Baryon Resonances Research Articles

Dynamically generated resonances from the vector octet-baryon decuplet interaction and their radiative decays into γ-baryon decuplet

The dynamically generated resonances from vector meson-baryon decuplet are studied using Lagrangians of the hidden gauge theory for vector interactions. One shows that some of the generated states can be associated with some known baryon resonances in the PDG data, while others are predictions for new states. Furthermore, we calculate the radiative decay widths of these resonances into a photon and a baryon decuplet.

Bonn–Gatchina partial wave analysis

The present status of the Bonn–Gatchina partial wave analysis of the photoproduction and pion induced data is presented. An observation of signals which can be associated with new baryon resonances and possible interpretations of the baryon spectrum is discussed.

Photoproduction of multi-meson final states — recent results from the Crystal-Barrel/TAPS experiment at ELSA-

Multi-meson final states such as γp → pπ0π0 and γp → p π0η have been investigated with the Crystal-Barrel/TAPS experiment at ELSA. Baryon cascades via Δπ and Δη and also via higher mass baryon resonances are clearly observed. Based on this data and on data from other reactions a partial wave analysis has been performed from which partial decay widths of various N* and Δ* states into the different pπ0π0 and pπ0η decay channels have been determined, leading partly to unexpected results. In addition, polarisation observables have been investigated. The beam asymmetry ∑ has been determined for both reactions and double polarisation data using a longitudinally polarised target and a circularly or linearly polarised photon beam has been taken. Given the angular coverage of the Crystal-Barrel/TAPS experiment this data covers almost the full angular range and phase space. This data will provide new and important information for the partial wave analyses and bring us one step closer towards a complete experiment.

A new N* resonance as a hadronic molecular state

We report our recent work on a hadronic molecule state of the KK̄N system with I = 1/2 and JP = 1/2+. We assume that the Λ(1405) resonance and the scalar mesons, f0 (980), a0 (980), are reproduced as quasi-bound states of K̄N and KK̄, respectively. Performing non-relativistic three-body calculations with a variational method for this system, we find a quasibound state of the KK̄N system around 1910 MeV below the three-body breakup threshold. This state corresponds to a new baryon resonance of N* with JP = 1/2+. We find also that this resonance has the cluster structure of the two-body bound states keeping their properties as in the isolated two-particle systems. We also briefly discuss another hadronic molecular state composed by two K̄ and one N, which corresponds to a Ξ* resonance.

Strategies for baryon resonance analysis

The analytic properties of scattering amplitudes provide a meeting point for experimental and theoretical investigations of baryon resonances. Pole positions and residues allow for a parameterization of resonances in a well-defined way which relates different reactions. The recent progress made within the Jülich model is summarized.

Charmed baryon resonances with heavy-quark symmetry

We study charmed baryon resonances that are generated dynamically from a coupled-channel unitary approach that implements heavy-quark symmetry. Some states can already be identified with experimental observations, such as Λc(2595), Λc(2660), σc(2902) or Λc(2941), while others need a compilation of more experimental data as well as an extension of the model to include higher order contributions. We also compare our model to previous SU(4) schemes.

Forward pion-nucleon charge exchange reaction and Regge constraints

We present our recent study of pion-nucleon charge exchange amplitudes above 2 GeV. We analyze the forward pion-nucleon charge exchange reaction data in a Regge model and compare the resulting amplitudes with those from the Karlsruhe–Helsinki and George-Washington-University partial-wave analyses. We explore possible high-energy constraints for theoretical baryon resonance analyses in the energy region above 2 GeV. Our results show that for the pion-nucleon charge exchange reaction, the appropriate energy region for matching meson-nucleon dynamics to diffractive scattering should be around 3 GeV for the helicity flip amplitude.

Baryon resonance analysis from MAID

The unitary isobar model MAID2007 has been used to analyze the recent data of pion electroproduction. The model contains all four-star resonances in the region below W = 2 GeV and both single-Q2 and Q2 dependent transition form factors could be obtained for the Delta, Roper, D13(1520), S11(1535), S31(1620), S11(1650), D15(1675), F15(1680) and P13(1720). From the complete world data base, including also π− data on the neutron, also Q2 dependent neutron form factors are obtained. For all transition form factors we also give convenient numerical parameterizations that can be used in other reactions. Furthermore, we show how the transition form factors can be used to obtain empirical transverse charge densities and our first results are given for the Roper, the S11 and D13 resonances.

Reaction dynamics for photoproductions of baryon resonances

The study of exotic structure of hadrons is fully achieved when reaction dynamics of the associated hadron productions is well understood. We employ as the standard mechanism the effective Lagrangian method and investigate several physical observables within the framework. The parameters are constrained by microscopic description of hadrons. We discuss photoproductions of kaon associated with the ground state Λ(1116) and its resonances Λ(1405) and Λ(1520). In the former example we emphasize the meson cloud effect which significantly renormalizes the phenomenological parameters, while in the latter we discuss the features of the standard method. Finally we discuss briefly the production of ηπ associated with the nucleon resonance N(1535) for the study of chiral symmetry of baryons.

A proposed U.S./China theoretical/experimental collaborative effort on baryon resonance extraction

In this paper we discuss the reasons for our work towards establishing a new collaboration between Jefferson Lab (JLab) and the Institute of High Energy Physics (IHEP) in Beijing. We seek to combine experimentalists and theorists into a dedicated group focused on better understanding the current and future data from JLab and from the Beijing Electron Positron Collider (BEPC). Recent JLab results on the extraction of single- and double-polarization observables in both the 1π- and 2π-channel show their high sensitivity to small production amplitudes and therefore their importance for the extraction of resonance parameters. The Beijing Electron Spectrometer (BES) at the BEPC has collected high statistics data on J/Ψ production. Its decay into baryon-antibaryon channels offers a unique and complementary way of probing nucleon resonances. The CEBAF Large Acceptance Spectrometer, CLAS, has access to N* form factors at high Q2, which is advantageous for the study of dynamical properties of nucleon resonances, while the low-background BES results will be able to provide guidance for the search for less-dominant excited states at JLab. Moreover, with the recently approved experimental proposal Nucleon Resonance Studies with CLAS12 and the high-quality data streaming from BES-III and CLAS, the time has come for forging a new Trans-Pacific collaboration of theorists and experimentalists on NSTAR physics.

Dynamically generated open-charm baryons beyond the zero-range approximation

The interaction of the low-lying pseudoscalar mesons with the ground-state baryons in the charm sector is studied within a coupled-channel approach using a $t$-channel vector-exchange driving force. The amplitudes describing the scattering of the pseudoscalar mesons off the ground-state baryons are obtained by solving the Lippmann-Schwinger equation. We analyze in detail the effects of going beyond the $t=0$ approximation. Our model predicts the dynamical generation of several open-charm baryon resonances in different isospin and strangeness channels, some of which can be clearly identified with recently observed states.

Pion-nucleon scattering in Kadyshevsky formalism. II. Baryon exchange sector

In this article, which is the second part in a series of two, we construct tree-level baryon exchange and resonance amplitudes for $\ensuremath{\pi}N/\mathit{MB}$ scattering in the framework of the Kadyshevsky formalism. We use this formalism to formally implement absolute pair suppression, where we make use of the method of Takahashi and Umezawa. The resulting amplitudes are Lorentz invariant and causal. We continue studying the frame dependence of the Kadyshevsky integral equation using the method of Gross and Jackiw. The invariant amplitudes, including those for meson exchange, are linked to the phase shifts using the partial-wave basis.

The phase and pole structure of the N*(1535) in $ \pi$ N $ \rightarrow$ $ \pi$ N and $ \gamma$ N $ \rightarrow$ $ \pi$ N

The nature of some baryonic resonances is still an unresolved issue. The case of the N*(1535) is particularly interesting in this respect due to the nearby etaN threshold and interference with the N*(1650). The N*(1535) has been described as a threshold effect, as a genuine 3-quark resonance, or as dynamically generated from the interaction of the octet of baryons with the octet of mesons. In the scheme of dynamical generation, predictions for the interaction of the N*(1535) with the photon can be made. In this study, we simultaneously analyze the role of the N*(1535) in the piN-->piN and gammaN-->piN reactions and compare to the respective amplitudes from partial wave analyses. This test is very sensitive to the meson-baryon components of the N*(1535).

Baryon resonances in the mean field approach and a simple explanation of the Θ+ pentaquark

We suggest to classify baryon resonances as single-quark states in a mean field, and/or as its collective excitations. Identifying the Roper resonance N(1440), the nucleon resonance N(1535), and the singlet hyperon Lambda(1405) as single-quark excitations, we find that there must be an exotic S=+1 baryon resonance Theta+ (the "pentaquark") with a mass about 1440+1535-1405=1570 MeV and spin-parity one-half-plus. We argue that Theta+ is an analog of the Gamov--Teller excitation long known in nuclear physics.

FORMATION OF η-MESIC NUCLEI BY (π, N) REACTION AND CHIRAL SYMMETRY FOR BARYONS

We calculate formation spectra of η-nucleus systems in (π, N) reactions with nuclear targets, which can be performed at existing and/or forthcoming facilities, including J-PARC, in order to investigate η-nucleus interactions. Based on the N*(1535) dominance in the ηN system, η-mesic nuclei are suitable systems for study of in-medium properties of the N*(1535) baryon resonance, such as reduction of the mass difference of N and N* in nuclear medium, which affects level structure of the η and N*-hole modes. We find that clear information on the in-medium N*- and η-nucleus interactions can be obtained through the formation spectra of the η-mesic nuclei.

Differential cross sections for the reactionsγp→pηandγp→pη'

High-statistics differential cross sections for the reactions gamma p -> p eta and gamma p -> p eta-prime have been measured using the CLAS at Jefferson Lab for center-of-mass energies from near threshold up to 2.84 GeV. The eta-prime results are the most precise to date and provide the largest energy and angular coverage. The eta measurements extend the energy range of the world's large-angle results by approximately 300 MeV. These new data, in particular the eta-prime measurements, are likely to help constrain the analyses being performed to search for new baryon resonance states.

Formation ofη-mesic nuclei by the (π,N) reaction and properties ofN*(1535)in medium

We calculate formation spectra of the $\ensuremath{\eta}$-nucleus systems in the ($\ensuremath{\pi},N$) reactions with nuclear targets, which can be performed at existing and/or forthcoming facilities, including the Japan Proton Accelerator Research Complex, to investigate the $\ensuremath{\eta}$-nucleus interaction. Based on the ${N}^{*}(1535)$ dominance in the $\ensuremath{\eta}N$ system, the $\ensuremath{\eta}$-mesic nuclei are suitable systems for the study of in-medium properties of the ${N}^{*}(1535)$ baryon resonance, such as reduction of the mass difference of $N$ and ${N}^{*}$ in the nuclear medium, which affects the level structure of the $\ensuremath{\eta}$ and ${N}^{*}$-hole modes. We find that clear information on the in-medium ${N}^{*}$- and $\ensuremath{\eta}$-nucleus interactions can be obtained through the formation spectra of the $\ensuremath{\eta}$-mesic nuclei. We also discuss the experimental feasibilities by showing several spectra of the ($\ensuremath{\pi},N$) reactions calculated with possible experimental settings. Coincident measurements of the $N\ensuremath{\pi}$ pairs from the ${N}^{*}$ decays in nuclei help us to reduce backgrounds.

Baryon resonances as hadronic molecule states with kaons

We investigate hadronic molecule states of \(K \bar K N\) and \(\bar K \bar K N\) systems with I = 1/2 and J P = 1/2 + , assuming that Λ(1405) and the scalar mesons, f 0(980), a 0(980), are reproduced as quasi-bound states of \(\bar KN\) and \(K \bar K\). Performing non-relativistic three-body calculations for these systems, we find weakly bound states for \(K \bar K N\) and \(\bar K \bar K N\) around 1900 MeV, which correspond to new baryon resonances of N * and Ξ * with J P = 1/2 + . We find that these resonances have cluster structure of the two-body bound state keeping its properties as in the isolated two-particle system.

Meson and Baryon resonances

In this talk I review recent advances on the structure of the meson and baryon resonances which can be dynamically generated from the interaction of mesons or mesons and baryons. Particular emphasis is put on results involving vector mesons, which bring new light into the nature of some of the observed higher mass mesons and baryons and make predictions for new states.

Five-quark Components in the Proton and the Lowest 1/2 − Baryon Resonances

From recent studies of strangeness in the proton and properties of the lowest spin-parity 1/2 − baryons, N*(1535) and Δ ∗ + + ( 1620 ) , new pictures for the internal structure of the five-quark components in the proton and the lowest 1/2 − baryon resonances are emerging. While the proton and the lowest 1/2 − baryon nonet may have large diquark-diquark-antiquark components, the lowest 1/2 − baryon decuplet is proposed to have vector-meson-baryon components.