Approach to Perturbative QCD Results in Transition Amplitudes of NucleonNegative-Parity Resonances

The scaling behaviors of the nucleon resonance transition amplitudes from perturbative QCD (PQCD) are utilized to parametrize the amplitudes of the first negative-parity nucleon resonance . Our analysis indicates that the constraints of the transition amplitude for the resonance at the limit by QCD sum rule calculations are not applicable at a moderate range of compared with the present available data if the contribution of is dominant in the limit.

Recent results on nucleon resonance electrocouplings from the studies of π^+π^-p electroproduction with the CLAS detector

Recent results on nucleon resonance studies in π＋π-p electro- production off protons with the CLAS detector are presented. The analysis of CLAS data allowed us to determine all essential contributiag mechanisms, providing a credible separation between resonant and non-resonant parts of the cross sections in a wide kinematical area of invariant masses of the final hadronic system 1.3 〈 W 〈 1.8 GeV and photon virtutualities 0.2 〈 Q2 〈 1.5 GeV2. Electrocouplings of several excited proton states with masses less than 1.8 GeV were obtained for the first time from the analysis of π＋π-p exclusive electroproduction channel.

Nucleon resonances in πN→η'N and J/ψ→pη'

We are aiming to study the J/ψ →pqη′ decay in an isobar model and the effective Lagrangian approach on the basis of the coupling constants extracted from the πN→η′ reaction. After a careful exploration of the contributions of the S11（1535）, P11（1710）, P13（1900）, S11（2090） and Pl,（2100） resonances, we conclude that either a subthreshold resonance or a broad P-wave state in the near threshold range seems to be indispensable to describe the present data of the πN→η′.Furthermore, at least one broad resonance above rl＇N threshold is preferred. With this detailed analysis, we give the invariant mass spectrum and Dalitz plot of the J/ψ →pqη′decay for the purpose of assisting the future detailed partial wave analysis. It is found that the J/ψ →pqη′ data are useful for disentangling the above or below threshold resonant contribution, though it still further needs the differential cross section data of πN→η′ to realize some of the resonant and non-resonant contribution.

Nucleon Resonances in the π^-p → K^0Λ Reaction near Threshold

We investigate the two-body reaction π^-p→ K°A within the effective Lagrangian approach and the isobar model. In addition to the ＂background＂ contributions from t-channel K＊ exchange, u-channel N＊（1192） and ∑＊（1385） exchanges, and s-channel nucleon pole terms, the contributions from the nucleon resonances N＊（1535）, N＊（1650）, and N＊ （1720） are investigated. It is shown that the inclusion of these nucleon resonances contributions leads to a good descrip- tion of the experimental totM and differential cross sections data at low energy region. The s-channel N＊ （1535）, N＊ （1650）, and N＊（1720） resonances and the t-channel K＊ exchange give the dominant contributions below W = 1.76 GeV, while the s-channel nucleon pole, u-channel ∑（1192） and ∑＊（1385） exchanges give the minor contributions.

Single π0electro-production in the resonance region with CLAS＊

We report the analysis status of single π0electroproduction in the resonance region to study the electromagnetic excitation of the nucleon resonances. The study is aimed at understanding of the internal structure and dynamics of the nucleon. The experiment was performed using an unpolarized cryogenic hydrogen target and 2.0 and 5.8 GeV polarized electron beam during the ele and e1-6 run periods with CLAS at Jefferson Lab. The new measurements will produce a data base with high statistics and large kinematic coverage for the hadronic invariant mass （W） up to 2.0 CeV in the momentum transfer （Q2） range of 0.3--6.0 GeV2. Preliminary results will be presented and compared with the various model calculations.

Dispersive analysis of low energyγN→πN process and studies on the N^*(890)resonance

We present a dispersive representation of the γN→πN partial-wave amplitude based on unitarity and analyticity.In this representation,the right-hand-cut contribution responsible for πN final-state-interaction effects is taken into account via an Omnes formalism with elastic πN phase shifts as inputs,while the left-hand-cut contribution is estimated by invoking chiral perturbation theory.Numerical fits are performed to pin down the involved subtraction constants.Good fit quality can be achieved with only one free parameter,and the experimental data regarding the multipole amplitude E0+ in the energy region below the △(1232) are well described.Furthermore,we extend the γN →μN partial-wave amplitude to the second Riemann sheet to extract the couplings of the N*(890).The modulus of the residue of the multipole amplitude E0+(S11pE) is 2.41 mfm·GeV2,and the partial width of N*(890)→ yN at the pole is approximately 0.369 MeV,which is almost the same as that of the N*(1535) resonance,indicating that N*(890) strongly couples to the πN system.

Analysis of data for γp → f1(1285)p photoproduction

The photoproduction of the f1(1285)meson off the proton target is investigated within an effective Lagrangian approach.The t-channelρ-andω-exchange diagrams,u-channel nucleon-exchange diagram,generalized contact term,and s-channel pole diagrams of the nucleon and a minimal number of nucleon resonances are taken into account in constructing the reaction amplitudes to describe the experimental data.Three different models,that is,the Feynman,Regge,and interpolated Regge models,are employed,where the t-channel reaction amplitudes are constructed in Feynman,Regge,and interpolated Regge types,respectively.The results show that neither the Feynman model with two nucleon resonances nor the interpolated Regge model with one nucleon resonance can satisfactorily reproduce the available data forγp→f1(1285)p.Nevertheless,in the Regge model,when any one of the N(1990)7/2^(+),N(2000)5/2^(+),N(2040)3/2^(+),N(2060)5/2^(-),N(2100)1/2^(+),N(2120)3/2^(-),N(2190)7/2^(-),N(2300)1/2^(+),and N(2570)5/2^(-)resonances is considered,the data can be well described.The resulting resonance parameters are consistent with those advocated in the Particle Data Group(PDG)review.Further analysis shows that,in the high-energy region,the peaks of γp→f1(1285)p differential cross sections at forward angles are dominated by the contributions from t-channelρ-andω-exchange diagrams,while in low-energy region,the s-channel pole diagrams of resonances also provide significant contributions to theγp→f1(1285)p cross sections.

The N* physics program at Jefferson Lab

Recent measurements of nucleon resonance transition form factors with CLAS at Jefferson Lab are discussed. The new data confirm the assertion of the symmetric constituent quark model of the Roper as the first radial excitation of the nucleon. The data on high Q2 nπ＋ production better constrain the branching ratios liNK and [3Nn. For the first time, the longitudinal transition amplitude to the S11（1535） was extracted from the nπ＋ data. Also, new results on the transition amplitudes for the D13（1520） resonance are presented showing a rapid transition from helicity 3/2 dominance seen at the real photon point to helicty 1/2 dominance at higher Q2. I also discuss the status of the search for new excited nucleon states.

Investigation of the anomaly in η-photoproduction off the neutron

Quasi-free photoproduction of wmesons off the neutron and off the proton has been studied using a deuterium target and bremsstrahlung photons produced by MAMI-C with incident energies up to 1.5 GeV. The wmesons were detected in coincidence with the recoil nucleons thus a fully exclusive measurement was performed. Preliminary results show a bump-like structure in the excitation function for the neutron close to W ≈ 1675 MeV which is not seen for the proton. Considering the experimental resolution and using a Breit-Wigner fit the width of this structure was approximated below 50 MeV.

Near-threshold η production in pp collisions

We study near-threshold η meson production in pp collisions within an effective Lagrangian approach combined with the isobar model, by allowing for the various intermediate nucleon resonances due to the π, η, and ρ-meson exchanges. It is shown that the ρ-meson exchange is the dominant excitation mechanism for these resonances,and the contribution from the N＊（1720） is dominant. The total cross section data can be reasonably reproduced,and the anisotropic angular distributions of the emitted η meson are consistent with experimental measurements.Besides, the invariant mass spectra of pp and pη explain the data well at excess energy of 15 Me V, and are basically consistent with the data at excess energy of 40 Me V. However, our model calculations cannot reasonably account for the two-peak structure in the pη distribution at excess energies of 57 and 72 Me V, which suggests that a more complicated mechanism is needed at higher energy region.

Analytical investigation of γN→N^*,Δ^* transition helicity amplitudes

We study the structure of nonstrange baryons by analytically calculating the electromagnetic transition helicity amplitudes of the nucleon andΔresonances.We employ an improved hypercentral constituent quark model and obtain the corresponding eigenenergies and eigenfunctions in closed forms.Then,we calculate the transverse and longitudinal helicity amplitudes for nucleon andΔresonances.The comparison of evaluated observables and experimental data indicates good agreement between the proposed model and available data.

N*-Program at COSY-Jülich

The paper gives a brief introduction into the COSY-facility, the currently used detector systems （ANKE, TOF and WASA） and the targets （cluster jet, pellet and ABS）, and it presents some recent results, which were obtained in the field of baryon excited states.