Narrow Results

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 isobar models η-MAID and η′-MAID have been used to analyze new data on quasi-free η photoproduction on the deuteron from Bonn and recent η′ data on the proton from Jlab. In η photoproduction on the neutron a bump around W = 1700 MeV was observed which could possibly arise from a narrow P₁₁ state that is discussed as a non-strange member of the Θ⁺ antidecuplett. In η′ photoproduction on the proton resonance contributions are found that can be attributed to missing resonances in the energy region around W = 1900 MeV.

Excitation of the nucleon at low Q² and energies up to several GeV reveals clearly measurable structures that indicate the existance of resonances. A major goal of experiments carried out at the accelerator facility ELSA in Bonn/Germany is to explore this resonance structure, which gives valuable input to models describing the QCD structure of the nucleon. Resonance contributions for nuclear reaction channels, such as single pion, two pion and eta production from the experiments can be given using the Bonn-Gatchina partial wave analysis¹. The talk at the conference MESON 2008 in Krakow gave an overview of results from experiments at ELSA and plans for the near future.

During the last few years photoproduction of mesons off light target nuclei (²H, ³He, ⁷Li) has been systematically studied at the tagged photon beams of the Bonn ELSA accelerator and the Mainz MAMI accelerator. The experiments used the combined Crystal Barrel/TAPS setup in Bonn and the Crystal Ball/TAPS setup in Mainz. Most measurements investigated meson photoproduction off the neutron for many different reaction channels in order to establish the iso-spin structure of electromagnetic resonance couplings. This project used mostly deuteron targets. The measurements with the He and Li targets aimed at the search for η-mesic nuclei and hadron in-medium properties.

We have performed the tree-level calculations of the eta meson photoproduction on proton for photon laboratory energies between 714 and 1500 MeV. The resonances N(1535), N(1710), N(1650), N(1440) and N(1520) were assumed. The form factors were introduced in the hadron vertices to involve the structure of hadrons. The influence of various form factor recipes was investigated. In order to keep gauge invariance, the appropriate contact term was introduced. The parameters of our model (coupling constants, cutoffs) were fitted to recent experimental data of the cross sections and some polarization observables.

Recent experimental and theoretical advances in investigating electromagnetic meson production reactions in the nucleon resonance region are reviewed. We give a description of current experimental facilities with electron and photon beams and present a unified derivation of most of the phenomenological approaches being used to extract the resonance parameters from the data. The analyses of π and η production data and the resulting transition form factors for the Δ(1232)P₃₃, N(1535)S₁₁, N(1440)P₁₁, and N(1520)D₁₃ resonances are discussed in detail. The status of our understanding of the reactions with production of two pions, kaons, and vector mesons is also reviewed.

The differential and total cross sections were measured for the γp → ηp and γd → ηpn reactions at E_γ ≤ 1150 MeV by using an electro-magnetic (EM) calorimeter SCISSORS II at the Laboratory of Nuclear Science (LNS), Tohoku University. The total cross section on the deuteron shows a bump around E_γ = 1 GeV, while no bump is observed in the same energy region of that on the proton. This bump is attributed to be a nucleon resonance excited from the neutron, and it is a candidate of anti-decuplet penta-quark baryons with hidden strangeness.

It was difficult, however, to detect all the γ's coming from η decay since the solid angle of SCISSORS II was only 12.6% in total. Statistics of the detected η produced events is poor and systematic uncertainty of the obtained cross section is not small due to low acceptance. A new EM calorimeter complex called FOREST with a solid angle of about 4π sr has been constructed. The spin and parity of the relevant resonance are expected to be determined by the experiments with FOREST.

Recent advances in the study of excited nucleons are discussed. Much of the progress has been achieved due to the availability of high precision meson production data in the photoproduction and electroproduction sectors, the development of multi-channel partial wave analysis techniques, and advances in Lattice QCD with predictions of the full excitation spectrum.

We report our recent work on a dynamical investigation of the and states in a chiral quark model. Our results show that the interaction is attractive, which consequently results in a bound state with the binding energy of about 5-42 MeV, unlike the case of state, which has a repulsive interaction and thus is unbound.

The transition γ_vpN* amplitudes (electrocouplings) for prominent excited nucleon states obtained in a wide area of photon virtualities offer valuable information for the exploration of the N* structure at different distances and allow us to access the complex dynamics of non-perturbative strong interaction. The current status in the studies of γ_vpN* electrocouplings from the data on exclusive meson electroproduction off protons measured with the CLAS detector at Jefferson Lab is presented. The impact of these results on exploration of the N* structure is discussed.

The Λ(1520) photoproduction from proton target is investigated within the effective Lagrangian method and well compared with the experimental data. The D₁₃ state N(2120) plays the most important role among the nucleon resonances. Besides N(2120), a nucleon resonance predicted by the constituent quark model is found also important to reproduce the experimental data. Other nucleon resonances give negligible contributions in the channel considered in this work.

The mechanism of K* vector meson photoproduction off the proton target, γp → K^*+Λ, is deeply studied by using an effective Lagrangian method. Based on the (t,s,u)-channel Born diagrams, we included higher resonance terms, i.e., S₁₁(2090) and G₁₇(2190), along with the D₁₃(2080) and D₁₅(2200), which are already considered in a previous theoretical analysis. The total and differential cross section are computed and we obtain quite precise results in comparison with the recent CLAS data.

The unitary and analytic model for nucleons works very well. The model for spin ${\frac{1}{2}}^{+}$ baryons can predict static parameters for $\mathrm{\Lambda }$, ${\mathrm{\Sigma }}^{+}$, ${\mathrm{\Sigma }}^0$, ${\mathrm{\Sigma }}^{-}$, $\mathrm{\Xi }$, ${\mathrm{\Xi }}^{-}$. We prepared the scheme how to construct the model also for $\mathrm{\Delta }$ nucleon resonance. It was used to describe transition form factor $G_{\mathrm{\text{M}}}^{\ast }$ and the experimentally measured ratio $R_{\mathrm{\text{SM}}}$ with the help of relations for magnetic dipole transition and charge quadrupole form factors.

The differential cross-section data from the CLAS Collaboration for $\gamma p\to K^{\ast +}\mathrm{\Lambda }$ have been analyzed based on an effective Lagrangian approach. The $t$-channel $K$, $\kappa$, $K^{\ast }$ exchanges, the $s$-channel $N$ and near-threshold $N\ast$’s exchanges, the $u$-channel $\mathrm{\Lambda }$, $\mathrm{\Sigma }$, ${\mathrm{\Sigma }}^{\ast }(1385)$ exchanges, and the generalized contact term are considered in constructing the reaction amplitude. It is found that by including the $N(2060){5/2}^{-}$ resonance, which is responsible for the shape of the angular distribution of $\gamma p\to K^{\ast +}\mathrm{\Lambda }$ near the $K^{\ast }\mathrm{\Lambda }$ threshold, and one of the $N(2000){5/2}^{+}$, $N(2040){3/2}^{+}$, $N(2100){1/2}^{+}$, $N(2120){3/2}^{-}$ and $N(2190){7/2}^{-}$ resonances, one can describe the cross-section data for this reaction reasonably well. More experimental data on spin observables are needed to further pin down the resonance contents and associated resonance parameters in this reaction.

The interaction mechanism of K^*Λ photoproduction is re-analyzed within a Regge-inspired effective Lagrangian approach. The result shows that the differential cross-section data for γp → K^*+Λ can be satisfactorily described by introducing the only N(2060)5/2⁻ resonance in the s channel, which is quite different from our earlier work performed in an effective Lagrangian approach [A. C. Wang et al., Phys. Rev. C 96, 035206 (2017)], where the amplitudes are computed by evaluating Feynman diagrams and it is found that apart from the N(2060)5/2⁻ resonance, at least one additional resonance is indispensable for reproducing the data. Although the individual terms of the background contributions on the angular distributions are found to be highly model dependent, the extracted mass of N(2060)5/2⁻ from different models tends to be coincident, independent on how the t-channel amplitudes are constructed, whereas the width does not.

Lattice results on the N to Δ electromagnetic, axial-vector and pseudoscalar form factors are evaluated using dynamical staggered sea quarks and domain wall valence quarks for pion masses in the range of 580-350 MeV, as well as, dynamical and quenched Wilson fermions for similar pion masses.

The differential and total cross sections were measured for the γp → ηp and γd → ηpn reactions at E_γ ≤ 1150 MeV by using an electro-magnetic (EM) calorimeter SCISSORS II at the Laboratory of Nuclear Science (LNS), Tohoku University. The total cross section on the deuteron shows a bump around E_γ = 1 GeV, while no bump is observed in the same energy region of that on the proton. This bump is attributed to be a nucleon resonance excited from the neutron, and it is a candidate of anti-decuplet penta-quark baryons with hidden strangeness.

It was difficult, however, to detect all the γ's coming from η decay since the solid angle of SCISSORS II was only 12.6% in total. Statistics of the detected η produced events is poor and systematic uncertainty of the obtained cross section is not small due to low acceptance. A new EM calorimeter complex called FOREST with a solid angle of about 4π sr has been constructed. The spin and parity of the relevant resonance are expected to be determined by the experiments with FOREST.

Measurements of transition form factors for the electro-excitation of nucleon resonances in the Q₂ = 5 - 14 Gev² region can provide one with the information on quark wave functions at small transverse separations. In particular a comparison of form factors for the states of opposite parity can give insight in the mechanisms of chiral symmetry breaking. I discuss perspectives of the theoretical description of such reactions using a combination of lattice calculations and light-cone sum rules.

A constituent quark model based on the spectator formalism is applied to the γN → N* transition for the three cases, where N* is the nucleon, the Δ and the Roper resonance. The model is covariant, and therefore can be used for the predictions at higher four-momentum transfer squared, Q². The baryons are described as an off-mass-shell quark and a spectator on-mass-shell diquark systems. The quark electromagnetic current is described by quark form factors, which have a form inspired by the vector meson dominance. The valence quark contributions of the model are calibrated by lattice QCD simulations and experimental data. Contributions of the meson cloud to the inelastic processes are explicitly included.