Baryon Resonances Research Articles

Odd parity bottom-flavored baryon resonances

The LHCb Collaboration has recently observed two narrow baryon resonances with beauty. Their masses and decay modes look consistent with the quark model orbitally excited states ${\ensuremath{\Lambda}}_{b}(5912)$ and ${\ensuremath{\Lambda}}_{b}^{*}(5920)$, with quantum numbers ${J}^{P}=1/{2}^{\ensuremath{-}}$ and $3/{2}^{\ensuremath{-}}$, respectively. We predict the existence of these states within a unitarized meson-baryon coupled-channel dynamical model, which implements heavy-quark spin symmetry. Masses, quantum numbers and couplings of these resonances to the different meson-baryon channels are obtained. We find that the resonances ${\ensuremath{\Lambda}}_{b}^{0}(5912)$ and ${\ensuremath{\Lambda}}_{b}^{0}(5920)$ are heavy-quark spin symmetry partners, which naturally explains their approximate mass degeneracy. Corresponding bottom-strange baryon resonances are predicted at ${\ensuremath{\Xi}}_{b}(6035.4)$ (${J}^{P}=\frac{1}{2}\genfrac{}{}{0}{}{{}^{\ensuremath{-}}}{}$) and ${\ensuremath{\Xi}}_{b}(6043.3)$ (${J}^{P}=\frac{3}{2}\genfrac{}{}{0}{}{{}^{\ensuremath{-}}}{}$). The two ${\ensuremath{\Lambda}}_{b}$ and two ${\ensuremath{\Xi}}_{b}$ resonances complete a multiplet of the combined symmetry SU(3)-flavor times heavy-quark spin.

Separated structure functions for exclusiveK+ΛandK+Σ0electroproduction at 5.5 GeV measured with CLAS

We report measurements of the exclusive electroproduction of $K^+\Lambda$ and $K^+\Sigma^0$ final states from an unpolarized proton target using the CLAS detector at the Thomas Jefferson National Accelerator Facility. The separated structure functions $\sigma_U$, $\sigma_{LT}$, $\sigma_{TT}$, and $\sigma_{LT'}$ were extracted from the $\Phi$-dependent differential cross sections acquired with a longitudinally polarized 5.499 GeV electron beam. The data span a broad range of momentum transfers $Q^2$ from 1.4 to 3.9 GeV$^2$, invariant energy $W$ from threshold to 2.6 GeV, and nearly the full center-of-mass angular range of the kaon. The separated structure functions provide an unprecedented data sample, which in conjunction with other meson photo- and electroproduction data, will help to constrain the higher-level analyses being performed to search for missing baryon resonances.

Heavy-quark spin symmetry for charmed and strange baryon resonances

We study charmed and strange baryon resonances that are generated dynamically by a unitary baryon-meson coupled-channels model which incorporates heavy-quark spin symmetry. This is accomplished by extending the SU(3) Weinberg-Tomozawa chiral Lagrangian to SU(8) spin-flavor symmetry plus a suitable symmetry breaking. The model generates resonances with negative parity from the s-wave interaction of pseudoscalar and vector mesons with 1/2+ and 3/2+ baryons in all the isospin, spin, and strange sectors with one, two, and three charm units. Some of our results can be identified with experimental data from several facilities, such as the CLEO, Belle, or BaBar Collaborations, as well as with other theoretical models, whereas others do not have a straightforward identification and require the compilation of more data and also a refinement of the model.

Updated study of S-wave K¯N scattering and related spectroscopy at NLO UChPT

Abstract The meson–baryon scattering with strangeness −1 is studied within the coupled-channel unitary chiral perturbation theory. A large amount of experimental data are analyzed. This strongly constrains the free parameters in our calculation and also the resulting meson–baryon scattering amplitudes. Several different strategies to perform the fits to data are discussed, which allows one to make the important check on the stability of the results obtained. Our final threshold parameters for K ¯ N scattering and baryon resonance properties are stable and confident.

Odd-parity Dynamically Generated Baryon Resonances with Beauty Flavor

We study baryon resonances with heavy flavor in a molecular approach, thus as dynamically generated by baryon-meson scattering. This is accomplished by using a unitary coupled-channel model taking, as bare interaction, the extension of the Weinberg-Tomozawa SU(3) Lagrangian. A special attention is payed to the inclusion of heavy-quark spin symmetry and to the study of the generated baryon resonances that complete the heavy-quark spin multiplets. Our model reproduces the \Lambda_b(5912) and \Lambda_b(5920) baryons, which were recently observed by the LHCb collaboration. According to our analysis, these two states are heavy-quark spin symmetric partners. We also make predictions for few \Xi_b baryon resonances, which belong to the same SU(3)xHQSS multiplets as the \Lambda_b particles.

Magnetic moments of the low-lying $$\tfrac{1} {2}^ -$$ octet baryon resonances

The magnetic moments of the negative parity octet resonances with spin {1/2}: $N^*$(1535), $N^*$(1650), $\Sigma^*$(1620), and $\Xi^*$(1690) have been calculated within the framework of the chiral constituent quark model. In this approach, the presence of the polarized $q\bar{q}$ pairs (or the meson cloud, in other words) is considered by using the Lagrangian for Goldstone boson emission from the constituent quarks. Further, the explicit contributions coming from the spin and orbital angular momentum, including the effects of the configurations mixing between the states with different spins, are obtained. The motivation for these calculations comes from the recent interest in experimental measurement of the magnetic moment of the ${S_{11}(1535)}$ resonance and of similar calculations being done within lattice quantum chromodynamics approaches. Our results can be compared with those expected to come from these sources.

Effects of a spin-flavour-dependent interaction on light-flavoured baryon helicity amplitudes

This paper is a continuation of a previous work about the effects of a phenomenological flavour-dependent force in a relativistically covariant constituent quark model based on the Salpeter equation on the structure of light-flavoured baryon resonances. Here the longitudinal and transverse helicity amplitudes as studied experimentally in the electro-excitation of nucleon- and Δ-resonances are calculated. In particular the amplitudes for the excitation of three- and four-star resonances as calculated in a previous model A are compared to those of the novel model C as well as to existing and partially new experimental data such as, e.g., determined by the CB-ELSA Collaboration. A brief discussion on some improvements to model C is given after the introduction.

Beam polarization asymmetry and the electromagnetic production of kaons from protons

The beam polarization asymmetry in the reaction $ep\ensuremath{\rightarrow}{e}^{\ensuremath{'}}{K}^{+}\ensuremath{\Lambda}$ has been investigated in a tree-level effective Lagrangian model. The model incorporates most of the well-established baryon resonances with spins up to $\frac{5}{2}$, four less well-established nucleon resonances with larger mass, and the two kaon resonances ${K}^{\ensuremath{\star}}(892)$ and $K1(1270)$. The off-shell structure of the electromagnetic vertices was accounted for by the inclusion of electromagnetic form factors at those vertices. The free parameters of the model were fitted in a previous study to a large pool of photoproduction data from the CLAS, GRAAL, SAPHIR, and LEPS collaborations and to CLAS data for the virtual photoproduction structure functions ${\ensuremath{\sigma}}_{U}$, ${\ensuremath{\sigma}}_{T}$, ${\ensuremath{\sigma}}_{L}$, ${\ensuremath{\sigma}}_{TT}$, and ${\ensuremath{\sigma}}_{LT}$. Using this model, results were obtained for the beam polarization asymmetry structure function ${\ensuremath{\sigma}}_{L{T}^{\ensuremath{'}}}$ and compared with CLAS data. Two new fits to the combined photoproduction and electroproduction data with the ${\ensuremath{\sigma}}_{L{T}^{\ensuremath{'}}}$ data included were then generated. The first of these includes contributions from all of the resonances included in the previous study; the second excludes contributions from the $N(2080)$ and $N(2200)$ resonances. The results of both fits for both photoproduction and electroproduction observables are compared with the results of the previous fit and the data.

Dilepton production in pion-nucleon collisions in an effective field theory approach

We present a model of electron-positron pair production in pion-nucleon collisions in the exclusive reaction \pi N -> Ne^+e^-. The model is based on an effective field theory approach, incorporating 16 baryon resonances below 2 GeV. Parameters of the model are fitted to pion photoproduction data. We present the resulting dilepton invariant mass spectra for \pi^- p collisions up to sqrt(s)=1.9 GeV center-of-mass collision energy. These results are meant to give predictions for the planned experiments at the HADES spectrometer in GSI, Darmstadt.

Production of negative parity baryons in the holographic Sakai-Sugimoto model

We extend our investigation of resonance production in the Sakai-Sugimoto model to the case of negative parity baryon resonances. Using holographic techniques we extract the generalized Dirac and Pauli baryon form factors as well as the helicity amplitudes for these baryonic states. Identifying the first negative parity resonance with the experimentally observed S_{11}(1535), we find reasonable agreement with experimental data from the JLab-CLAS collaboration. We also estimate the contribution of negative parity baryons to the proton structure functions.

Systematic study of longitudinal and transverse helicity amplitudes in the hypercentral constituent quark model

We report in a systematic way the predictions of the non-relativistic hypercentral Constituent Quark model for the electromagnetic excitations of baryon resonances. The longitudinal and transverse helicity amplitudes are calculated with no free parameters for fourteen resonances, both for proton and neutron. The calculations lead to an overall fair description of data, specially in the medium $Q^2$ range, where quark degrees of freedom are expected to dominate.

Scattering phases for meson and baryon resonances on general moving-frame lattices

A proposal by L\"uscher enables one to compute the scattering phases of elastic two-body systems from the energy levels of the lattice Hamiltonian in a finite volume. In this work we generalize the formalism to S--, P-- and D--wave meson and baryon resonances, and general total momenta. Employing nonvanishing momenta has several advantages, among them making a wider range of energy levels accessible on a single lattice volume and shifting the level crossing to smaller values of $m_\pi L$.

Erratum to: Dilepton production in proton-induced reactions at SIS energies with the GiBUU transport model

We present dilepton spectra from p + p , d + p and p + Nb reactions at SIS energies, which were simulated with the GiBUU transport model in a resonance model approach. These spectra are compared to the data published by the HADES and DLS Collaborations. It is shown that the ρ spectral function includes non-trivial effects already in elementary reactions, due to production via baryon resonances, which can yield large contributions to the dilepton spectrum. Dilepton spectra from nuclear reactions in the energy range of the HADES experiment are thus found to be sensitive also to properties of nucleon resonances in the nuclear medium.

Recent progress in N* physics from Kaon photoproduction experiments at CLAS

The N* programme at CLAS in Jefferson Lab is dedicated to the study of the spectrum of baryon resonances and the search for missing resonances. Recent developments in polarized beams and targets at CLAS have made it possible to measure many of the single and double polarization observables which are necessary to disentangle the contributing processes. In particular, CLAS is well on the way to making the first complete measurement on pseudoscalar meson production. The current status of the N* program is presented together with preliminary results from recently completed Kaon Photoproduction experiments.

Well-Established Nucleon Resonances Revisited by Double-Polarization Measurements

The first measurement is reported of the double-polarization observable G in the photoproduction of neutral pions off protons, covering the photon energy range from 620 to 1120 MeV and the full solid angle. G describes the correlation between the photon polarization plane and the scattering plane for protons polarized along the direction of the incoming photon. The observable is highly sensitive to contributions from baryon resonances. The new results are compared to the predictions from SAID, MAID, and BnGa partial wave analyses. In spite of the long-lasting efforts to understand γp→pπ(0) as the simplest photoproduction reaction, surprisingly large differences between the new data and the latest predictions are observed which are traced to different contributions of the N(1535) resonance with spin parity J(P)=1/2(-) and N(1520) with J(P)=3/2(-). In the third resonance region, where N(1680) with J(P)=5/2(+) production dominates, the new data are reasonably close to the predictions.

Generalized baryon form factors and proton structure functions in the Sakai–Sugimoto model

We investigate the production of positive parity baryon resonances in proton electromagnetic scattering within the Sakai–Sugimoto model. The latter is a string model for the non-perturbative regime of large Nc QCD. Using holographic techniques we calculate the generalized Dirac and Pauli form factors that describe resonance production. We use these results to estimate the contribution of resonance production to the proton structure functions. Interestingly, we find an approximate Callan–Gross relation for the structure functions in a regime of intermediate values of the Bjorken variable.

First measurement of proton-induced low-momentum dielectron radiation off cold nuclear matter

We present data on dielectron emission in proton induced reactions on a Nb target at 3.5 GeV kinetic beam energy measured with HADES installed at GSI. The data represent the first high statistics measurement of proton-induced dielectron radiation from cold nuclear matter in a kinematic regime, where strong medium effects are expected. Combined with the good mass resolution of 2%, it is the first measurement sensitive to changes of the spectral functions of vector mesons, as predicted by models for hadrons at rest or small relative momenta. Comparing the e+e− invariant mass spectra to elementary p+p data, we observe for e+e− momenta Pee<0.8 GeV/c a strong modification of the shape of the spectrum, which we attribute to an additional ρ-like contribution and a decrease of ω yield. These opposite trends are tentatively interpreted as a strong coupling of the ρ meson to baryonic resonances and an absorption of the ω meson, which are two aspects of in-medium modification of vector mesons.

Recent Results for the Unquenched Quark Model

We discuss a new generation of unquenched quark models for baryons is presented in which the effects of quark-antiquark pairs are taken into account in an explicit form by means of a 3 P0 quark-antiquark creation mechanism. This provides the possibility to address many open problems in baryons structure and spectroscopy. The applications to magnetic moments and to the spin and flavor content of baryons will be reviewed here. 1 Open Problems in Baryons Physics There is compelling evidence for the existence of exotic degrees of freedom, i.e. other than valence quarks, in baryons, in particular for the inclusion of the effects of quark-antiquark pairs (virtual intermediate meson- baryon states). The importance of these quark-antiquark configurations (or higher Fock components in baryon wave functions) is evident from measurements of the ¯ d/ ¯ u asymmetry in the nucleon sea (1), parity-violating electron scattering experiments (2,3), the proton spin crisis (4-6), as well as from analysis of helicity ampli- tudes (7,8) and strong couplings of baryon resonances (9-11). The coupling with the continuum and higher Fock configurations can also be the key solution of the missing resonances problem, which are resonances predicted by all quark models based on three constituent quarks, such as the (non)relativistic quark model, bag models and chiral soliton models, but which have never been observed (12). This problem is common to all quark models even below 2 GeV, as well as for most quark-diquark models (see (13) and references therein for a review on quark-diquark models), even though in this case the number of missing states is much smaller, since they are based on a reduced number of effective degrees of freedom. The aim of this article is to present a short overview of constituent quark models (CQM) and the need for unquenching of the quark model. The recently introduced unquenched quark model for baryons (14,15) will be reviewed and briefly and some recent results for the magnetic moments, and the spin and flavor contents of octet baryons will be discussed.

Charmed and strange baryon resonances with heavy-quark spin symmetry

We study charmed and strange baryon resonances that are generated dynamically by a unitary baryon-meson coupled-channel model which incorporates heavy-quark spin symmetry. This is accomplished by extending the SU(3) Weinberg-Tomozawa chiral Lagrangian to SU(8) spin-flavor symmetry plus a suitable symmetry breaking. The model produces resonances with negative parity from s-wave interaction of pseudoscalar and vector mesons with $1/2^+$ and $3/2^+$ baryons. Resonances in all the isospin, spin, and strange sectors with one, two, and three charm units are studied. Our results are compared with experimental data from several facilities, such as the CLEO, Belle or BaBar Collaborations, as well as with other theoretical models. Some of our dynamically generated states can be readily assigned to resonances found experimentally, while others do not have a straightforward identification and require the compilation of more data and also a refinement of the model. In particular, we identify the $\Xi_c(2790)$ and $\Xi_c(2815)$ resonances as possible candidates for a heavy-quark spin symmetry doublet.

Study of exclusive one-pion and one-eta production using hadron and dielectron channels in pp reactions at kinetic beam energies of 1.25 GeV and 2.2 GeV with HADES

We present measurements of exclusive \pi^{+,0} and \eta\ production in pp reactions at 1.25 GeV and 2.2 GeV beam kinetic energy in hadron and dielectron channels. In the case of \pi^+ and \pi^0, high-statistics invariant-mass and angular distributions are obtained within the HADES acceptance as well as acceptance corrected distributions, which are compared to a resonance model. The sensitivity of the data to the yield and production angular distribution of \Delta(1232) and higher lying baryon resonances is shown, and an improved parameterization is proposed. The extracted cross sections are of special interest in the case of pp \to pp \eta, since controversial data exist at 2.0 GeV; we find \sigma =0.142 \pm 0.022 mb. Using the dielectron channels, the \pi^0 and \eta\ Dalitz decay signals are reconstructed with yields fully consistent with the hadronic channels. The electron invariant masses and acceptance corrected helicity angle distributions are found in good agreement with model predictions.