Hyperon Resonances Research Articles

Hyperon And Hyperon Resonance Properties From Charm Baryon Decays At BABAR

We present studies of hyperon and hyperon resonance production in charm baryon decays at BABAR. Using two-body decays of the Ξ0c and Ω0c, we are able to show, for the first time, that the spin of the Ω− is 3/2. The analysis procedures are extended to three-body final states and properties of the ξ(1690)0 are extracted from a detailed isobar model analysis of the Λ+c → ΛKSK+ Dalitz plot. Similar techniques are then used to study Ξ(1530)0 production in Λ+c decay.

ΞandΩbaryons in the Skyrme model

The mass spectrum and magnetic moments of \Xi and \Omega baryon resonances are investigated in the bound state approach of the Skyrme model. The empirical hyperon spectrum shows that several hyperon resonances share a pattern of (approximately) equal mass spacings between the states of same spin but of opposite parity. It is found that this pattern can be explained mostly by the energy difference between the P-wave and S-wave kaons bound to the soliton. Although one cannot exclude the possibility that these states can be described as pion-hyperon resonances, the present approach predicts that \Xi(1620) and \Xi(1690) have J^P = 12^-, while \Xi(1950) has J^P = 12^+. The differences with the quark model predictions for the \Xi and \Omega baryon spectrum are pointed out. Several relations for the masses and magnetic moments of those resonances are also obtained.

K¯ photoproduction from protons

We study the contribution of the Drell mechanism driven by K+ and K- exchange to the reaction γN → K¯N. Our calculation implements the full KN and ¯N reaction amplitudes in the form of partial-wave amplitudes taken from a meson-exchange model (KN) and a partial-wave analysis (¯N), respectively. Comparing our results to data of the LAMP2 Collaboration we observe that the Drell mechanism alone cannot describe the large Λ(1520) photoproduction rate observed experimentally. We argue that the discrepancy could be due to significant contributions from K * meson exchange with subsequent excitation of the Λ(1520)-resonance. After adding such contributions to our model a good agreement of the LAMP2 experiment is achieved. When applying the same model to the recent SAPHIR data we find an excellent description of the K+ p spectrum and find indications for a hyperon resonance with M R = 1617MeV and ΓR = 117MeV in the K- p mass distribution.

Photoproduction of Ξ off nucleons

The photoproduction reaction $\ensuremath{\gamma}N\ensuremath{\rightarrow}KK\ensuremath{\Xi}$ is investigated based on a relativistic meson-exchange model of hadronic interactions. The production amplitude is calculated in the tree-level approximation from relevant effective Lagrangians, whose (coupling constant) parameters are mostly fixed from the empirical data and/or quark models together with SU(3) symmetry considerations. Gauge invariance of the resulting amplitude is maintained by introducing the contact currents by extending the gauge-invariant approach of Haberzettl for one-meson photoproduction to two-meson photoproduction. The role of the intermediate low-lying hyperons and of the intermediate higher-mass hyperon resonances are analyzed in detail. In particular, the basic features of the production of ${\ensuremath{\Xi}}^{\ensuremath{-}}(1318)$ in $\ensuremath{\gamma}p\ensuremath{\rightarrow}{K}^{+}{K}^{+}{\ensuremath{\Xi}}^{\ensuremath{-}}$ and their possible manifestations in the forthcoming experimental data are discussed.

Helicity amplitudes and electromagnetic decays of hyperon resonances

We present results for the helicity amplitudes of the lowest-lying hyperon resonances Y*, computed within the framework of the Bonn constituent-quark model, which is based on the Bethe-Salpeter approach. The seven parameters entering the model were fitted to the best known baryon masses. Accordingly, the results for the helicity amplitudes are genuine predictions. Some hyperon resonances are seen to couple more strongly to a virtual photon with finite Q^2 than to a real photon. Other Y*'s, such as the S_{01}(1670) Lambda resonance or the S_{11}(1620) Sigma resonance, couple very strongly to real photons. We present a qualitative argument for predicting the behaviour of the helicity asymmetries of baryon resonances at high Q^2.

Comparative Regge analysis of Λ,ΣΛ(1520) and Θ+ production in γp,πp and pp reactions

Using the Quark-Gluon Strings Model --combined with Regge phenomenology-- we perform a comparative analysis of Λ, Σ0, Λ(1520) and Θ+ production in binary reactions induced by photon, pion and proton beams on the nucleon. We find that the existing experimental data on the γp↦K+Λ differential and total cross-sections can be described very well by the model for photon energies 1-16 GeV and - t < 2 GeV2 assuming a dominant contribution of the K* Regge trajectory. Moreover, using the same parameters we also reproduce the total γp↦K+Σ0 and γp↦K+Λ(1520) cross-sections suggesting a “universality” of the Regge model. In order to check the consistency of the approach we evaluate the differential and total cross-sections for the reaction π-p↦K0Λ which is also found to be dominated by the K* Regge trajectory. Using the apparent “universality” of the Regge model we extend our scheme to the analysis of the binary reactions γp↦¯0Θ+, π-p↦K-Θ+ and pp↦Σ+Θ+ as well as the exclusive and inclusive Θ+ production in the reactions pp↦p¯0Θ+ and pp↦Θ+X. Our detailed studies demonstrate that Θ+ production does not follow the “universality” principle, thus suggesting an essentially different internal structure of the exotic baryon relative to conventional hyperons or hyperon resonances.

Indication of an excited hyperon state in pp collisions with ANKE at COSY-Jülich

The reaction p p → p K + Y has been studied with the ANKE spectrometer at COSY-Julich in order to investigate heavy hyperon production. The missing mass spectra M M ( p K + ) have been analyzed and compared with Monte Carlo simulations. Indications for a hyperon resonance Y 0 ∗ ( 1480 ) have been found.

Kaon and Antikaon Properties in Cold Nuclear Medium

We present results of a self-consistent calculation for the kaon and antikaon spectral functions in cold nuclear matter, using as input the kaon-nucleon and antikaon-nucleon scattering amplitudes of the vacuum. We investigate the effect of in-medium pion dressing on the antikaon-nucleon scattering amplitudes and antikaon spectral function. We find the influence of pion dressing to be minor on the antikaon spectral function and limited on the hyperon resonances causing only a small additional broadening. An exception is the \Sigma(1690). At nuclear saturation density an attractive mass shift of about 20 MeV and width of about 130 MeV is obtained. The kaon shows a repulsive mass increase of 36 MeV and a small width of the quasiparticle peak at saturation density.

STRANGENESS PHYSICS WITH THE WASA DETECTOR AT COSY

The talk discusses the possible contributions of the WASA detector, relocated to COSY, to the exploration of the strangeness degree of freedom in hadron physics. New opportunities are opened particularly in the study of the hyperon-nucleon interaction, and in the study of hyperon resonances both in vacuum and in nuclear matter.

Hyperon resonances in coupled channels and Crystal Ball data

The known resonance parameters of the Λ and Σ hyperons were determined almost entirely from energy-dependent partial-wave analyses of K ¯ N scattering data. Some of these analyses attempted to obtain a consistent set of parameters for reactions involving different final states ( K ¯ N , πΛ, πΣ), but unitarity was not enforced and various features (e.g., backgrounds) were not treated consistently. Now that K − p → neutrals data are beginning to be published by the Crystal Ball Collaboration, it is timely to take a fresh look at the strangeness −1 hyperons. First results are presented for a new unitary, multichannel description of several K ¯ N reactions.

Dynamical generation of hyperon resonances

In this talk we report on how, using a chiral unitary approach for the meson–baryon interactions, two octets of J π = 1 / 2 − baryon states and a singlet are generated dynamically, resulting in the case of strangeness S = − 1 in two poles of the scattering matrix close to the nominal Λ ( 1405 ) resonance. We suggest experiments which could show evidence for the existence of these states.

Chiral Symmetry, Strangeness and Resonances

We review the important role played by the chiral SU(3) symmetry in predicting the properties of antikaons and hyperon resonances in cold nuclear matter. Objects of crucial importance are the meson-baryon scattering amplitudes obtained within the chiral coupled-channel effective field theory. The formation of baryon resonances as implied by chiral coupled-channel dynamics is discussed. Results for antikaon and hyperon-resonance spectral functions are presented for isospin symmetric and asymmetric matter.

Photoproduction of pseudoscalar mesons

Experiments that study the photoproduction of pseudoscalar mesons; pions, etas and kaons, have the potential to increase our knowledge of baryon and hyperon resonance properties. Recent experiments at JLab, Mainz, GRAAL, and Bonn are beginning to produce results in the form of polarization and asymmetry measurements and determinations of the differential and integrated cross sections. These new data are essential to the performance of Partial-Wave Analyses that are less model dependent and coupled-channels calculations that incorporate unitarity dynamically, combining hadronic reaction channels together with electromagnetic processes. This approach is necessary to extract resonance properties and may lead to the identification of missing, but predicted, resonances. Some recent experimental and phenomenological results for single and double pseudoscalar meson photoproduction are discussed.

Self-Consistent Antikaon Dynamics in Isospin-Asymmetric Nuclear Medium

We investigate properties of antikaons and hyperon resonances in isospin-asymmetric nuclear medium, using a self-consistent, covariant scheme based on vacuum antikaon-nucleon scattering amplitude.

Strange hadron resonances and QGP freeze-out

We describe how the abundance and distribution of hyperon resonances can be used to probe freeze-out conditions. We demonstrate that resonance yields allow us to measure the time scales of chemical and thermal freeze-outs. This should permit a direct differentiation between the explosive, sudden and staged adiabatic freeze-out scenarios. We then discuss the meaning of recent experimental results and suggest further measurements.

Low lying S=−1 excited baryons and chiral symmetry

The s-wave meson-baryon interaction in the $S = -1$ sector is studied by means of coupled-channels, using the lowest-order chiral Lagrangian and the N/D method to implement unitarity. The loops are regularized using dimensional renormalization. In addition to the previously studied $\Lambda (1405)$, employing this chiral approach leads to the dynamical generation of two more s-wave hyperon resonances, the $\Lambda(1670)$ and $\Sigma(1620)$ states. We make comparisons with experimental data and look for poles in the complex plane obtaining the couplings of the resonances to the different final states. This allows us to identify the $\Lambda (1405)$ and the $\Lambda(1670)$ resonances with $\bar{K}N$ and $K\Xi$ quasibound states, respectively.

An isobaric model for kaon photoproduction

The kaon photoproduction up to E γ Lab =2.0 GeV is analyzed by an isobaric model based on effective Lagrangians. Both pseudovector and pseudoscalar coupling schemes for kaon–baryon–baryon (baryon spin =1/2) interactions are considered with form factors. A vector meson ( K ∗(890) ), an axial-vector meson ( K 1(1270) ), nucleon resonances ( J⩽5/2), and hyperon resonances ( J⩽3/2) are treated as participating particles. Coupling constants relevant to the particle, most of which are still unknown, are determined through a systematic fitting of the differential cross section, the total cross section, the single polarization observable, and the branching ratio in radiative kaon capture to their experimental data. Our model constructed this way reproduces all the experimental data well.

The role of hyperon resonances in p(γ, K+)Λ processes

We discuss the role of hyperon resonances in the u-channel when modeling p (gamma,K+)Lambda processes in an effective Lagrangian approach. Without the introduction of hyperon resonances, one is forced to use soft hadronic form factors with a cutoff mass which is at best two times the kaon mass. After inclusion of the hyperon resonances in the u-channel, we obtain a fair description of the data with a cutoff mass of the order of 1.8 GeV.

The baryon spectroscopy from [formula omitted] decays

J ψ decay at Beijing Electron-Positron Collider (BEPC) is well known as the best place for looking for glueballs. But in fact it is also an excellent place for studying N ∗ and hyperon physics. For J ψ → N Nππ , and N Nππ the πN and ππN systems are limited to be pure isospin 1 2 due to isospin conservation. This is a big advantage in studying N ∗ resonances from J ψ decays, compared with the πN and γN experiments which suffer difficulty on the isospin decomposition of 1 2 and 3 2 . There are also plenty other decay modes of J ψ involving baryon-antibaryon pairs plus mesons which can be used to study N ∗ and hyperon resonances, e.g., looking for the “missing” N ∗ states from J ψ → ppω , ppη′ , pK − Λ , etc.; studying N ∗(1535) → ηp from J ψ → ppη and studying Λ(1405) → Σπ from J ψ → Λ Σ −π + . It provides a new way to study the internal structure of baryon resonances. The newest progress and outlook in this field are presented.

Scalar and vector meson production and two-step processes in the ( K−, K+) reaction on 12C

The double strangeness exchange reaction ( K −, K +) is investigated with respect to the sub-threshold production of scalar and vector mesons ( f 0/ a 0/ φ) decaying into K + K − and the two-step processes induced by intermediate mesons and Ξ − hyperons at p k − = 1.66 GeV/ c using a scintillating fiber active target. The differential cross section ( 〈 dσ dΩ L 〉) averaged over the angular interval (2.3° ⩽ θ K + L ⩽ 14.7°) for the sub-threshold f 0/ a 0/ φ meson production with the K + K − decay is 11 ± 6 μb/sr at 0.6 ⩽ p K 1 < 0.95 GeV/ c. The present result differs significantly from the theoretical calculation which predicts the contribution of the f 0/ a 0/ φ meson production to be predominant in the ( K −, K +) reaction below p K + = 0.95 GeV/ c. We found a sizable contribution from two-step ( K −, K + processes, characterized by production of two S = −1 hyperons, consistent with the result of the intra-nuclear cascade (INC) model calculation with respect to the meson-induced hyperon (or hyperon resonance) pair production in the momentum region 0.6 ⩽ p K + < 0.95 GeV/ c. The observed enhancement of the cross section for the two-step ΛΛ production beyond the prediction of the INC model at p K + ⋍ 1.1 GeV/c could be due to the Ξ − p → ΛΛ reaction in 12C.