Narrow Results

2π-photoproduction is one of the promising reactions to search for new baryon resonances and to investigate the properties of resonances in general. Recent data taken by various experiments show interesting resonance structures. First preliminary partial wave analysis results of the γp→pπ⁰π⁰-CB-ELSA data indicate the observation of baryon resonances not only decaying via Δπ but also via higher mass states like e.g. D₁₃(1520)π. Recently, data on polarisation observables became available, like e.g. the double polarisation data from MAMI, or the single polarisation data from GRAAL and CLAS.

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.

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.

The spectroscopy of light and strange baryons had been an important aspect with still unknown resonances and intrinsic baryonic properties. This paper focuses on the $\mathrm{\Delta }$ baryons treating $u$ and $d$ quarks with different constituent masses. The theoretical framework for calculating the resonance masses is hypercentral Constituent Quark Model (hCQM) with confining potential of the linear form. To account for the spin–orbit splitting in the correct order, first- and second-order corrections of the order $1∕m$ and $1∕m^2$ have been incorporated to the potential and spin-dependent term, respectively. The results obtained have been compared with many varied approaches as well as experimental available states. In addition to mass spectra, Regge trajectories for (n, $M^2$) and ($J$, $M^2$) have been plotted. The magnetic moment as well as transition magnetic moment have been calculated using effective quark mass. The radiative decay width has been studied for ${\mathrm{\Delta }}^{+}$ and ${\mathrm{\Delta }}^0$ channels.

The hypercentral Constituent Quark Model (hCQM) has been employed to obtain the resonance masses of hadrons ranging from light, strange to heavy sector. This work is dedicated towards incorporating the higher order correction in the mass so as to improve the spin-order hierarchy. The potential is Coulomb-like part and linear term as confinement part along with the spin–spin interaction terms. In this paper, the light, strange baryon masses have been revisited with correction term with $\frac{1}{m}$ and $\frac{1}{m^2}$ and compared with earlier results as well as various other approaches. The slope and intercepts have been recalculated for obtained masses which are expected to aid in experimental findings. The recent results are found to be in good accordance with the experimental range as well as theoretical models.

New measurements of transverse target and beam-target asymmetries in pion photo-production were performed using the Crystal Ball at the Mainz Microtron MAMI. Preliminary results are discussed in the context of model-independent partial wave analyses for pseudo-scalar meson photo-production below E_cm = 2 GeV.

General properties of the S-matrix, such as constraints from two- and three-body unitarity as well as gauge invariance, are discussed and illustrated for the example of a dynamical coupled channel approach. The Jülich model has been updated to analyze πN, ηN, and KY production as well as pion photoproduction. Partial wave amplitudes and resonance properties are determined.

The coupled-channel unitary Lagrangian (Giessen) model is extended to study two pion production within the isobar approximation. The effect from the t-channel pion exchange to the πN → σN reaction is found to be small which is in line with the previous conclusions by the Crystal Ball collaboration. The calculations with the genuine Roper resonance are found to be in nice agreement with both the Crystal Ball measurements of the 2πN production and the GWU single energy solutions in the elastic πN channel.

The reactions πN → πN, ηN, KΛ and KΣ are described in a combined analysis, based on the Jülich dynamical coupled-channel framework. Unitarity, analyticity and other general requirements of the S-matrix are preserved in this Lagrangian-based, field-theoretical approach. The reliable extraction of resonance parameters is possible in terms of poles and residues.

The new method for a combined analysis of meson production data is presented. This method fully satisfies the unitarity and analyticity conditions and is suitable for an analysis of modern high statistic data. The results obtained in the framework of this approach are compared with the results of the K-matrix analysis. Including in the analysis of the meson production reaction the data from the γ-neutron collision allows us to extract the information about the helicity couplings and of nucleon states.

A large part of the experimental program in Hall B of the Jefferson Lab is dedicated to baryon spectroscopy. Meson photoproduction experiments are essential part of this program. CEBAF Large Acceptance Spectrometer (CLAS) and availability of circularly and linearly polarized tagged photon beams in combination with longitudinally and transversely polarized frozen spin targets provide unique conditions for this type of experiments. For the first time, a complete or nearly complete measurement became possible and will allow model independent extraction of the reaction amplitude. The measurements were complete with both proton and deuteron targets. An overview of the collected experimental data will be presented.

Double polarization experiments using a longitudinally or transversely polarized frozen-spin-butanol target and a linearly or circularly polarized photon beam were performed with the CBELSA/TAPS experiment at the electron accelerator ELSA. With its nearly 4π angular coverage, this setup is very well suited to study neutral meson photoproduction off the nucleon up to beam energies of 3.2 GeV. Results obtained for the double polarization observable E in neutral pion and eta photoproduction show the large sensitivity of the data on the contributing resonances. If the data are compared to the predictions of state of the art partial wave analyses, large discrepancies are observed.

We discuss our relativistic quark-diquark model, developed in the framework of point form dynamics. Our results for the nonstrange baryon spectrum and for the nucleon electromagnetic form factors are discussed.

We present a selection of recent results on light hadron spectroscopy from the BESIII collaboration. The topics include the full partial wave analysis on J/ψ radiative decays, including J/ψ → γηη and γωϕ. Besides, we also cover the topics about baryon spectroscopy, including the partial wave analysis of and .

Baryon spectroscopy and structure studies are the key to a deeper understanding of the processes which happened during the first few microseconds of the universe. The understanding of the fundamental degrees of freedom inside a proton or a neutron and how they change with varying quark masses can help us to obtain a deeper understanding on the origin of confinement and its connection to chiral symmetry breaking. While photoproduction experiments are have established as a very effective tool to search and identify new resonances, a hard scale, provided by electroproduction is needed to probe the internal structure of the N* states and to reveal their true nature. The paper will give an overview on the spectroscopy of light baryon states, with a special focus on recent progress in hadron structure studies with CLAS at JLAB. The CLAS spectrometer offers a unique dataset to determine the electrocouplings of N* and Δ resonances using both single and double pion electroproduction. As an outlook, an overview of the planed baryon resonance studies with CLAS12 at the recently upgraded 12 GeV CEBAF electron accelerator will be presented.