Baryons 2002

Preface.

INTERNATIONAL ADVISORY COMMITTEE.

CONTENTS.

We review some of the highlights from recent work concerning the structure of baryons. We concentrate especially on those developments where the interaction with lattice QCD has led to new insights.

The basis of constituent quark models of the baryon spectrum is reviewed, with a description of the effective degrees of freedom, their confinement, and various models of their short-distance interactions. It is argued that such models should not be compared with the results of analyses of hadron scattering data without considering the effects of quark-antiquark pairs on the spectrum, and a model of such effects is described.

Recent electroproduction results in the domain of s-channel nucleon resonance excitation are presented, and preliminary data in the search for missing states will be discussed. I also address a new avenue to pursue N* physics using exclusive deeply virtual Compton scattering, recently measured for the first time at JLab and DESY.

Recent lattice QCD calculations of the baryon spectrum are outlined.

I describe methods for dealing with b and c quarks within the lattice QCD approach and summarise recent results for phenomenologically important quantities.

The problem of understanding the nuclear effects observed in lepton-nucleus deep-inelastic-scattering (the EMC effect) is still with us. Standard nuclear models (those using only hadronic degrees of freedom) are not able to account for the EMC effect. Thus it is necessary to understand how the nuclear medium modifies quark wave functions in the nucleus. Possibilities for such modifications, represented by the quark meson coupling model, and the suppression of point-like-configurations are discussed, and methods to experimentally choose between these are reviewed.

A review is given of new experimental results that provide information on the spin structure of the nucleon. Inclusive measurements on a longitudinally polarized deuterium target have completed a set of high precision studies of the polarized structure function g₁(x) carried out at HERMES. A high luminosity experiment at SLAC on transversely polarized nucleon targets has produced data on the structure function g₂(x). Measurements of the double-spin asymmetry for the photoproduction of pairs of oppositely charged high-p_T hadron can be interpreted in terms of evidence for a positive gluon polarization. Exploratory measurements of the beam-spin asymmetry in deeply-virtual Compton scattering (at both HERMES and JLab) were successful, implying that this channel can - in principle - be used to study the total angular momentum carried by the quarks. Small single-target spin asymmetries observed at HERMES in semi-inclusive pion production experiments indicate that the only hitherto unmeasured leading order structure function, the transversity spin structure function h₁(x), is non-zero.

Recent HERA data on small-x structure functions as well as DIS diffraction are presented. The relationship between these processes and possible indications of dynamics beyond the DGLAP formalism are discussed.

After contrasting the low energy effective theory for the baryon sector with one for the Goldstone sector, I use the example of pion nucleon scattering to discuss some of the progress and open issues in baryon chiral perturbation theory.

An overview of recent results in threshold pion production is given. While the photoproduction data are in impressive agreement with calculations in the framework of Chiral Perturbation Theory, electroproduction data at low four-momentum transfer shows severe deviations from these calculations.

I present an overview of both the technical issues involved and the progress made so far in the study of hadron structure using lattice QCD.

We present an overview of most recent experimental results of photonuclear reactions in the resonance energy region. High precision and polarization observables are the key issues in the study of N* resonance properties.

I explain how instantons break chiral symmetry and how do they bind quarks in baryons. The confining potential is possibly irrelevant for that task.

We report here on a new measurement of the parity violating (PV) Asymmetry in the scattering of polarized electrons on unpolarized protons performed at the MAMI accelerator facility in Mainz. This experiment is the first to use counting techniques in a parity violation experiment. The kinematics of the experiment is complementary to the earlier measurements of the SAMPLE collaboration at the MIT Bates accelerator and the HAPPEX collaboration at Jefferson Lab. After discussing the experimental context of the experiments, the setup at MAMI and preliminary results are presented.

Theoretical approaches to investigate the structure of nucleon resonances from the pion electroproduction are briefly reviewed. Application of the dynamical approach on the Δ₃₃(1232) region and the consequences on the γ + N → Δ transition form factors are discussed in comparison with the recent data.

The baryon spectroscopy program using the Crystal Ball detector at Brookhaven National Laboratory (BNL) is presented. Precise measurements for π⁻p and K⁻p interactions to neutral final states have been obtained. The measurements were performed in the C6 beam line at the BNL AGS, which has a maximum beam momentum of about 750 MeV/c. Data were taken at incident beam momenta as low as 150 MeV/c for pions and 500 MeV/c for kaons. New experiments have been approved that will extend the pion measurements to even lower momenta and to obtain additional data with kaon beams.

A new mass formula is suggested describing the spectrum of light baryons. The formula uses 3 baryon masses (N, Δ, Ω) and the slope of meson Regge trajectories as input quantities and no free parameter.

Spin structure functions in the deep inelastic region have been extensively measured over the past three decades. On the contrary much less is known in the region of the nucleon resonances and at low to intermediate Q². A large experimental program is in progress at Jefferson Lab to study this kinematic region using polarized electrons impinging on polarized proton, deuteron, and helium-3 targets. Preliminary results on the first moment of the spin structure function g₁ and on the Gerasimov-Drell-Hearn integral for proton and neutron are presented. In addition, first double polarization data on exclusive pion production are discussed.

Quark-hadron duality and its potential applications are discussed. We focus on theoretical efforts to model duality.

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.

We review the status of hybrid baryons. The only known way to study hybrids rigorously is via excited adiabatic potentials. Hybrids can be modelled by both the bag and flux–tube models. The low–lying hybrid baryon is with a mass of 1.5 – 1.8 GeV. Hybrid baryons can be produced in the glue–rich processes of diffractive γN and πN production, Ψ decays and annihilation.

The electromagnetic form factors of the proton and neutron contain all the information about the charge and current distribution of these baryons, and thus provide strong constraints on the fundamental theory of strong interactions. Indeed, over the last several years, there have been a number of high profile experiments which aim to measure the nucleon form factors at increasingly large momentum transfers, which corresponds to probing the nucleon at very small distance scales. At the same time, there has been a monumental effort on the theoretical front to develop a coherent picture which would account for the observed behaviours. In this paper, I will present the current state of experimental knowledge of the nucleon form factors, as well as discuss the theoretical and phenomenological efforts in this area.

Virtual Compton Scattering off the proton: γ*p → γp is a new field of investigation of nucleon structure. Several dedicated experiments have been performed at low c.m. energy and various momentum transfers, yielding specific information on the proton. This talk reviews the concept of nucleon Generalized Polarizabilities and the present experimental status.

I review some basic facts about generalized parton distributions, and then report on selected issues where progress has been made recently: the description and physical interpretation of GPDs, and the theory of processes where they can be measured.

No abstract received.

A comprehensive program to measure polarized structure functions in the resonance region and beyond using the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab has completed taking data. One of the experimental goals is to measure the first moment(Γ₁) of the polarized structure function, g₁, as a function of Q² by scattering polarized electrons from polarized proton (NH₃) and deuteron (ND₃) targets. Deep inelastic scattering measurements obtain positive values for the first moment at large Q². The first moment is constrained by the Gerasimov-Drell-Hearn sum rule ¹ to have a negative slope at the photon point. The CLAS measurements encompass the transition from the low Q² region dominated by resonances and other non-perturbative effects to the high Q² region where quasi-free partons dominate the interaction. Inclusive results for the double spin asymmetry A₁ and the first moment of g₁ are reported which span a range in Q² from 0.2 to 1.5 (GeV/c)² and up to W=2.5 GeV, thus probing a range in Bjorken-x from 0.04 to 0.8.

We report on new measurements in Hall C at JLAB of longitudinal and transverse separated proton structure functions in the resonance region (1 < W² < 4 GeV²) and spanning the four-momentum transfer range 0.2 < Q² < 4.0 (GeV/c)².

The Collins function belongs to the so-called time-reversal odd fragmentation functions. In spite of this property, we explicitly generate a non-zero Collins function in the framework of a simple field theoretical model by calculating the fragmentation of a quark into a pion at the one-loop level. We also estimate the Collins function at a low energy scale using the chiral invariant model of Manohar and Georgi. Different spin and/or azimuthal asymmetries measurable in semi-inclusive DIS and e⁺e⁻ annihilation, which contain the Collins function, are briefly discussed as well. In particular, the measurement of a purely azimuthal asymmetry in e⁺e⁻ annihilation can allow the extraction of the Collins function from data.

Power corrections to the Q² behavior of the Nachtmann moments of the proton polarized structure function are investigated at large Bjorken x by developing a phenomenological fit of both the resonance (including the photon point) and deep inelastic data up to Q² ~ 50 (GeV/c)². The leading twist is treated at NLO in the strong coupling constant and the effects of higher orders of the perturbative series are estimated using soft-gluon resummation techniques. In case of the first moment higher-twist effects are found to be quite small for Q² ≳ 1 (GeV/c)², and the singlet axial charge is determined to be a₀[10 (GeV/c)²] = 0.16 ± 0.09. In case of higher order moments, which are sensitive to the large-x region, higher-twist effects are significantly reduced by the introduction of soft gluon contributions, but they are still relevant at Q² ~ few (GeV/c)² at variance with the case of the unpolarized transverse structure function of the proton. This finding suggests that spin-dependent correlations among partons may have more impact than spin-independent ones. It is also shown that the parton-hadron local duality is violated in the region of polarized electroproduction of the Δ(1232) resonance.

We report the first measurement of the beam-spin asymmetry in the electroproduction of positive pions above the baryon resonance region at CLAS at beam energy of 4.25 GeV. At large fractions of virtual photon momentum carried by the pion the amplitude of measured sin ϕ modulation is 0.037 ± 0.006.

An extensive experimental program to measure the spin structure of the nucleons is underway in Hall B at Jefferson Lab using a polarized electron beam incident on polarized hydrogen and deuterium targets, consisting of frozen NH₃ and ND₃ material. Spin degrees of freedom offer the possibility to test, in an independent way, existing models of resonance electroproduction. The most accessible resonance is the Δ(1232) since it does not overlap with other states and decays strongly via π emission. The present analysis select the Δ⁺(1232) in the exclusive channel from data of the EG1 run period, taken in the Fall of 1998, to extract single and double spin asymmetries in a Q² range from 0.5 to 1.5 GeV²/c². Results of the asymmetries are presented as a function of the center of mass decay angles of the π⁰ and compared with the unitary isobar model and two dynamic models.

Electroproduction of the Δ(1232) is well suited for the study of mechanisms responsible for resonance formation and decay. The Q² dependence of the quadrupole electric (E₁₊) and scalar (S₁₊) multipoles in the γ*N → Δ → Nπ transition is especially sensitive to details of the quark wave functions and the evolution from pion to quark degrees of freedom. New π⁰ electroproduction data taken with CLAS at Jefferson Lab are compared to recent models which incorporate the dynamical effects of the pion cloud. The ratios E₁₊/M₁₊ and S₁₊/M₁₊ are extracted using a partial wave analysis over the interval Q²=0.4-1.8 GeV².

Exclusive H(e,e'K)Y data were taken in January, March and April of 2001 at the Jefferson Lab Hall A. The electrons and kaons were detected in coincidence in the hall's two High Resolution Spectrometers (HRS). The kaon arm of the pair had been specially outfitted with two aerogel Čerenkov threshold detectors, designed to separately provide pion and proton particle identification thus allowing kaon identification. The data show the cross section's dependence on the invariant mass, W, and 4-momentum transfer, Q², along with results of systematic studies. Ultimately the data will be used to perform a Rosenbluth Separation as well, separating the longitudinal from the transverse response functions. Preliminary data on this L/T ratio are presented.

Recent recoil polarization measurements in Hall A at Jefferson Lab show that the ratio of the electric to magnetic form factors for the proton decreases significantly with increasing Q². This contradicts previous Rosenbluth measurements which indicate approximate scaling of the form factors . The cross section measurements were reanalyzed to try and understand the source of this discrepancy. We find that the various Rosenbluth measurements are consistent with each other when normalization uncertainties are taken into account and that the discrepancy cannot simply be the result of errors in one or two data sets. If there is a problem in the Rosenbluth data, it must be a systematic, ∊-dependent uncertainty affecting several experiments.

The Gari-Krümpelmann (GK) models of nucleon electromagnetic form factors, in which the ρ, ω, and ϕ vector meson pole contributions evolve at high momentum transfer to conform to the predictions of perturbative QCD (pQCD), was recently extended to include the width of the ρ meson by substituting the result of dispersion relations for the pole and the addition of ρ′ (1450) isovector vector meson pole. This extended model was shown to produce a good overall fit to all the available nucleon electromagnetic form factor (emff) data. Since then new polarization data shows that the electric to magnetic ratios R_p and R_n obtained are not consistent with the older G_Ep and G_En data in their range of momentum transfer. The model is further extended to include the ω′ (1419) isoscalar vector meson pole. It is found that while this GKex cannot simultaneously fit the new R_p and the old G_En data, it can fit the new R_p and R_n well simultaneously. An excellent fit to all the remaining data is obtained when the inconsistent G_Ep and G_En is omitted. The model predictions are shown up to momentum transfer squared, Q², of 8 GeV²/c².

The electric form factor of the neutron, G_E,n, is measured at Q² = 0.6 – 0.8 (GeV/c)² in a new experiment at Mainz University. A neutron polarimeter, optimized to withstand high electromagnetic background rates, was built for the Three Spectrometer Hall of the A1 collaboration. We present the experimental setup and the status of the data analysis.

The ratio of the electric to the magnetic form factor of the neutron, G_En/G_Mn, was measured via recoil polarimetry from the quasielastic reaction at three values of Q² [ viz., 0.45, 1.15, and 1.47 (GeV/c)²] in Hall C of the Thomas Jefferson National Accelerator Facility. Preliminary data indicate that G_En follows the Galster parameterization up to Q² = 1.15 (GeV/c)² and appears to rise above the Galster parameterization at Q² = 1.47 (GeV/c)².

In the G⁰ experiment, parity violation asymmetries in elastic electron scattering from the nucleon will be measured. The primary purpose of the experiment is to separate the s quark contributions and from the overall charge and magnetization densities of the proton. With this aim in view, a dedicated apparatus has been constructed and is currently being installed at JLab / Hall C.

The canonical quantization procedure in SU(2) Skyrme model ensures the existence of stable soliton solution with nucleons quantum numbers. An interesting consequence of the canonical ab initio quantization of the model is the natural appearance of a finite effective pion mass even for the chirally symmetric Lagrangian. The explicit expression for electric and magnetic form factors of nucleon have been derived. The calculated form factors are close to empirical ones.

We present a calculation of the nucleon form factors in the light cone sum rule approach¹ employing higher twist distribution amplitudes that were recently obtained using conformal symmetry of QCD.² Comparing our predictions with the most recent measurements at Jefferson Lab we find that the experimental data are well described by soft contributions that include nonleading helicity structures in the nucleon distribution amplitudes.

Results for all elastic electroweak nucleon form factors are presented for the chiral constituent quark model based on Goldstone-boson-exchange dynamics. The calculations are performed in a covariant framework using the point-form approach to relativistic quantum mechanics. The direct predictions of the model yield a remarkably consistent picture of the electroweak nucleon structure.

Hard exclusive leptoproductions of real photons, lepton pairs and mesons are the most promising tools to unravel the three-dimensional picture of the nucleon, which cannot be deduced from conventional inclusive processes like deeply inelastic scattering.

Recent results from the Deeply Virtual Compton Scattering (DVCS) program at Jefferson Lab will be presented. Approved dedicated DVCS experiments at 6 GeV will be discussed.

We show that electromagnetic gauge invariance requires a "spin rotation" of the quarks in the usual twist–2 contribution to the amplitude for Deeply Virtual Compton Scattering. This rotation is equivalent to the inclusion of certain kinematical twist–3 ("Wandzura–Wilczek type") terms, which have been derived previously using other methods. The new representation of the twist–3 terms is very compact and allows for a simple physical interpretation.

The HERMES collaboration is planning to use a Recoil Detector in combination with a high density unpolarized gas target to measure hard exclusive reactions. This paper outlines the design of the detector and gives projections for measurements of the beam spin and beam charge asymmetries in Deeply Virtual Compton Scattering (DVCS).

We describe a dispersion relation formalism to the virtual Compton scattering reaction off the proton as a new tool to analyze VCS experiments above pion threshold, where one observes increasing effects of the generalized polarizabilities.

The photoinduced production of neutral mesons off protons has been studied with the Crystal–Barrel-detector at the electron stretcher facility ELSA. First data demonstrate that reactions with multi–photon final states can be reconstructed with high efficiency. Preliminary results on γp → pπ⁰π⁰ and γp → pπ⁰η show evidence for successive decays of high mass states through different intermediate resonances.

The associated strangeness production in elementary proton induced reactions is studied exclusively at the external COSY beam using the time-of-flight spectrometer TOF. The complete measurement of all primary and decay particle tracks allows the extraction of total and differential cross sections as well as Dalitz plots and invariant mass spectra of the subsystems for the channels pp→ K⁺Λp, K⁰Σ⁺p, K⁺Σ⁰p and K⁺Σ⁺n. For all channels the full phase space is covered from the reaction threshold up to the COSY-limit of about 3.5 GeV/c. Especially the analysis of the Dalitz plots of the channel pp→ K⁺Λp show a strong influence of N*- resonances. In parallel the production of the ω - meson is studied in the reaction pp→ ppω.

No abstract received.

We study dynamical relativistic corrections to electromagnetic baryon transition operators in the quark model associated to Fock-space components. We generate such components by means of a relativistic ³P₀ pair creation mechanism and we find sizeable contributions to photoproduction amplitudes of resonances.

The ³P₀ decay model is briefly reviewed. Possible improvements, partly motivated by the examination of a microscopic description of a quark-anti-quark pair creation, are considered. They can provide support for the one-body character of the model which, otherwise, is difficult to justify. To some extent, they point to a boost effect that most descriptions of processes involving a pair creation cannot account for.

The evidence and the theoretical justification of chiral symmetry restoration in high-lying baryons is presented.

Virtual Compton Scattering off the proton has been studied at Q²-values of 1.0 and 1.9 (GeV/c)² in Hall A at the Thomas Jefferson National Accelerator Facility (JLab). Data were taken below and above the pion production threshold as well as in the resonance region. Results obtained below pion threshold at Q² = 1.0 (GeV/c)² are presented in this paper.

The Experiment (E93-050) at Jefferson Lab measured the ep → epγ and ep → epπ⁰ cross sections in the nucleon resonance region, from threshold to W = 1.9 GeV at Q² = 1 GeV² for backward emission of the γ or π°.

The hypercentral constituent quark model contains a spin independent three-quark interaction inspired by lattice QCD calculations which reproduces the average energy of SU(6) multiplets ¹. The splittings are obtained with a residual generalized SU(6)-breaking interaction including an isospin dependent term ². The long standing problem of the Roper is absent and all the 3- and 4-star states are well reproduced. The model has been used in a systematic way for transverse and longitudinal electromagnetic transition form factor of the 3- and 4- star and also for the missing resonances. The prediction of the electromagnetic helicity amplitudes agrees quite well with the data except for low Q², showing that it can supply a realistic set of quark wave functions. In particular we report the calculated helicity amplitude A_1/2 for the S11(1535), which is in agreement with the TJNAF data ³.

We report on a new experiment to search for the neutron electric dipole moment which has the potential to lower the current limit by a factor of 50 to 100. A unique approach to this measurement is described including the results of recent measurements at LANSCE of the mass diffusion coefficient for ³He in superfluid ⁴He.

Corrections to the masses of baryons from baryon-meson loops are calculated using a pair-creation model to give the momentum-dependent vertices, and a model which includes configuration mixing to describe the wave functions of the baryons. A large set of baryon-meson intermediate states are employed, with all allowed SU(3)_f combinations, and excitations of the intermediate baryon states up to and including the second band of negative-parity excited states. It is found that roughly half of the splitting between the nucleon and Delta ground states arises from loop effects, and the resulting splittings of negative-parity excited states are sensitive to configuration mixing caused by the residual interactions. With reduced-strength one-gluon-exchange interactions between the quarks, there is a reasonable correspondence between model masses and the bare masses required to fit the masses extracted from data analyses.

No abstract received.

The s-wave meson-baryon interaction is described using the lowest-order SU(3) chiral Lagrangian in a unitary coupled-channels Bethe-Salpeter equation. The entire octet of ground-state J^P = 1/2⁻ resonances, the Λ(1670), Σ(1620), Ξ(1620) and N(1535) states, along with the SU(3) singlet Λ(1405), is found to be dynamically generated through meson-baryon rescattering.

The differential cross sections and the single beam asymmetries have been measured for the single pion electroproduction on the proton at Q² = 0.4 GeV² using CLAS. The large phase space coverage of CLAS and the high statistical accuracy of the data allowed us to perform the analysis of single pion electroproduction observables and to obtain the A_1/2 amplitudes for the S₁₁(1535). The preliminary results of this analysis are in good agreement with the η-meson production data.

The polarized longitudinal-transverse structure function σ_LT′ in the reaction has been measured for the first time in the Δ(1232) resonance region for invariant mass W = 1.1 − 1.3 GeV and at four-momentum transfer Q² = 0.40 and 0.65 GeV². Data were taken at the Thomas Jefferson National Accelerator Facility with the CEBAF Large Acceptance Spectrometer (CLAS) using longitudinally polarized electrons at an energy of 1.515 GeV. This newly measured σ_LT′ provides new and unique information on the interference between resonant and non-resonant amplitudes in the Δ(1232) resonance region. The comparison to recent phenomenological calculations shows sensitivity to the description of non-resonant amplitudes and higher resonances.

No abstract received.

New cross section measurements for the η electro-production are reported for total center of mass energy from threshold to 2 GeV and invariant momentum transfer Q² between 0.13 and 3.2 (GeV/c)². This large kinematic range and increased statistics allow to study the response functions and Q² evolution of photo-coupling amplitude.

Differential cross sections for the reaction γp → ηp have been measured for incident photon energies from 0.75 to 1.95 GeV. Data were taken using the CLAS spectrometer and tagged photon facility at Jefferson Lab. The data provide the first extensive measurements of angular distributions for this process above the S₁₁(1535) resonance in the range up to W = 2.1 GeV.

We use wavelet analysis (WA) to reduce statistical noise in experimental data to clear out resonances contribution. With these "cleaned up" data, we find ρ′ and ω′ parameters with generalized coupled-channel Breit–Wigner method that preserves unitarity in case of overlapping states.

The mass spectrum of the 70-plet of negative parity baryons is studied using the 1/N_c expansion of QCD. It is found that the Λ(1405) is well described as a three-quark state and a spin-orbit partner of the Λ(1520). Singlet states with higher orbital angular momentum L are also briefly discussed.

Electroprodution data from CLAS at Jefferson Lab are being analyzed in order to identify resonant couplings to baryon resonances decaying into πΔ, ρN, or ωN. A multi-dimenional fit to the new two-pion electroproduction data not only provides better determined photocouplings of several resonances, but also implicates a significant correction of the hadronic couplings of resonances at W ≈ 1.7 GeV. The electroproduction of ω shows strong indications of u- and s-channel contributions at W below 2 GeV.

We show that in a quark model scheme the use of a screened confining potential, suggested by lattice and QCD non-perturbative calculations, instead of an infinitely rising one with the interquark distance may give rise to a one-to-one correspondence between the predicted three-quark bound states and the observed baryon resonances. This points out to the use of the baryon spectrum as a quite stringent test of the long-range confining potential.

Very little is known about the doubly-strange Ξ hyperons. SU(3)_F symmetry, based on QCD, implies the existence of many Ξ states yet to be found. A complete study of the excited Ξ spectrum can also be used to study other related areas of nuclear physics, such as the s – d quark mass difference and Ξp scattering. We will report on a new approach to Ξ physics, using the photoproduction process γp → K⁺K⁺Ξ⁻, in which the Ξ is cleanly tagged by the missing mass of the (K⁺K⁺) system. We show the current status of this study with the CLAS detector at Jefferson Laboratory, and discuss how it relates to the above topics. We also comment on the future of this program.

An extensive program dedicated to the study of open strangeness systems was established in Hall B at Jefferson Lab. This program takes full advantage of the excellent characteristics of the CEBAF accelerator combined with the almost complete angular coverage of the CLAS detector. A general overview of the program is given, as well as results for the angular dependence of the electroproduction of kaon–hyperon final states.

At the LEPS beam line at SPring-8, K⁺ photoproduction off the proton is studied for 1.9 GeV < W < 2.3 GeV and 0° < θ_cm(K⁺) < 6C°. Since data are taken with transversely polarized photons, the photon-polarization assymetry can be measured. In contrast to the cross section, this observable is very sensitive to the details of the reaction process, such as the possible contribution from so-called 'missing' resonances and thus provides a good way to distinguish between various theoretical descriptions. In the paper, the current status of the analysis and expected impact of the results are discussed.

Kaon photoproduction off the proton is studied in an effective Lagrangian approach. The dominant resonance contributions in the reaction dynamics are identified, including a search for signals of "missing" resonances. Special attention is paid to the issue of the elusive role played by background contributions.

Results of the Dressed K-matrix Model are presented for nucleon Compton scattering, nucleon polarisabilities and related sum rules. Effects of the meson loop dressing on the Δ resonance are shown.

We explain the early onset of shadowing in nuclear photoabsorption within a multiple scattering approach and discuss its relation to in-medium modifications of the ρ⁰.

The production and decay of vector mesons (ρ, ω) in pA reactions at COSY energies is studied with particular emphasis on their in-medium spectral functions. It is explored within transport calculations, if hadronic in-medium decays like π⁺π⁻ or π⁰γ might provide complementary information to their dilepton (e⁺e⁻) decays. Whereas the π⁺π⁻ signal from the ρ-meson is found to be strongly distorted by pion rescattering, the ω-meson Dalitz decay to π⁰γ appears promising even for more heavy nuclei. The perspectives of scalar meson (f₀, a₀) production in pp reactions are investigated within a boson-exchange model indicating that the f₀-meson is hard to detected in these collisions in the or ππ decay channels whereas the channels and look very promising.

We report a helicity analysis of subthreshold ρ⁰ production on ²H, ³He and ¹²C nuclei at low photoproduction energies. The results are indicative of a large longitudinal ρ polarization (l = 1, m = 0) and are consistent with a strong helicity-flip mechanism of ρ production. The analysis supports an in-medium modification of the ρ⁰ spectral function.

A covariant and unitary approach to pion- and photon-nucleon scattering taking the πN, ρN, ωN, ηN, πΔ, KΛ and KΣ channels into account is presented. It is argued that the s- and d-wave nucleon N(1535), N(1650), N(1520) and N(1700) resonances and as well as the isobar Δ(1620) and Δ(1700) resonances should be generated in terms of coupled channel dynamics. A fair description of the experimental data relevant for the properties of slow vector-mesons in nuclear matter is obtained. The ρ- and ω-meson spectral functions are evaluated in nuclear matter according to the low-density theorem. They are determined by the vector-meson nucleon scattering amplitudes as obtained in the coupled channel analysis.

Polarization transfer in the reaction was measured in Jefferson Lab experiment 93-049. The ratio of the polarization transfer coefficients, (P'_xP'_z)_He, is on average significantly reduced as compared to the same ratio in elastic scattering. This is so far unaccounted for by relativistic DWIA calculations, and favors the inclusion of a predicted medium modification of the proton form factor.

The total cross sections of the (γ, η) reaction on C have been measured for photon energies between 620 and 1100 MeV at Tohoku University, in order to study the property of the S₁₁(1535) resonance in the nuclear medium. Model calculations based on the quantum molecular dynamics (QMD) have been performed. The comparison between the calculation and the experimental data suggests that the resonance property of S₁₁(1535) might be changed in nuclear interior.

Preliminary differential cross sections of the reactions A(γ, π° π°) and A(γ, π° π°⁺ + π°π⁻) with A=¹H, ¹²C, and ^natPb are presented. A significant nuclear-mass dependence of the ππ invariant-mass distribution is found in the π°π° channel. The dependence is not observed in the π°π^± channel. It is consistent with an in-medium modification of the ππ interaction in the I=J=0 channel, changing width and pole position of a ππ resonant state.

Recent inclusive electron–nucleus scattering data have been utilized for precision tests of quark–hadron duality. The data are in the resonance and quasielastic regions and cover a range in Q² from 0.5 to 7 (GeV/c)². The Q² dependence of the moments of the F₂ structure function were investigated and indicate that duality holds for nuclei, even at low Q².

A detailed study of inclusive deep inelastic scattering from mirror A = 3 nuclei at large values of x_Bjorken is presented. The main purpose is to estimate the theoretical uncertainties on the extraction of from such measurements. Within the convolution approach we confirm the cancellation of nuclear effects at the level of ≈ 1% for x ≲ 0.75 in overall agreement with previous findings. However, within models in which modifications of the bound nucleon structure functions are accounted for to describe the EMC effect in nuclei, we find that the nuclear effects may be canceled at a level of ≈ 3% only, leading to an accuracy of ≈ 12% in the extraction of at x ≈ 0.7 ÷ 0.8. Another consequence of bound nucleon modifications is that the iteration procedure does not improve the accuracy of the extraction of .

The influence of the nuclear medium on the production of charged hadrons in semi-inclusive deep inelastic scattering (DIS) has been studied by the HERMES experiment at DESY with a 27.5 GeV positron beam. A large reduction of the differential multiplicity of charged hadrons and identified charged pions from krypton relative to that from deuterium is observed. The reduction is larger than that seen in previously published HERMES data on nitrogen. The data are compared to two theoretical models. Both describe well the reduction of the multiplicity ratio at low values of the virtual photon energy ν and at high values of the fractional energy transfer z to the hadron. The A dependence of the data is also addressed.

The quasielastic (e,e'p) reaction was studied on targets of deuterium, carbon, and iron up to a value of momentum transfer Q² of 8.1 (GeV/c)². A nuclear transparency was determined by comparing the data to calculations in the Plane-Wave Impulse Approximation. The dependence of the nuclear transparency on Q² and the mass number A was investigated in a search for the onset of the Color Transparency phenomenon. We find no evidence for the onset of Color Transparency within our range of Q².

Inclusive electron scattering from nuclei at low momentum transfer (corresponding to x ≥ 1) and moderate Q² is dominated by quasifree scattering from nucleons. In the impulse approximation, the cross section can be directly connected to the nucleon momentum distribution via the scaling function F(y). The breakdown of the y-scaling assumptions in certain kinematic regions have prevented extraction of nucleon momentum distributions from such a scaling analysis. With a slight modification to the y-scaling assumptions, it is found that scaling functions can be extracted which are consistent with the expectations for the nucleon momentum distributions.

Medium dependencies of bound nucleons are studied in a fully relativistic and unfactorized framework for the description of exclusive processes. The theoretical framework, which is based on the eikonal approximation, can accommodate both optical potential and Glauber approaches for the treatment of final state interactions. It is discussed how both approaches compare to one another. Calculations for ¹²C(e, e′p) nuclear transparencies are presented. The issue of measuring the predicted medium modifications for the bound nucleon's electromagnetic form factors is addressed by presenting results.

The cross section ratios of inclusive electron scattering from the nuclei ⁴He, ¹²C, ⁵⁶Fe and ³He are measured for the first time. It is shown that these ratios as a function of Q² and x_B are scaled in Q² > 1.4 (GeV/c) and x_B > 1.5 range. This scaling was predicted within Short Range Correlation (SRC) model. The values of these ratios in the scaling region can be used to derive the probabilities of SRC in heavy nuclei. Our analysis demonstrate that for nuclei with A ≥ 12 these probabilities are 4.5-5.2 times larger than in deuterium, while for ⁴He it is larger by factor of 2.6-3.3 only.

We have measured the ³He(e,e′pp)n reaction at 2.2 and 4.4 GeV over a wide kinematic range. The kinetic energy distribution for 'fast' nucleons (p > 250 MeV/c) peaks where two nucleons each have 20% or less and the third or 'leading' nucleon carries most of the transferred energy. These fast nucleon pairs (both pp and pn) are back-to-back and carry very little momentum along , indicating that they are spectators. Experimental and theoretical evidence indicates that we might have measured NN correlations in ³He(e,e′pp)n by striking the third nucleon and detecting the spectator correlated pair.

The A(e, e′K⁺)YX reaction has been investigated in Hall C at Jefferson Laboratory for 6 different targets. Data were taken for Q² ≈ 0.35 and 0.5 GeV² at a beam energy of 3.245 GeV for ¹H, ²H, ³He, ⁴He, C and Al targets. The missing mass spectra are fitted with Monte Carlo simulations taking into account the production of Λ and Σ⁰ hyperon production off the proton, and Σ⁻ off the neutron. Models for quasifree production are compared to the data, excess yields close to threshold are attributed to FSI. Evidence for Λ-hypernuclear bound states is seen for ^3,4He targets.

The first Λ-hypernuclear spectroscopy study using an electron beam has been carried out at Jefferson Lab. The hypernuclear spectrometer system (HNSS) was used to measure spectra from the reaction with close to 1 MeV resolution, the best energy resolution obtained so far in hypernuclear spectroscopy with magnetic spectrometers. This paper describes the HNSS and preliminary results for the system. A program of hypernuclear physics experiments is planned for the future with much higher yield and even better energy resolution.

No abstract received.

The electroproduction of the η′ meson on nucleons is investigated within a relativistic chiral unitary approach based on coupled channels. The s wave potentials for electroproduction and meson-baryon scattering are derived from a chiral effective Lagrangian which includes the η′ as an explicit degree of freedom and incorporates important features of the underlying QCD Lagrangian such as the axial U(1) anomaly. The effective potentials are iterated in a Bethe-Salpeter equation and cross sections of η′ electroproduction from nucleons are obtained. The investigation of the η′-nucleon system may offer new insights into the role of gluons in chiral dynamics.

A combined chiral and 1/N_c expansion of the Bethe-Salpeter interaction kernel leads to a good description of the kaon-nucleon, antikaon-nucleon and pion-nucleon scattering data typically up to laboratory momenta of . The covariant on-shell reduced coupled channel Bethe-Salpeter equation with the interaction kernel truncated to chiral order Q³ and to the leading order in the 1/N_c expansion is evaluated.

The longest range weak pion-nucleon coupling constant, , is important for nuclear parity violation. However, after considerable effort in the past two decades, its value is still poorly known largely due to many-body theoretical uncertainties. Prospects of a new measurement of in a theoretically clean process are presented. A measurement of the parity-violating asymmetry in pion photoproduction off the proton is related to in a low-energy theorem for the photon polarization asymmetry at threshold in the chiral limit. At present two completed experiments - photon circular polarization for ¹⁸F and the anapole moment of ¹³³Cs - have been interpreted to give very different values of . This experiment will be the first attempt to measure in the single nucleon system. A reliable measurement of provides a crucial test of the meson-exchange picture of the weak NN interaction. Such a test of the meson-exchange picture will shed light on low energy QCD.

The π⁰ → γγ width is determined to next to leading order in the combined chiral and 1/N_c expansions. It is shown that corrections driven by chiral symmetry breaking produce an enhancement of about 4.5% with respect to the width calculated in terms of the chiral-limit amplitude leading to Γ_{π⁰ → γγ} = 8.10 ± 0.08 MeV. This theoretical prediction will be tested via π⁰ Primakoff production by the PRIMEX experiment at Jefferson Lab.

A dependence of the value of the pion-nucleon sigma term on higher partial waves is discussed. Two recent predictions of a high value of the sigma term are scrutinized. It has been concluded that tha main reason for obtaining high values of the sigma term are input D waves that are not consistent with analyticity.

A new polarized target using HD in the solid phase has been developed for studies of the nucleon spin structure at Q² = 0 using pion photo-production. In combination with the high quality LEGS photon beam and a large solid angle spectrometer this target allows practically background-free measurements on the proton and on the neutron. The first beam-target double-polarization data taken with this target are reported here.

We present a calculation of the lowest-lying baryon masses in the quenched approximation to QCD. The calculations are performed using a non-perturbatively improved clover fermion action, and a splitting is found between the masses of the nucleon and its parity partner. An analysis of the mass of the first radial excitation of the nucleon finds a value considerably larger than that of the parity partner of the nucleon, and thus little evidence for the Roper resonance as a simple three-quark state.

No abstract received.

We present our results for the heavy meson spectrum from quenched lattice QCD calculations. By employing a fully relativistic anisotropic lattice action with very fine temporal resolution, we are able to calculate the heavy quark spectrum for a wide range of quark masses including both charm and bottom. Higher excitations such as the exotic hybrids and orbitally excited mesons are also obtained for the charmonium spectrum. Using several different lattice spacings, we perform a continuum extrapolation of the spectrum.

We present some piloting calculations of the masses of the doubly heavy baryons in the framework of the simple approximation within the nonperturbative string approach. The simple analytical results for dynamical masses of heavy and light quarks and eigenvalues of the effective QCD Hamiltonian are presented.

N* masses in the spin-1/2 and spin-3/2 sectors are computed using two non-perturbative methods: lattice QCD and QCD sum rules. States with both positive and negative parity are isolated via parity projection methods. The basic pattern of the mass splittings is consistent with experiments. The mass splitting within the same parity pair is directly linked to the chiral symmetry breaking QCD.

List of Participants.

Author Index.

Agenda.