International Journal of Modern Physics A

Recent results on the study of the electromagnetic structure of nucleon resonances, the spin structure of protons and neutrons at small and intermediate photon virtuality, and the search for exotic pentaquark baryons are presented.

The photon beam at the LEPS facility is produced by backward-Compton scattering of laser photons from 8 GeV electrons at the SPring-8. The status and prospects of the experimental study on the Θ⁺ at LEPS are reported. Further evidences for pentaquark baryons and counter-evidences are also discussed.

I first give a brief overview of the status of exotic hadron search so far. Then I review the recent theoretical development on pentaquarks. Finally I emphasize that none of theoretical formalisms predicts the existence of Θ pentaquark very reliably. None of them explains its narrow width in a natural way.

In this contribution, we will present our first preliminary measurement of the differential cross section for the reaction γn→ηn. Comparison of the reactions γp→ηp for free and bound proton (D₂ target) will also be discussed.

A general review of BES physics results are given. New results from BES on J/ψ, ψ(2S) and ψ(3770) decays are presented. Enhancement of the mass spectrum at the threshold is observed in the decay of J/ψ to . Analyzes of sigma and kappa signals are reported in J/ψ to ωπ⁺π^- and J/ψ to respectively. Results of new ψ(2S) decay channels are reported and compared with 12% rule. Some results from ψ(3770) scanning and D decays are reported. The status of BEPCII/BESIII project is briefly reported.

At ELSA interesting results on baryon resonances have been obtained by the CB-ELSA, the CBELSA/TAPS and the SAPHIR collaborations. New resonances were found, in particular a new D₁₅(2070) decaying into pη, was recently observed by the CB-ELSA experiment. The availability of a polarized beam and a polarized target did allow to measure the GDH sum rule up to 2.9 GeV. In the future double polarization experiments will be performed using the Crystal Barrel detector together with new forward detector components. These polarization observables will provide important additional information for the partial wave analyses performed to extract the contributing resonances and their parameters from the data.

Differential and total cross sections are presented for π^-- and K^--induced reactions on a proton target leading to all-neutral final states. Also shown are rates for rare and upper limits for forbidden eta-meson decays to test Chiral Perturbation Theory, the π⁰π⁰ interactions and C and CP invariance.

The COSY accelerator in Jülich is presented together with its internal and external detectors. The physics programme performed recently is discussed with emphasis on strangeness physics.

The experimental program on meson production in light-ion collisions at the CELSIUS cooler/storage ring of the The Svedberg Laboratory in Uppsala, is reviewed. Production of pions and eta-mesons have been studied in pp and pd collisions using the PROMICE/WASA detector. This program is now continued at CELSIUS with the WASA4π detector together with studies of meson decays.

The nucleon electromagnetic form factors have been studied in the past extensively from unpolarized electron scattering experiments. With the development in polarized beam, recoil polarimetry, and polarized target technologies, polarization experiments have provided more precise data on these quantities. In this talk, I review recent experimental progress on this subject.

Calculations of form factors in different forms of relativistic kinematics are presented. They involve the instant, front and point forms. In the two first cases, different kinematical conditions are considered while in the latter case, both a Dirac-inspired approach and a hyperplane-based one are incorporated in our study. Numerical results are presented for the pion form factors with emphasis on both the low and high Q² range. A new argument is presented, explaining why some approaches do considerably much better than other ones when only a single-particle current is considered.

The measurement of spin observables in the Λ hyperon production has shown how poor is our understanding of the spin effects in the nucleon structure and in the hadronisation processes. New possibilities are offered by future facilities.

The basic ideas and methods of chiral perturbation theory are briefly reviewed. I discuss the recent attempts to build an effective Lagrangian in the resonance region and summarize the known large–N_C constraints on the low-energy chiral couplings.

In this talk we report on the use of a chiral unitary approach for the interaction of the octets of meson and baryon and the octet of mesons with the decuplet of baryons. Two octets of J^π=1/2^- baryon states and a singlet are generated dynamically in the first case, resulting in the case of strangeness S=-1 in two poles of the scattering matrix close to the nominal Λ(1405) resonance. In the second case many resonances are also generated, among them an exotic baryon with S=1 corresponding to a ΔK resonance. We make suggestions of experiments which could show evidence for the existence of these states.

I review the present status of QCD investigations on chiral Lagrangian for pseudo scalar mesons.

The CHAOS Collaboration at TRIUMF has measured differential cross sections for π^±p elastic at incident pion kinetic energies of 19.9, 25.8, 32.0, 37.1, 43.3, 57.0, and 67.0 MeV. The data analysis at the five lowest energies has been completed at the University of Tübingen. In order to measure at forward angles, in the Coulomb-Nuclear Interference region, the CHAOS detector was modified from its standard configuration with a specially developed range telescope. μ^±p scattering at forward angles was also measured simultaneously, and used for validation of acceptance simulations.

The new pionic hydrogen experiment at PSI aims at a determination of the strong interaction ground state shift and width of the pionic hydrogen atom with the final goal to extract isospin separated scattering lengths with accuracies on the percent level. Its achievements compared to previous efforts are indicated and the inherent difficulties of the experimantal approach as well as the chosen solutions are discussed. First results are presented.

A model independent expression for the electromagnetic corrections to a phenomenological hadronic pion-nucleon (πN) scattering length a^h, extracted from pionic hydrogen, is obtained. In a non-relativistic approach and using an extended charge distribution, these corrections are derived up to terms of order α² log α in the limit of a short-range hadronic interaction. We infer which gives for the πNN coupling through the GMO relation .

A self-consistent analysis of pion scattering and pion photoproduction within a coupled channels dynamical model is presented. The results indicate the existence of a third and a fourth S₁₁ resonance with the masses 1846±47 and 2113±70 MeV. In the case of pion photoproduction, we found that background contributions to the imaginary part of the S-wave multipole become large and negative in the region of the S₁₁(1535) resonance. Due to this fact, much larger resonance contributions are required in order to explain the results of the recent multipole analysis. For the first S₁₁(1535) resonance we obtain a value of 72±2×10^-3GeV^-1/2 for the electromagnetic helicity amplitude A_1/2.

A model for two-pion production in pion-induced reactions including the excitation of baryon resonances is presented and applied to the recent Brookhaven data. The impact of the Θ⁺(1540) resonance on differential and integrated cross sections for kaon reactions on nuclei is analysed using the Jülich kaon-nucleon model. An upper limit for the width of the pentaquark candidate of 1 MeV results.

The dynamical model of pion electroproduction developed by the authors has been extended to study the pion weak production reaction. The axial vector N-Δ transition form factor obtained from the analysis of existing neutrino reaction data and the role of dynamical pion cloud on are discussed.

The Crystal Ball Spectrometer was 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 program and 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 → ηn 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 investigated by studying the 2π° production mechanism both in the fundamental interaction and in nuclei.

An overview is given of selected recent HERMES results involving hadron production. Topics including transversity, hard exclusive production of pion pairs, observation of the Θ⁺ pentaquark state and quark fragmentation in nuclei will be presented.

Preliminary results are presented of the reaction γ*N→Δ→πN that was studied at the Jefferson Laboratory for transferred four momentum of 1.0 (GeV/c)². Angular distributions of the cross sections and polarizations were measured over much of the available phase space. Out of the eighteen response functions involved in this reaction, fourteen separate responses and two additional response combinations were extracted. All resonant and background multipole amplitudes were decomposed for s- and p-waves as well as several of the d-wave multipole amplitudes. The responses and multipoles were compared to model calculations. Significant differences were observed between the experimental values and those of the models for several of the responses and multipoles. The ratios of the electric and longitudinal quadrupole amplitudes to the magnetic dipole amplitude were extracted. These values are different from previously reported values.

The issue of hadron deformation has been a subject of intense scientific interest during the last two decades. The detailed study of the N→Δ transition is the best method of experimental investigation of this issue. A brief overview of the experimental and theoretical landscape on the field will be presented followd by a presentation of the most recent results from the Bates N→Δ program. The Bates data yield precise quadrupole to dipole amplitude ratios: EMR=(-2.3±0.3_stat+sys±0.6_model)% and CMR=(-6.1±0.2_stat+sys±0.5_model)% for . The derived amplitudes give credence to the conjecture of deformation in hadrons favoring, at low Q², the dominance of mesonic effects.

In α-p and p-p scattering at energies in the GeV region a strong "scalar" N* excitation has been observed in the mass region of the lowest P₁₁ resonance, with characteristic properties quite different from the Roper resonance seen in π-N and γ-N. From the t-dependence in p-p scattering a P₁₁ assignment is clearly supported. Further, from exclusive p+p→ppπ⁺ π^- data a dominant 2π-N decay of this resonance is deduced. In a DWBA description the differential cross sections were found to be sensitive to the nucleon transition density. To describe all p-p data at beam momenta between 9 and 30 GeV/c a surface peaked transition density had to be utilized. Equivalent information on the charge transition density can be obtained from the longitudinal electron scattering amplitude S_1/2, deduced from polarized e-p scattering experiments. Combining the analysis of (p,p') and (e,e'), for the "scalar" P₁₁ we obtain the following results: a) a valence quark excitation is clearly seen, b) strong sea-quark effects are observed, which are significantly larger than the valence quark contribution, c) the sea-quark transition density shows "breathing", indicating that these effects are already present in the g.s. nucleon.

The studies on the multi-quark candidates, light scalar mesons and excited baryon states at BES are presented, based on 5.8×10⁷ J/ψ data collected with BESII detector. The measurements of some J/ψ and η_c decays are presented too. We also report the searches for the lepton flavor violation and pentaquark states in J/ψ decays.

J/ψ strong decay is one of the best places to study the hadronic structure and the strong interaction. In this talk, as the background contribution, the proton pole diagram contribution to the branching ratio and the angular distribution in decays is analyzed. The applicability of quark models is explored via process. The structures of N*s, especially Roper resonance, are discussed by studying J/ψ decays. The productions of possible multi-quark systems via charmonium and bottomonium strong/radiative decays are also investigated.

Glueballs and hybrids are predicted to exist but searches for them have failed to provide conclusive evidence. One–gluon exchange is not an important part of strong interactions in this energy regime. Instead, quarks seem to interact indirectly, via changes of the QCD vacuum. Strong interactions seem to be governed by instanton–induced interactions; the chiral soliton model gives a more suitable interpretation of the Θ⁺(1540) than models based on the dynamics of four quarks and one antiquark.

The subject of Generalized Parton Distributions is briefly reviewed. The general theoretical framework and the recently published data are discussed.

After a series of experiments measuring the quark contribution ΔΣ to the nucleon spin in the 90's, a new generation of experiments is exploring the gluon contribution. First results are already obtained for the value of ΔG/G at a few values of x_g. More results are expected in the future from COMPASS at CERN and RHIC spin at Brookhaven.

The nucleon spin structure has been shown to be described by a dynamical constituent quark model with ~85% q³ and Fock components. As a QCD system there is no gauge invariance and canonical commutation relation simultaneously satisfied decomposition of the nucleon spin operator. To keep gauge invariance but give up the canonical commutation relation of the angular momentum operator will ruin the multiple moments expansion and the partial wave decomposition. A weak gauge invariant condition is proposed where the canonical commutation relation of angular momentum operator is intact and the gauge invariance is only true for special sets of matrix elements instead of the operators.

The pp→ppx(x=γγ, π⁰π⁰, π⁺π^-) reactions have been measured at CELSIUS in the energy range T_p=775-1360 MeV using the WASA 4π detector with hydrogen pellet target. Close to threshold the ppπ⁺π^- data are in accordance with the Roper resonance concept and/or a dynamic formation of the σ meson. At higher energies the formation of the ΔΔ system becomes increasingly dominant. The ppπ⁰π⁰ channel basically shows similar features, however, develops a peculiar enhancement at small invariant masses M_π⁰π⁰ - in resemblance of Bose-Einstein correlations. Alternatively, the enhancement could be also indicative of a (dynamic) isospin breaking due to ππ rescattering in the σ channel. The latter is favored by the results on pd→³Hex (x=π⁰π⁰, π⁺π^-), where the observed π⁰π⁰ enhancements are still much larger. In the γγ channel a line at M_γγ=2m_π is observed. The possible nature of this line is discussed.

In this report, the theoretical framework of the chiral SU(3) quark model is introduced briefly. By using this model the NN, YN and KN scattering processes, the structures of dibaryon states and pentaquark states are studied. A new interesting dibaryon candidate (ΩΩ)_0⁺ is predicted.

Medium-modification of hadronic interactions is defined as the differences between hadron-hadron interaction in the nuclear medium and the corresponding interaction in free space. Deeply penetrating hadrons provide such information and we discuss here pionic atoms and scattering by nuclei of 21.5 MeV pions. Brief mention is made also of the interaction of 500-700 MeV/c K⁺ with nuclei.

The total and differential cross section for exclusive vector meson electro- and photoproduction above the resonance region (2<W<3 GeV) were measured at Jefferson Laboratory covering a wide kinematic range (0<Q²<5 GeV²/c² and 0<-t<5 GeV²). These measurements provide a unique tool to investigate the transition between soft and hard physics. Our data indicates that this regime can be successfully and consistently described by using the so called effective partonic degrees of freedom.

This paper gives an introduction to the hadron physics program and the planned PANDA experiment at the future FAIR facility located at GSI in Germany.

A strongly momentum-dependent dressed-quark mass function is basic to QCD. It is central to the appearance of a constituent-quark mass-scale and an existential prerequisite for Goldstone modes. Dyson-Schwinger equation (DSEs) studies have long emphasised this importance, and have proved that QCD's Goldstone modes are the only pseudoscalar mesons to possess a nonzero leptonic decay constant in the chiral limit when chiral symmetry is dynamically broken, while the decay constants of their radial excitations vanish. Such features are readily illustrated using a rainbow-ladder truncation of the DSEs. In this connection we find (in GeV): f_ηc(1S)=0.233, m_ηc(2S)=3.42; and support for interpreting η(1295), η(1470) as the first radial excitations of η(548), η′(958), respectively, and K(1460) as the first radial excitation of the kaon. Moreover, such radial excitations have electromagnetic diameters greater than 2 fm. This exceeds the spatial length of lattices used typically in contemporary lattice-QCD.

The chiral quark model posits that pions couple to the axial current of constituent quarks. This chiral pion coupling governs the pion decays of baryon resonances and also implies pion and multipion interactions between the quarks. The qualitative features of this model for the description of pionic decay widths and baryon spectra are outlined. When the covariant quark model wave function for the nucleon is chosen so as to describe the empirical nucleon form factors the pion decay widths of the positive parity resonances are underpredicted. This indicates the presence of significant multiquark components in these resonances.

The status of conventional baryon flux-tubes and hybrid baryons is reviewed. Recent surprises are that a model prediction indicates that hybrid baryons are very weakly produced in glue-rich Ψ decays, and an analysis of electro-production data concludes that the Roper resonance is not a hybrid baryon. The baryon decay flux-tube overlap has been calculated in the flux-tube model, and is discussed here. The behavior of the overlap follows naïve expectations.

Light and heavy pentaquarks are described within a constituent quark model based on a spin-flavor hyperfine interaction. In this model the lowest state acquires positive parity. The masses of the light antidecuplet members are calculated dynamically using a variational method. It is shown that the octet and antidecuplet states with the same quantum numbers mix ideally due to SU(3)_F breaking. Masses of the charmed antisextet pentaquarks are predicted within the same model.

The πN scattering measurements from last couple of decades are not in very good agreement with each other. In fact, using the different data sets one finds different values for the pion-nucleon coupling constant. An analysis with theoretical constraints is the only way to produce accurate partial waves. In this analysis, the fixed-t dispersion relations are used to ensure analyticity in the invariant amplitudes and to decrease the effects of inaccuracies in the data base. Pietarinen's expansion is the method used to enforce the dispersion constraints. The strength of the analyticity constraints is illustrated with C^± amplitudes in the forward direction.

Measurements of differential cross sections for pion-nucleon charge exchange, π^-p→π⁰n, in the region of the P₁₁(1440), or Roper, resonance are presented. These data were obtained as part of the baryon spectroscopy program using the Crystal Ball detector at the Alternating Gradient Synchrocyclotron (AGS) at Brookhaven National Laboratory (BNL). Data were taken in 1998 with a liquid hydrogen target and in 2002 with CH₂ and C targets.

Total cross sections of the single charge exchange reaction π^-p→π⁰n have been measured at PSI from about 40 to 250 MeV using a transmission technique. Preliminary results show an excellent agreement with predictions from the SAID FA02 phase shift analysis for energies above 70 MeV.

Pion charge-exchange (π^-p→π⁰n) differential cross sections are one of the basic sources of information on neutral pion-nucleon resonances. We present new, high statistics, high quality charge-exchange differential cross-section data obtained in the incident π^- momentum range 650–750 MeV/c. The total uncertainty of the measurements is 6% or less. 648 new data points were obtained using the Crystal Ball multiphoton spectrometer covering 94% of the full solid angle. Our data show deviations from the results of the partial–wave analysis for some momenta, in particular at very backward angles. We also report on a search for the cusp effect in π^-p→π⁰n at the opening of the π^-p→ηn channel.

The s-wave interactions of the baryon decuplet with the octet of pseudoscalar mesons is studied in a unitarized coupled channel approach. We obtain a fair agreement for mass and width of several resonances. In particular, the Ξ(1820), the Λ(1520) and the Σ(1670) states are well reproduced. Other resonances are predicted and also the couplings of the observed resonances to the various channels are evaluated.

By applying a spatially extended sink operator, the nucleon two-point functions are calculated in Coulomb gauge from quenched lattice QCD with overlap fermions. The Bethe-Salpeter wavefunctions of nucleon and Roper resonance are extracted at different quark masses. The typical size of nucleon is about 0.7 fm, which seems insensitive to the quark masses involved in this work. It is also found that the wavefunction of Roper has a radial node.

The πN system in is limited to be pure isospin 1/2 due to isospin conservation. This is a big advantage in studying N*→πN from the J/ψ decays, compared with πN and γN experiments which mix isospin 1/2 and 3/2 for πN system. Based on the 58 million J/ψ data collected at the Beijing Electron Positron Collider, and other decay channels are analyzed. In the decay , three peaks show up very clearly at 1400 MeV/c², 1500 MeV/c² and 1670 MeV/c² in the πN invariant mass spectrum with additional peak around 2030 MeV/c² which may be caused by a new N* resonance.

The reaction NN→NNη is revisited and in particular the short-range mechanism is investigated. It is found that this particular mechanism has a small contribution in contradiction to what was found by earlier investigations.

We suggest a unified description of baryon form factors in terms of a two-component model with an intrinsic q³ structure and a meson cloud, parameterized in terms of vector mesons. We use this model to study the N-Δ form factors. The results are in excellent agreement with data. The N-Δ transition appears to be dominated by coupling to the ρ meson.

We consider the Quasilocal Quark Model of NJL type (QNJLM) as an effective theory of non-perturbative QCD including scalar (S), pseudoscalar (P), vector (V) and axial-vector (A) four-fermion interaction with derivatives. In the presence of a strong attraction in the scalar channel the chiral symmetry is spontaneously broken and as a consequence the composite meson states are generated in all channels. With the help of Operator Product Expansion the appropriate set of Chiral Symmetry Restoration (CSR) Sum Rules in these channels are imposed as matching conditions to QCD at intermediate energies. The mass spectrum and some decay constants for ground and excited meson states are calculated.

We report a study on the non-pertribative structure of the polarized nucleon sea using the meson cloud model. It is found that the SU(2) flavour symmetry and strange-antistrange symmetry in the polarized nucleon sea are broken. Our calculations are in agreement with the recent results from HERMES Collaboration.

The baryonic decays of J/ψ provide a new way to study the internal structure of baryons. A simple diquark model applied to the calculation of the decay cross-sections is compared with the ordinary constituent quark model. Various models also give different predictions for the rates involving the N*(1440) resonance in the final state.

Following a recently proposed confinement generating mechanism, we provide a new string inspired model with a massive dilaton and a new dilaton coupling function ⁵. By solving analytically the equations of motion, a new class of confining interquark potentials is derived which includes several popular potential forms given in the literature.

There has been a discrepancy between values of the pion-nucleon sigma term extracted by two different methods for many years. Analysis of recent high precision pion-nucleon data has widened the gap between the two determinations. It is argued that the two extractions correspond to different quantities and that the difference between them can be understood and calculated.

A brief review of the pion-nucleon sigma-term is given.

Dispersion relations along interior hyperbolas and a set of hyperbolas passing through the Cheng-Dashen point are used to calculate the pion-nucleon sigma term. The t-channel input is updated using the recent GWU partial wave solution and ππ phase shifts from calculations based on Roy equations. Obtained values for the sigma term are still within the error bars of the previous Karlsruhe result.

The COSY-11 collaboration has conducted experiments aiming at the determination of the excitation function and phase-space population of the pp→ppη reaction close to the kinematical threshold. The precise data obtained with the stochastically cooled proton beam of the cooler synchrotron COSY and the high resolution zero-degree magnetic spectrometer allowed for the observation of the significant deviations – in the shape of the excitation function and two-particle invariant masses – from the predictions based on the assumption that the reaction phase space is homogenously populated. Comparison of the shape of the excitation function for the pp→ppη and pp→ppη′ reaction allows to distinquish in the model independent way an influence originating from the proton-proton and proton-η interaction. For the comparison the full data set from experiments performed at COSY and other laboratories is used.

The S, P, D, F wave KN phase shifts have been studied in the chiral SU(3) quark model by solving a resonating group method equation. The numerical results of different partial waves are in agreement with the experimental data except for the cases of P₁₃ and D₁₅, which are less well described when the laboratory momentum of the kaon meson is greater than 400 MeV.

The KLOE experiment at the Frascati ϕ-factory DAΦNE has collected about 0.5 fb^-1 of data till the end of the year 2002. These data allow to perform a wide physics program, ranging from the physics of charged and neutral kaons to radiative ϕ-decays. Results are presented for the K_L lifetime and the semileptonic processes K_S,L→πeν. From the light meson spectroscopy program, results on the decays ϕ→f₀(980)γ, a₀(980)γ as well as ϕ→ηγ, η′γ are presented.

π⁺π^- transitions of heavy quarkonia, especially the ϒ(3S)→ϒ(1S)π⁺π^- decay process are studied. An effective Lagrangian is employed and the S wave ππ final state interaction (FSI) is included in the framework of the Chiral Unitary Theory (ChUT). It is found that when an additional intermediate state with J^P=1⁺ and I=1 is introduced, the experimental data can be well-explained, and a consistent description of bottomonium π⁺π^- transitions can be obtained. As a consequence, the mass and the width of the intermediate state are predicted. From the quark content analysis, this state should be a state.

We make a theoretical study of the J/Ψ decays into ωππ, ϕππ, and using the techniques of the chiral unitary approach stressing the important role of the scalar resonances dynamically generated through the final state interaction of the two pseudoscalar mesons. We also discuss the importance of new mechanisms with intermediate exchange of vector and axial-vector mesons and the role played by the OZI rule in the J/Ψϕππ vertex, quantifying its effects. The results nicely reproduce the experimental data for the invariant mass distributions in all the channels considered.

We study the radiative ρ, ω and ϕ decay into π⁰π⁰γ and π⁰ηγ taking into account mechanisms in which there are two sequential vector-vector-pseudoscalar or axial-vector–vector–pseudoscalar steps followed by the coupling of a vector meson to the photon, considering the final state interaction of the two mesons. Other mechanisms in which two kaons are produced through the same sequential mechanisms or from vector meson decay into two kaons which undergo final state interaction leading to the final pair of pions or π⁰η, are also considered. The results of the parameter free theory, together with the theoretical uncertainties, are compared with the latest experimental results at Frascati and Novosibirsk.

We study coupled channel effects upon isospin I=2 and I=0 ππ S-wave interaction. With introduction of the ππ→ρρ→ππ coupled channel box diagram contribution into ππ amplitude in addition to ρ and f₂(1270) exchange, we reproduce the ππ I=2 S-wave and D-wave scattering phase shifts and inelasticities up to 2 GeV quite well in a K-matrix formalism. For I=0 case, the same ππ→ρρ→ππ box diagram is found to give the largest contribution for the inelasticity among all possible coupled channels including ππ→ωω→ππ, . We also show why the broad σ appears narrower in production processes than in ππ scattering process.

We show how deviations from the dimensional scaling laws for exclusive processes may be related to a breakdown in the locality of quark-hadron duality, i.e. the "restricted locality". For exclusive reactions like meson photo- and electroproduction above the resonance region, we explore the effects arising from such a local duality breaking and propose that it can be a possible source for oscillations about the smooth quark counting rule predicted by pQCD in the 90-degree differential cross sections.

The recent measurement of quark-hadron duality is shortly discussed by using a simple constituent quark model. Comparing with the moments of the structure functions in the resonance region and in the deep inelastic scattering region, we study the occurances of Bloom-Gilman quark-hadron duality for the nucleon structure function F₂ and g₁. Local duality is also analyzed.

Preliminary results on the beam asymmetry Σ for η and π⁰ photoproduction on the proton and the neutron in the deuteron in a quasi free kinematics regime have been obtained. Very preliminary results will be presented on the status of art of the GRAAL search for the two members of the exotic penta-quarks anti-decuplet, the P11 and the Θ⁺.

In this paper we calculate spin dependence of parton distributions and proton structure function. The polarized valon distributions in a proton and the polarized parton distributions inside the valon are necessary to obtain the polarized parton distributions in a proton. The results of our calculations for the proton structure function, are in good agreement with the available experimental data. We obtain the spin contribution and angular momentum of the valons to the proton.

We calculate nuclear parton distribution functions (PDFs), using the constituent quark model. We find the bounded valon distributions in a nuclear to be related to free valon distributions in a nucleon. By using improved bounded valon distributions for a nuclear with atomic number A and the partonic structure functions inside the valon, we can calculate the nuclear structure function in x space. The results for nuclear structure-function ratio at some values of A, are in good agreement with the experimental data.

Based on the quark mass density- and temperature- dependent model we suggest a model for nuclear matter where the meson field is introduced to be directly coupled to the quarks. The dynamic formation of the nucleon bag, the saturation properties of nuclear matter as well as equation of state for this model are studies.

The cross section σ(e⁺e^-→π⁺π^-γ) has been measured with the KLOE detector at the electron-positron collider DAΦNE, at an energy W=m_ϕ=1.02 GeV. We perform this measurement as a function of the π⁺π^- invariant mass, M_ππ. The photon in the above process is due to Initial State Radiation. Dividing by a theoretical radiator function, we obtain the cross section σ(e⁺e^-→π⁺π^-) for the mass range with an overall accuracy of 1.3%. We also calculate the hadronic contribution of the two-pion channel to the anomalous magnetic moment of the muon for the given mass range: .

Relativistic effects in two photon decay of 0^-+ quarkonium are investigated with a relativistic phenomenological approach. Comparing with the non-relativistic approximation, the relativistic phenomenological approach gives corrections coming from three sources: relative momentum distribution, relative energy distribution and description of quark spinors in the meson. These relativistic effects are studied in detail for and systems.

Exclusive ω electroproduction off the proton was measured at the highest possible four-momentum transfer with the (close to) 6 GeV beam now available at CEBAF. Cross sections are presented, together with an analysis of the ω spin density matrix elements. Indications are that π⁰ exchange in the t-channel (or rather the exchange of the corresponding saturating Regge trajectory) seems to dominate the process γ*p→ωp, even for photon virtuality Q² as large as 5 GeV². Contributions of the handbag type, related to Generalized Parton Distributions in the nucleon, are therefore difficult to extract from this particular process.

The reaction p(e,e′π⁺)X⁰ was studied with two high resolution magnetic spectrometers to search for narrow baryon resonances. A missing mass resolution of 2.0 MeV was achieved. A search for structures in the mass region of 0.97<M_X⁰<1.06 GeV yielded no significant signal. The yield ratio of p(e,e′π⁺)X⁰/p(e,e′π⁺)n was determined to be (-0.35±0.35)×10^-3 at 1.004 GeV and (0.34±0.42)×10^-3 at 1.044 GeV. This measurement clearly demonstrated the potential of high resolution missing mass searches in coincidence experiments.

Flavor symmetric sea quarks could contribute to the ratio of in the proton, as well as flavor asymmetric sea quarks. We clarify the isoscalar mesonic contribution, especially σ meson cloud effects in the meson cloud model. We also provide the meson numbers in the proton with different meson cutoff values. Our calculations show that a physical proton picture favors smaller cutoff of the examined range 1.0<Λ_ω<1.5GeV.

To obtain the improved QCD predictions for the moments of the xF₃ structure function in νN scattering, three-loop Wilson coefficients with the corresponding NNLO anomalous dimension coefficients is used. A comparison between QCD predictions and the xF₃ data of the CCFR collaboration has been done in which the Bernstein Polynomial method is used in the complete renormalization group improvement approach. A direct fits for over the corresponding range of Q² with five effective quark flavours, is performed.

The analyzing powers A_N and spin correlation parameters A_NN, A_SS, and A_SL of the elastic proton-proton scattering have been measured with an internal polarized atomic beam target at the cooler synchrotron COSY. Excitation functions were taken continuously during the acceleration and deceleration of the internal beam for kinetic energies ranging between 0.45 and 2.5 GeV and scattering angles 30°≤Θ_c.m.≤90°. The analyzing power excitation functions provide a new polarization standard in the GeV region. The spin correlation data on A_SS disagree strikingly with existing phase shift solutions. The addition of the new polarization data to the world database removes some of the ambiguities in the phase shift solutions and the direct reconstruction of scattering amplitudes. The data are compared with a recent extension of a meson-exchange potential model to higher energies.

A model for nuclei described directly in terms of quarks has been developed in both relativistic and non-relativistic forms. It describes nuclear binding energy and structure for small nuclei (A=3,4) systematically correctly, including the EMC effect. With one free parameter each for strange and for nonstrange states, it also well describes low energy baryon-nucleon scattering, phase shifts and potentials. It predicts low mass, narrow dibaryon and pentaquark states. To be consistent with reported states, new physics may be required that is not included in any quark model to date.

The strangeness s=+1 pentaquark states and the six-quark quark clusters qqqqqq are investigated. By analyzing the inherent nodal structure of the systems, the low-lying states are picked out. By comparing the accessibility of the nodeless states and referring the presently obtained K-N interaction potential, we propose that the quantum numbers of the lowest-lying pentaquark state Θ⁺ may be (J^P, T)=(½⁺,0) and L=1. Meanwhile, the possible dibaryon states are briefly discussed.

Investigations of the mass and decays of the J^PC=1^-+ hybrid are reviewed, including calculation of the π₁(1^-+)→ηπ, η′π decay widths within the QCD sum rules technique. In this calculation, the recently-proposed η, η′ quark mixing scheme is employed. The results indicate that the decay width Γ_π1→ηπ≈250 MeV is large compared with the decay width Γ_{π1→η′π}≈20 MeV. Inspired by these results, some phenomenological approaches are suggested to gain an understanding of the underlying mechanism of ηπ and η′π hybrid decays.

The splittings between positive parity pentaquarks containing a single heavy (c, b) antiquark and four light (u, d) quarks are investigated in models having spin-dependent interactions generated by either effective Goldstone boson exchange or effective color magnetic exchange. Model-independent features of these splittings are emphasized. Overlaps to the "fall-apart" decay configurations ND, ND* or NB, NB*, relevant to decay couplings for the various 2-body pentaquark decays, are also computed.

Using large N_c counting rule, it is argued that tetra-quark resonances do not exist. Also it is pointed out that there exists the violation of exchange degeneracy in the exotic KN scattering channel. It implies either the failure of resonance saturation assumption or it suggests the existence of exotic baryon resonances in such a channel.

We observe a narrow enhancement near 2m_p in the invariant mass spectrum of pairs from radiative decays. In the case of S-wave fit, the peak mass is below 2m_p at and the total width is Γ<30MeV at the 90% confidence level. And we also find near-threshold enhancement in the invariant mass spectrum of in decays. The mass is m=2075±12(stat)±5(syst)MeV and width is Γ=90±35(stat)±9(syst)MeV. A near-threshold enhancement is also studied and it has m=1.50~1.65GeV, Γ=70~110MeV with .

The BES collaboration has recently observed a strong enhancement close to the proton-antiproton, threshold in the J/ψ decays into . Such a structure can be explained by a traditional nucleon-antinucleon, , model. The near threshold ¹¹S₀ bound state and/or the well-established ¹³P₀ resonant state found in this interaction can adequately describe the BES data.

We give the comparisons between the chiral SU(3) quark model and the extended chiral SU(3) quark model. The results show that the phase shifts of NN scattering are very similar. However, the short range mechanisms of nucleon-nucleon interaction are totally different. In the chiral SU(3) quark model, the short range interaction is dominantly from OGE, and in the extended chiral SU(3) quark model, it is dominantly from vector meson exchanges.

Spin transfer in high energy fragmentation process is determined by the hadronization mechanism and spin structure of hadrons. It can be studied by measuring the polarizations of hyperons and/or vector mesons in e⁺e^- annihilation, in the current fragmentation region of polarized deeply inelastic lN-scatterings, and high p_T-jets in polarized pp-collisions. Theoretical calculations have been made using different models. In this talk, I will briefly summarize the main features of the models, the results obtained and the comparison with available data. They can be used for future tests by experiments.

Multijet production rates in neutral current deep inelastic scattering (DIS) have been measured in the range of exchanged boson virtualities 10<Q²<5000 GeV². The data were taken at the ep collider HERA with centre-of-mass energy using the ZEUS detector and correspond to an integrated luminosity of 82.2 pb^-1. Jets were identified in the Breit frame using the k_T cluster algorithm in the longitudinally invariant inclusive mode. Measurements of differential multijet cross sections are presented as functions of jet transverse energy , pseudorapidity and Q² with and . Next-to-leading-order QCD calculations describe the data well. The value of the strong coupling constant α_s(M_Z), determined from the ratio of the trijet to dijet cross sections, is .

I briefly review some of the interesting recent results on neutron spin structure that have been obtained using the Jefferson Lab Hall A facility with a polarized ³He target.

A relativistic plane wave formalism for quasifree eta meson photoproduction is presented. The sensitivity of the unpolarized differential cross section, the recoil nucleon polarization, and the photon asymmetry is investigated. The results indicate that the S₁₁ resonance dominates the unpolarized differential cross section, but that its contribution to the spin observables are very small, for which the D₁₃ resonance plays the primary role.

The quantum vacuum in a many-body system of finite nuclei has been investigated within the relativistic Hartree approach which describes the bound states of nucleons and anti-nucleons consistently. The contributions of the Dirac sea to the source terms of the meson-field equations are taken into account up to the one-nucleon loop and one-meson loop. The tensor couplings for the ω- and ρ-meson are included in the model. The overall nucleon spectra of shell-model states are in agreement with the data. The calculated anti-nucleon spectra in the vacuum differ about 20 – 30 MeV with and without the tensor-coupling effects.