient p-nuclei. We present a new technique which simulates a thermal photon bath at typical temperatures of several billion Kelvin during a supernova explosion. We are able to determine astrophysical reaction rates for the relevant photon-induced reactions. Additionally, we give some examples for the relevance of photon-induced reactions in the astrophysical s-process.

The manuscript reviews some astrophysically important weak-interaction processes. These include electron captures, beta-decays and neutrino-induced reactions in a core-collapse supernova, and β-decays and neutrino-nucleus processes in r-process nucleosynthesis. Giant resonance response plays an essential role in all these applications.

The High Intensity Gamma Ray Source (HI_γS), a collaborative project between TUNL and the Duke Free Electron Laser Laboratory at Duke University, is described. The results of some initial experiments and plans for the future research program are discussed.

Optimum nuclear parton distributions are investigated by analyzing high-energy nuclear reaction data. Valence-quark distributions at medium x and antiquark distributions at small x are determined by the data of F₂ structure function ratios. However, gluon distributions cannot be determined well. If Drell-Yan data are included in the analysis, the antiquark distributions are restricted in the region x ~ 0.1.

A description of the Roper using the chiral chromodielectric model is presented and the transverse A_1/2 and the scalar S_1/2 helicity amplitudes for the electromagnetic Nucleon–Roper transition are obtained for small and moderate Q². The sign of the amplitudes is correct but the model predictions underestimate the data at the photon point. Our results do not indicate a change of sign in any amplitudes up to Q² ~ 1 GeV². The contribution of the scalar meson excitations to the Roper electroproduction is taken into account but it turns out to be small in comparison with the quark contribution. However, it is argued that mesonic excitations may play a more prominent role in higher excited states.

In order to study the S₁₁(1535) resonance in the nuclear medium, total cross sections of the (γ, η) reaction on C have been measured for photon energies between 620 and 1100 MeV. This is the first result by using STB Tagger and SCISSORS in LNS, Sendai. Model calculations based on the quantum molecular dynamics (QMD) have been performed. The comparison of the calculation with the total cross section of the η photoproduction suggests that the resonance width of S₁₁(1535) in nuclei is more than 80 MeV broader than the natural width.

Vector meson photoproduction and electroproduction have been suggested as a tool to find or confirm the nucleon resonances. In order to extract more reliable informations on the nucleon resonances, understanding the non-resonant background is indispensable. We consider final state interactions in ω photoproduction as a background production mechanism. For the intermediate states, we consider nucleon–vector-meson and nucleon-pion channels. The role of the final state interactions is discussed in ω meson photoproduction near threshold.

We discuss the extention of the Gerasimov-Drell-Hearn (GDH) sum rule, which pertains to real photons, to include scattering due to virtual photons. We present data from Jefferson Laboratory experiment E94-010 which measured the inclusive scattering of polarized electrons from a polarized ³He target over the quasielastic and resonance regions. From these data we exctract the transverse-transverse interference cross section , and compute the Q² depenent extended GDH integral.

In order to further our understanding of the few body system, a new series of measurements on the reaction ³He(e,e'p) have been made at Jefferson Lab. Making use of beam energies as high as 4.8 GeV and of the two high resolution in Hall A, kinematics which were previously unattainable have been investigated. In this paper the first preliminary results of this experiment will be presented.

Finite Formation Time (FFT) effects in the exclusive reaction ⁴He(e,e′p)³H at high values of Q² are introduced and discussed. From its mechanism, the FFT effects attenuate the Glauber-type Final State Interaction(FSI) as the value of Q² grows. We investigate how and to what extent this effect becomes apparent. As a result it is shown that a dip in the distorted momentum distributions n_D(k) predicted by the Plane Wave Impulse Approximation (PWIA), which is filled by the Glauber-type Final State Interaction (FSI), is completely recovered at Q² ~ 20(GeV/c)².

The 90° differential cross sections have been measured for the two-body photodisintegration of ³H and ³He in the reactions ³H(γ,d)n and ³He(γ,d)p using tagged photons with energies between 18 and 50 MeV. The differential cross sections have been compared with recent predictions using different models of the nucleon-nucleon potential.

The production of charged hadrons in semi-inclusive deep inelastic scattering off nuclei has been studied by the HERMES experiment at DESY using 27.5 GeV positrons. A reduction of the multiplicity of charged hadrons and identified charged pions from nuclei relative to that from deuterium has been measured as a function of the relevant kinematic variables ν, z and . A larger reduction of the multiplicity ratio has been found for the krypton with respect to the one previously measured on nitrogen in agreement with a ~ A^2/3 power law. Both the krypton and nitrogen data show that the multiplicity ratio is the same for positive and negative pions, while a significant difference is observed between for positive and negative hadrons. This result can be interpreted in terms of a difference between the formation time of protons and pions. It has been also suggested that the observed differences between positive and negative hadrons can be attributed to a different modification of the quark and antiquark fragmentation functions in nuclei.

We have developed a relativistic formalism for studying quasi-free processes from nuclei. The formalism can be applied with ease to a variety of processes and renders transparent analytical expressions for all observables. We have applied it to kaon photoproduction and to electron scattering. For the case of the kaon, we compute the recoil polarization of the lambda-hyperon and the photon asymmetry. Our results indicate that polarization observables are insensitive to relativistic, nuclear target, and distortion effects. Yet, they are sensitive to the reactive content, making them ideal tools for the study of modifications to the elementary amplitude — such as in the production, propagation, and decay of nucleon resonances — in the nuclear medium. For the case of the electron, we have calculated the spectral function of ⁴He. An observable is identified for the clean and model-independent extraction of the spectral function. Our calculations provide baseline predictions for the recently measured, but not yet fully analyzed, momentum distribution of ⁴He by the A1-collaboration from Mainz. Our approach predicts momentum distributions for ⁴He that rival some of the best non-relativistic calculations to date.

We present theoretical investigation of quasielastic and Δ excitations for the ¹²C target induced by virtual photon in (e,e′) reaction. We attempt to describe the quasielastic and Δ regions in one formalism. We use the Tamm-Dancoff Approximation to deal with the nuclear many-body effects. The longitudinal and transverse structure functions are separately studied with and without including the particle-hole correlation. We investigate the contributions from different physical processes by decomposing the inclusive cross sections.

Kaon photo- and electroproduction processes on the deuteron are investigated theoretically. Modern hyperon-nucleon forces as well as an updated kaon production operator on the nucleon are used. Sizable effects of the hyperon-nucleon final state interaction are seen in various observables. Especially the photoproduction double polarization observables are shown to provide a handle to distinguish different hyperon-nucleon force models.

The advent of high-energy, CW-beams of electrons now allows electro-production and precision studies of nuclei containing hyperons. Previously, the injection of strangeness into a nucleus was accomplished using secondary beams of mesons, where beam quality and target thickness limited the missing mass resolution. We review here the theoretical description of the (e,e′K⁺) reaction mechanism, and discuss the first experiment demonstrating that this reaction can be used to precisely study the spectra of light hypernuclei. Future experiments based on similar techniques, are expected to attain even better resolutions and rates.

The differential cross section for the photoproductionof the φ (1020) near threshold (E_γ = 1.57GeV) is predicted to be sensitive to production mechanisms other than diffraction. However, the existing low energy data is of limited statistics and kinematical coverage. Complete measurements of φ meson production on the proton have been performed at The Thomas Jefferson National Accelerator Facility using a liquid hydrogen target and the CEBAF Large Acceptance Spectrometer (CLAS). The φ was identified by missing mass using a proton and positive kaon detected by CLAS in coincidence with an electron in the photon tagger. The energy of the tagged, bremsstrahlung photons ranged from φ-threshold to 2.4 GeV. A description of the data set and the differential cross section for (E_γ = 2.0 GeV) will be presented and compared with present theoretical calculations.

K⁺ photoproduction is expected to provide new insight into the structure of nucleon resonances. However, due to the lack of data and ambiguities in the theoretical descriptions, many questions remain unanswered. At the LEPS beam line at SPring-8, data for the K⁺ photoproduction of the proton have been taken between December 2000 and June 2001. Both differential cross section and photon-polarization asymmetries have been measured for the p(γ, K⁺)Λ and p(γ, K⁺)Σ⁰ reactions. The photon polarization asymmetries in particular will provide valuable information to further the understanding of these reaction channels and the presence of missing resonances. The center-of-mass energy range covered is 1.9 GeV < W < 2.3 GeV. The angular acceptance for detection of the K⁺ is between 0deg and 60deg in the center-of-mass frame. Here, we discuss the status of the analysis and the relevance of the observables that are accessible.

An extensive program of strange particle production off the proton is currently underway with the CEBAF Large Acceptance Spectrometer (CLAS) in Hall B at Jefferson Laboratory. Precision measurements of ground-state and low-lying excited-state hyperons are being carried out with both electron and real photon beams, both of which are available with high polarization at energies up to 6 GeV. This talk will focus on selected aspects of our strangeness physics program regarding electroproduction measurements of single and double-polarization observables.

The scalar mesons f₀(980) and a₀(985) are two of the best-verified states in the excited meson spectrum. They appear as narrow resonances in the 2π and ηπ mass spectra, respectively, observed in hadroproduction reactions and in the decays of heavier particles such as the J/Ψ and the Z⁰. Their relatively low mass and narrow width do not match well with quark-model expectations for the lightest scalar multiplet, fueling speculation that these states might be different in nature from conventional mesons. A variety of theoretical models have been put forward to explain these states, most notably the so-called “2-kaon molecule” model in which they appear as kaon-antikaon bound states. Within this picture, the radiative decays of the φ(1020) meson two two-pseudoscalar final states are expected to be dominated by f₀ and a₀ intermediate states, in spite of their appearance very near the edge of the available phase space. Recent experimental results from e⁺e⁺ colliders have recently provided striking verification of this effect. Meanwhile, new work within a more general theoretical approach to the two-meson scattering problem has shown that the appearance of narrow scalar resonances near the two-kaon threshold is not a unique prediction of the molecule model, but are a consequence of the general principles of unitarity and chiral symmetry. The relative merits of these different viewpoints are examined in light of the φ decay data.

At the GRAAL facility, a polarised and tagged ray beam is produced in the energy range from 500 MeV up to 1500 MeV. Results of beam polarisation asymmetries and cross sections for the photoproduction of η and π⁰ on the proton are presented. These very precise measurements cover the angular range 30°-150°, providing stringent constraints to theoretical models.

The self-consistent HF+RPA (TDA) model is applied to describe the excitation energies and the strength distributions of Gamow-Teller and spin-dipole resonances for charge exchange reactions on ²⁰⁸Pb. We found that the model predictions show reasonable agreement to recent experimental data. Isoscalar monopole resonances were also studied in relation to the nuclear compression modulus by using the same model. It is shown in drip line nuclei that strong peaks of the monopole resonances appear at the excitation energies not only in the giant resonance region but also near the threshold of the RPA response. The asymmetry term of nuclear compression modulus is discussed with giant monopole resonances in Ca-isotopes.

The high-energy solar neutrino flux is determined by the low-energy cross sections of the ⁷Be(p,γ)⁸B reaction. As well as many direct measurements, Coulomb dissociation with intermediate-energy ⁸B beams and low-energy proton-transfer reactions with ⁷Be beams have been investigated to determine indirectly the astrophysical S₁₇-factors of the ⁷Be(p,γ)⁸B reaction. The results of these studies are generally in good agreement, though recent demand, S-factor in 5% accuracy, requires more detailed understanding of the reaction mechanism.

The nuclei ⁴⁸Ca, ⁵⁸Ni and ⁹⁰Zr were investigated with high-resolution inelastic electron scattering at 180° in a momentum transfer range q ≃ 0.35 – 0.8 fm⁻¹. Complete M2 strength distributions could be extracted up to excitation energies of about 15 MeV utilizing a fluctuation analysis technique. Microscopic calculations including the coupling to 2p-2h states successfully describe the experimentally observed strong fragmentation of the M2 mode. A quantitative reproduction of the data requires a quenching similar to the M1 case and the presence of appreciable orbital strength which can be interpreted as a torsional elastic vibration (the so-called twist mode). A comparison to proton scattering in kinematics which favor spin-isospin excitation of J^π = 2⁻ states reveals direct evidence for dominantly orbital excitations in ⁵⁸Ni in line with microscopic predictions.

The magic numbers are the key concept of the shell model, and are different in exotic nuclei from those of stable nuclei. Its novel origin and robustness will be discussed in relation to the spin-isospin part of the nucleon-nucleon interaction.

Absolute cross sections of the photonuclear reaction of ³He and ⁴He are measured near the peak energies of the excitation functions. The results are compared with the recent calculations based on Faddeev-AGS formalism and Lorentz-kernel transform method.

The astrophysical factor S₁₇ for ⁷Be(p,γ)⁸B reaction is reliably extracted from the transfer reaction ⁷Be(d,n)⁸B at E = 7.5 MeV with the asymptotic normalization coefficient method. The transfer reaction is analyzed with CDCC based on the three-body model. This analysis is free from uncertainties of the optical potentials in the previous DWBA analyses.

Triaxial superdeformed bands in ¹⁶³Lu is analysed by applying the Holstein-Primakoff transformation both to the total angular momentum and to the single-particle angular momentum. Quite good agreements with the experimental values are reproduced in the energy difference between two superdeformed bands as a function of angular momentum, and also in the ratio of E2 transitions among these bands. The results coining from the exact diagonalization of the particle plus rotor Hamiltonian are compared with the approximate calculation.

A semimicroscopical approach is applied to describe the photoabsorption and partial photonucleon reactions accompanied by excitation of the giant dipole resonance (GDR). The approach is based on both continuum-RPA (CRPA) and a phenomenological description for doorway-state coupling to many-quasiparticle configurations. Apart from a phenomenological mean field, the separable isovector momentum-dependent forces and momentum-independent Landau-Migdal particle-hole interaction are used as the input quantities for CRPA calculations. The photoabsorption and partial (n,γ)-reaction cross sections in the vicinity of the GDR are satisfactorily described for ⁸⁹Y, ¹⁴⁰Ce and ²⁰⁸Pb target nuclei.

The electromagnetic M1 operator contains not only the usually dominant isovector (IV) spin (στ) term, but also IV orbital (ℓτ), isoscalar (IS) spin (σ) and IS orbital (ℓ) terms. On the other hand the Gamow-Teller (GT) operator contains only the στ term. Under the assumption that the isospin is a good quantum number, isobaric analog structure is expected for the nuclei with the same mass number A, and thus analogous transitions are found. For M1 transitions in the T = 1/2 mirror nuclei pairs ²³Na-²³Mg and ²⁷Al-²⁷Si, contributions of these terms are studied by comparing the strengths of the M1 γ and GT transitions obtained in high-resolution (³He,t) charge-exchange reactions. For M1 transitions in the T = 0 even-even nucleus ²⁴Mg, IV orbital and IV spin contributions were studied for the ΔT = 1, IV M1 transitions by comparing the M1 strengths of (e,e′) and GT strengths of (³He, t) charge-exchange reactions.

Change of the shell structure near drip-lines is pointed out and enhancement of low-energy dipole strength is shown in light drip-line nuclei. Change of the shell magicity can be explained by taking into account an important role of spin-isospin interaction. New aspects of quenching effects in Gamow-Teller (GT) transitions and magnetic moments are investigated for p-shell nuclei using improved shell-model interactions with enhanced spin-flip neutron-proton interactions and modified single-particle energies. Manifestation of less quenching due to a weakening of intermediate coupling nature of the Cohen-Kurath interction is shown in GT transitions; ¹²C → ¹²N, ¹¹B → ¹¹Be and ⁹Li → ⁹Be. Better agreement with experimental values is obtained by using the present interactions in most of GT transitions and magnetic moments in p-shell nuclei.

A brief review on photo-absorption and photo-fission of actinide nuclei at intermediate energies is given. To explain the experimental data for ²³⁷Np which contradict to universal behavior of the photo-absorption process vs the nuclear mass, we extrapolate the data on electromagnetic dissociation of relativistic heavy ions to the photo-fission cross section.

A microscopic coupled-channels model for Compton and pion scattering off the nucleon is introduced which is applicable at the lowest energies (polarizabilities) as well as at GeV energies. To introduce the model first the conventional K-matrix approach is discussed to extend this in a following chapter to the “Dressed K-Matrix” model. The latter approach restores causality, or analyticity, of the amplitude to a large extent. In particular, crossing symmetry, gauge invariance and unitarity are satisfied. The extent of violation of analyticity (causality) is used as an expansion parameter.

We propose a new formula for extracting the polarized gluon distribution Δg in the proton from the large–p_T light hadron pair production in deep inelastic semi-inclusive reactions. Though the process dominantly occurs via photon–gluon fusion(PGF) and QCD Compton, we can remove an effect of QCD Compton from the combined cross section of light hadron pair production by using symmetry relation among fragmentation functions and thus, rather clearly extract information of Δg.

The contribution of the strange-quark current to the electromagnetic form factors of the nucleon is studied using lattice QCD. The strange current matrix elements from our lattice calculation are analyzed in two different ways, the differential method used in an earlier work by Wilcox and a cumulative method which sums over all current insertion times. The preliminary results of our simulation indicate the importance of high statistics, and that consistent results between the varying analysis methods can be achieved. Although this simulation does not yet yield a number that can be compared to experiment, several criteria useful in assessing the robustness of a signal extracted from a noisy background are presented.

In recent years, the meaning of quantum entanglement has received renewed interest due to possible implications to the emerging fields of quantum computing and information theory. We have recently carried out feasibility studies of these measurements on the ¹S₀ pair of protons produced in the 12C(d,² He)¹²B reaction. This talk will present the motivation of these studies, present status and future prospects.

The leading and sub-leading order results for pion electroproduction in polarized and unpolarized semi-inclusive deep-inelastic scattering, are considered putting emphasis on transverse momentum dependent effects appearing in azimuthal asymmetries. In particular, single-, and double-spin asymmetries of the distributions in the azimuthal angle φ of the pion related to the lepton scattering plane are discussed. A possibility to measure spin-independent and double-spin analyzing powers, simultaneously, is also discussed.

The idea of the quark-hadron duality is developed and applied to specific integral sum rules for the photoexcitation of hadron resonances. Within the constituent quark model approach, the relations between different bremsstrahlung weihgted integrals of the nucleon resonance amplitudes and/or cross sections and correlation functions of the quark dipole moments in the nucleon ground state are obtained. These functions are of interest for checking the detailed quark-configuration structure of the nucleon state vector. Some applications of the presented approach in the meson sector are made, and the role of meson and scalar diquark cluster degrees of freedom in the electromagnetic hadron observables is discussed.

The Gerasimov-Drell-Hearn(GDH) sum rule relates the helicity dependent total photo-absorption cross sections of circularly polarized photons on longitudinally polarized nucleons to the statistic properties of the nucleon. Recently, a direct measurement for the proton has been carried out at Mainz in the the energy range 200 < ν < 800 MeV. The contribution to the GDH sum rule in this energy range was found to be 226 ± 5(stat) ± 12(sys) μb. The running GDH integral from 200 MeV is still increasing even at 800 MeV. For the measurements over 800 MeV, an experiment has been carried out by the same collaboration at Bonn. The preliminary results on the helicity dependent total photoabsorption cross-section difference in the energy range of 800 < ν < 1400 MeV show a structure in the 2nd resonance region around 1 GeV. And the data give positive contribution to the sum rule. In the higher energy regions, an experiment is planned using the Laser-Electron-Photon facility at SPring-8 (LEPS). The circularly polarized photon beam produced by the backward-Compton scattering of laser photons from 8 GeV electrons will allow measurements up to the maximum energy of 2.8 GeV. The experimental plan along with the setup will be shown.

Sum rules connecting parity violating(p.v.)threshold terms of the Compton amplitudes with the suitable integrals over p.v. parts of the total cross sections for photoproduction are given and discussed. They hold both for hadron and nuclear targets of arbitrary spin. Theoretical legitimacy of sum rules is satisfactory; they are valid in the lowest contributing order of the electroweak theory and are exact in strong interactions. Model dependent discussion presented in this talk will be concerned with application of p.v. sum rules to the cases of proton and deuteron targets.

We briefly review in the following the subject of the recently introduced “Generalized Parton Distributions”. We first outline the general theoretical formalism, then its relations to experimental observables. We also review the existing and short-term future projects at the various experimental facilities concerned by this physics. Finally, we discuss the recently published data which look extremely promissing for the development of this field.

We explain some of the challenges and recent discoveries in the struggle to connect hadron properties calculated in lattice QCD at large quark mass to the physical region. We suggest that formal expansions based on effective field theory need to be supplemented by some physical insight before the problem becomes tractable. However, once this is done, some surprisingly accurate results can be extracted.

If a baryon field belongs to a certain linear representation of chiral symmetry of SU(2) ⊗ SU(2), the axial coupling constant g_A can be determined algebraically from the commutation relations derived from the superconvergence property of pion-nucleon scattering amplitudes. This establishes an algebraic explanation for the values of g_A of such as the non-relativistic quark model, large-N_c limit and the mirror assignment for two chiral partner nucleons. For the mirror assignment, the axial charges of the positive and negative parity nucleons have opposite signs. Experiments of eta and pion productions are proposed in which the sign difference of the axial charges can be observed.

We present results for the photoproduction of ω and φ meson in the nucleon isotopic channles. A recently developed quark model with an effective Lagrangian is employed to account for the non-diffractive s- and u-channel processes; the diffractive feature arising from the natural parity exchange is accounted for by the t-channel pomeron exchange, while the unnatural parity exchange is accounted for by the t-channel pion exchange. In the ω production, the isotopic effects could provide more information concerning the search of “missing resonances”, while in the φ production, the isotopic effects could highlight non-diffractive resonance excitation mechanisms at large angles.

We investigate the role of the nucleon resonances with masses M_N* ≤ 1720 MeV in ω meson photoproduction at low energy with E_γ < 1.25 GeV. The amplitude is calculated within effective Lagrangian approach, which includes, at the tree level, leading t-channel pseudoscalar meson exchange, nucleon Born terms and nucleon resonance excitations. For the later part we use an effective Lagrangians motivated by previous studies of π and η photoproduction. The contributions from the nucleon resonances are found to be significant relative to the all other channels in changing the differential cross sections in a wide interval of t In particular, we suggest that a crucial test of our predictions can be made by measuring single and double spin asymmetries.

Within the French-Italian GRAAL collaboration, we have constructed a Polarized HD Target Factory (HYDILE), implanted at IPN Orsay. This project is very similar to the SPHICE one existing at BNL, and developed within the LEGS Spin Collaboration. The expected properties of the HD targets and the results already obtained for the polarization of HD samples at Orsay and BNL are given. New possibilities to measure polarization observables on the proton, the neutron and the deuteron are now opened. Most simple and double polarization measurements for the photoproduction of the low mass pseudoscalar and vector mesons could be performed, by using the high polarization (linear or circular) of the backscattered photon beams and the large effective polarization (longitudinal or tranverse) of the target, resulting essentially, from the good dilution factor. Polarized total cross sections concerning the isovector Drell-Hearn Gerasimov Sum Rule related to the proton neutron difference, could be measured with minimum systematic uncertainties: the target containing both polarized protons and neutrons. By using the available polarized photon beams from LEGS, GRAAL and Spring-8, measurements could be done from the π meson threshold up to 2.5 Gev, where the φ meson photoproduction cross section is close to its maximum value. Typical examples are given for GRAAL and Spring-8.

A polarized and tagged gamma ray beam is produced at GRAAL by the Compton scattering of laser light on the high energy electron circulating in the ESRF storage ring. We present results on the beam polarization asymmetries in the Compton Scattering and positive pion photo-production on proton target in the energy region 500-1500 MeV. These very precise results cover the angular range 30-150 degrees, providing stringent constraints to theoretical models.

Reaction spectroscopy of Λ hypernuclei by the (e,e'K⁺) reacation with unprecedented quality is under preparation, following the success of Jlab E89-009 experiment, which observied a sub-MeV spectrum in the reaction. The experiment, Jlab E01-011, utilizes the “tilt method”, in which the scattered electron spectromter is vertically tilted so that 0-degree bremsstrahlung electrons do not enter the spectrometer system. Together with a high-resolution kaon spectrometer(HKS) under construction, reaction spectroscopy of Λ hypernuclei with more than 50 times efficient and twice better resolution(3-400 keV(FWHM)) becomes possible. Hypernuclear spectroscopy by the (e,e'K⁺) reaction will be extended to the mass region beyond the p-shell region.

COSY Jülich is a race-track shaped synchrotron which accelerates and cools beams of protons (both polarized and unpolarized) and deuterons with momenta up to 3.6 GeV/c. Those beams are delivered to internal and external target positions for hadron physics experiments. Since magnetic and time-of-flight detectors based on organic scintillators are used in the experimental setups, all measurements are essentially “photon blind”. Recent improvements in the performance of high-density inorganic scintillators offer the possibility to design very compact large-acceptance electromagnetic calorimeters with excellent timing and good energy resolution, applicable also for photon energies below 1 GeV. Such a detection system, based on PbWO₄, is planned to be built for the internal magnetic spectrometer ANKE at COSY Jülich. The limited space and stray magnetic fields of ANKE place severe boundary conditions, which have to be taken into account for detector layout and the choice of photo sensors.

The set of experiments forming the g8 run took place this past summer (6/04/01 – 8/13/01) in Hall B of Jefferson Lab. These experiments made use of a beam of linearly-polarized photons produced through coherent bremsstrahlung and represent the first time such a probe has been employed at Jefferson Lab. Among the several new and upgraded Hall-B beamline devices commissioned prior to the production running of g8a were the photon tagger, the coherent bremsstrahlung facility (goniometer + an instrumented collimator), a photon profiler, and the PrimEx dipole + pair spectrometer telescopes. We essentially commissioned a new beam-line for photon running in Hall B. The scientific purpose of g8 is to improve the understanding of the underlying symmetry of the quark degrees of freedom in the nucleon, the nature of the parity exchange between the incident photon and the target nucleon, and the mechanism of associated strangeness production in electromagnetic reactions. With the high-quality beam of the tagged and collimated linearly-polarized photons and the nearly complete angular coverage of the Hall-B spectrometer, we seek to extract the differential cross sections and polarization observables for the photoproduction of vector mesons and kaons at photon energies ranging between 1.1 and 2.25 GeV. For the first phase of g8, i.e. g8a, we collected approximately 1.8 billion triggers for . In this paper, we report on the results of the commissioning of the beamline devices for the g8a run.

The following sections are included:

• LIST OF PARTICIPANTS