Hyperon Resonances Research Articles

The theoretical study of pp¯→Λ¯Ση reaction

We study the production of hyperon resonances in the pp¯→Λ¯Ση reaction within an effective Lagrangian approach. The model includes the production of Σ(1750) and Λ(1670) in the intermediate state excited by the K and K∗ meson exchanges between the initial proton and antiproton. Due to the large coupling of Σ(1750)Ση vertex, Σ(1750) is found a significant contribution near the threshold in this reaction. We provide total and differential cross section predictions for the reaction and discuss the possible influence of Σ(1750)Ση vertex coupling and model parameters, which will be useful in future experimental studies. This reaction can provide a platform for studying the features of Σ(1750) resonance, especially the coupling to Ση channel.

Nuclear medium effects on the properties of Λ(1405)

We study the Λ(1405) resonance with I(J P ) = 0(1/2−) in the context of the pentaquark hypothesis in the nuclear medium. To explore the influence of the nuclear medium on the physical parameters of Λ(1405), we propose a molecular-type structure involving admixtures of K − p and K¯0n , correlated with the nuclear matter density. Our analysis reveals a substantial shift in mass and residue, approximately 20% and 38%, respectively. These findings have significant implications for experimental researchers aiming to identify in-medium characteristics of hyperon resonances.

Theoretical study of the π+p→pa0+ϕ reaction

We study the production of hyperon resonances in the π+p→pa0+ϕ reaction within an effective Lagrangian approach. The model includes the production of Δ(1940) and Δ(1920) in the intermediate state excited by the ρ and p exchanges between the initial proton and π meson. Because of the large contribution of ρ exchange in Δ(1940) production, Δ(1940) is found to be a significant contribution near the threshold in this reaction. We provide total and differential cross section predictions for the reaction and discuss the possible influence of model parameters, off-shell effects, and branching ratios for Δ(1940)→pa0 and Δ(1920)→pa0 decays, which will be useful in future experimental studies. This reaction can provide a platform for studying the features of Δ(1940) resonance, especially the coupling to the pa0 channel. Published by the American Physical Society 2024

Intriguing aspects of light baryon resonances

We discuss that some light baryon resonances exhibit properties which cannot be described when attributing a three-valence quark structure to them. Besides pointing out the hadron resonances which clearly require description beyond the quark model, we focus on the third s11, N∗ state and its decay to final states consisting of the lightest hyperon resonances which have a partial width comparable to that for the decay to πN. Such properties of the mentioned nucleon resonance get manifested in the cross sections and other observables related to processes producing the lightest hyperon resonances. We show that all these findings arise from the strong association of the baryon resonances to the dynamics among the ground-state hadrons.

The FAIR Phase-0 Hyperon Program at HADES

Abstract. Hyperons are a unique probe to study the non-perturbative aspects of the strong interaction. At HADES, they are produced in proton or pion induced reactions at kinetic energies up to 4.5 GeV. Already in the past, HADES has shown its potential for hyperon physics, including λ polarization, λ-N interaction and measurements of the λ(1405) and λ(1520) line-shapes. The HADES detector has recently been extended with a forward detector, partly developed for the PANDA experiment, extending the acceptance for hyperon channels at forward angles. The PANDA@HADES initiative gives the opportunity for an even richer hyperon program. The current main objectives are the production of hyperon resonances, electromagnetic decays of hyperons with special focus on hyperon Dalitz decays and double strangeness production, including a λ – λ interaction study and ξ− production. First results from the ongoing studies promise a successful execution of the program. In the future, there is the possibility for a pion beam experiment with HADES, enabling further hyperon studies.

The decay of N∗ (1895) to light hyperon resonances

In this talk I review the findings of our recent works where we have studied the decay of N ∗ (1895) and the implications of such properties on the photoproduction of light hyperons. I discuss that meson-baryon interactions play an essential role in describing the nature of N ∗ (1895) and report the details of our investigation of its decays to different meson-baryon systems and to final states involving Λ(1405) and a proposed Σ(1400). We find that the width of N ∗ (1895) gets important contributions from the decay to light hyperon resonances. Such an information can be used to look for alternative processes to study N ∗ (1895) in experimental data.

K+Σ− photoproduction within an isobar model

We used an isobar model to study the K+Σ− photoproduction reaction on a neutron target with focus on the resonance region. In order to achieve a reasonable agreement with the data, we included spin-3/2 and spin-5/2 nucleon resonances in a consistent formalism together with a Δ resonance and two kaon resonances on top of the Born terms. The free parameters of the model were adjusted to the data from the CLAS and LEPS Collaborations on differential cross sections and photon beam asymmetry. The cornerstone of this analysis was an upgrade of the fitting method. Previously, we used only the plain χ2 minimization, which could not prevent us from overfitting the data. We, therefore, introduced a regularization method, the least absolute selection shrinkage operator (LASSO), which, together with information criteria, restricts the number of nonzero parameters and prevents us from overfitting the data. In our analysis, we arrived at two models, fit M, whose parameters were fitted with the MINUIT code only, and fit L, where we used the more advanced LASSO. Both models describe the data in a similar way and we observe only slight differences in the dσ/dΩ data description at very forward angles where the fit M is flat whereas the fit L produces two broad peaks, and in the photon beam asymmetries above 2 GeV at backward kaon angles where the fit M produces a bump. Surprisingly, no hyperon resonances are needed for the correct data description in these models. On the other hand, the N(1720)3/2+ nucleon resonance was found to be very important in both models.

Model selection for K+Σ− photoproduction within an isobar model

We utilize an isobar model to investigate the ${K}^{+}{\mathrm{\ensuremath{\Sigma}}}^{\ensuremath{-}}$ photoproduction off a neutron in the resonance region. Except for the Born terms, we include high-spin (spin-3/2 and spin-5/2) nucleon resonances in the consistent formalism together with a few $\mathrm{\ensuremath{\Delta}}$ and kaon resonances to achieve an acceptable agreement with data. Interestingly, we reveal that no hyperon resonances are needed to achieve a reasonable description of data. On the other hand the $N(1720)3/{2}^{+}$ resonance was found to be very important for correct description of data. The free parameters of the model were fitted to experimental data from the LEPS and CLAS Collaborations on either differential cross sections or photon beam asymmetry. The novel feature of the fitting procedure is the use of a regularization method, the least absolute shrinkage selection operator, and information criteria in order to choose the best fit.

Excitation of hyperon resonances in photoproduction

Excitation of hyperon resonances in photoproduction

Photoproduction of Λ* and Σ* resonances with JP=1/2− off the proton

We study the photoproduction of the $\Lambda(1405)$ and $\Sigma(1400)$ hyperon resonances, the latter of which is not a well established state. We evaluate the $s$-, $t$- and $u$-channel diagrams in the Born approximation by employing the effective Lagrangians. A new ingredient is the inclusion of a nucleon resonance $N^*(1895)$ that is dynamically generated with predictions for its coupling to the $K\Lambda(1405)$ and $K\Sigma(1400)$ channels. To extend the applicability of the model to energies beyond the threshold region, we consider a Regge model for the $t$-channel $K$- and $K^*$-exchanges. Our results are in good agreement with the CLAS data available on $\Lambda(1405)$, while for $\Sigma(1400)$ we predict observables for its production. We also provide polarization observables for both hyperon productions, which can be useful in future experimental investigations. The present study provides new information on the nucleon resonance $N^*(1895)$ which can be an alternative source for generating the hyperon resonances $\Lambda(1405)$ and $\Sigma(1400)$.

Electromagnetic transition form factors and Dalitz decays of hyperons

Dalitz decays of a hyperon resonance to a ground-state hyperon and an electron-positron pair can give access to some information about the composite structure of hyperons. We present expressions for the multi-differential decay rates in terms of general transition form factors for spin-parity combinations J^P = frac{1}{2}^pm , frac{3}{2}^pm of the hyperon resonance. Even if the spin of the initial hyperon resonance is not measured, the self-analyzing weak decay of the “final” ground-state hyperon contains information about the relative phase between combinations of transition form factors. This relative phase is non-vanishing because of the unstable nature of the hyperon resonance. If all form factor combinations in the differential decay formulae are replaced by their respective values at the photon point, one obtains a QED type approximation, which might be interpreted as characterizing hypothetical hyperons with point-like structure. We compare the QED type approximation to a more realistic form factor scenario for the lowest-lying singly-strange hyperon resonances. In this way we explore which accuracy in the measurements of the differential Dalitz decay rates is required in order to distinguish the composite-structure case from the pointlike case. Based on the QED type approximation we obtain as a by-product a rough prediction for the ratio between the Dalitz decay width and the corresponding photon decay width.

Photoproduction mechanism for charged reaction of K*A with nucleon

The photoproduction of vector meson K*(892) on the nucleon has been investigated in the framework of the effective Lagrangian model by utilizing the Feynman diagram of the lowest orders in calculating the transition amplitudes. Beside the Born and the contact terms, we consider the Σ*(1780) as an additional hyperon resonance alongside with the Σ*(1385) in the M-channel. It is found that the Σ*(1780) has remarkable effects in the backward scattering region where there are still discrepancies between the theoretical predictions and the experimental data.

Decay properties of N*(1895)

The nature of nucleon resonances is still being debated, while much experimental data are accumulated. In this work, we focus on the negative parity resonance ${N}^{*}(1895)$ which is located in the scattering region of various meson-baryon coupled channels, and such dynamics can be crucial in understanding its properties. To test the relevance of such hadron dynamics, we investigate the decay properties of ${N}^{*}(1895)$ in detail. We examine how a two pole nature of ${N}^{*}(1895)$ is compatible with its observed decay properties. Moreover, we find that the resonance decays into final states involving $\mathrm{\ensuremath{\Lambda}}(1405)$ and $\mathrm{\ensuremath{\Sigma}}(1400)$, where the latter is not yet observed experimentally. Such decay processes can be useful to study the properties of the aforementioned hyperon resonances.

Photoproduction of the Λ(1520) hyperon with a 9 GeV photon beam at GlueX

The GlueX experiment is located at the Thomas Jefferson National Accelerator Facility (JLab) in Newport News, VA, USA. It features a hermetic 4π detector with excellent tracking and calorimetry capabilities. Its 9 GeV linearly polarized photon beam is produced from the 12 GeV electron beam, delivered by JLab’s Continuous Electron Beam Accelerator Facility (CEBAF), via bremsstrahlung on a thin diamond and is incident on a LH2 target. GlueX recently finished its first data taking period and published first results.The main goal of GlueX is to measure gluonic excitations of mesons. These so-called hybrid or exotic mesons are predicted by Quantum Chromodynamics (QCD) but haven’t been experimentally confirmed yet. They can have quantum numbers not accessible by ordinary quark-antiquark pairs which helps in identifying them using partial wave analysis techniques. The search for exotic mesons requires a very good understanding of photoproduction processes in a wide range of final states, one of them being pK + K − which contains many meson and baryon reactions. The Λ(1520) is a prominent hyperon resonance in this final state and is the subject of this presentation.This talk will give an introduction to the GlueX experiment and show preliminary results for the photoproduction of the Λ(1520) hyperon. The measurement of important observables like the photon beam asymmetry and spin-density matrix elements will be discussed and an outlook to possible measurements of further hyperon states in the pK + K − final state will be given.

VarLambda and varSigma Excitations and the Quark Model

The spectrum of varLambda and varSigma excitations is reviewed taking into account (nearly) all hyperon resonances which were seen in early analyses or in one of the recent partial-wave analyses. The spectrum is compared with the old Isgur–Karl model and the Bonn model. These models allows us to discuss the SU(3) structure of the observed resonances. The SU(3) decomposition is compared with SU(6) relations between the different decay modes. Seven varLambda states are proposed to be classified as SU(3) singlet states. The hyperon spectrum is compared with the spectrum of N and varDelta resonances.

Photon induced KΛ production on the proton in the low energy region

The associated photoproduction of [Formula: see text] from the proton in the low energy region is studied using an isobar model in which the nonresonant contributions are obtained from the nonlinear sigma model with chiral SU(3) symmetry which predicts, in a natural way, the contact term with its coupling strength along with the coupling strengths of the various Born terms predicted by the nonlinear sigma model. The present model is an extension of the nonlinear sigma model with chiral SU(2) symmetry, used earlier to study the photo, electro, and neutrino productions of pions. In the resonance sector, the contributions from the well established nucleon resonances ([Formula: see text]) in the [Formula: see text] channel, the hyperon resonances ([Formula: see text]) in the [Formula: see text] channel, and the kaon resonances ([Formula: see text] and [Formula: see text]) in the [Formula: see text] channel having spin [Formula: see text] and mass [Formula: see text][Formula: see text]GeV with a significant branching ratio in [Formula: see text] decay mode, have been considered. The strong and electromagnetic couplings of the [Formula: see text] channel nucleon resonances are taken from experiments while the couplings for the resonances in the [Formula: see text] and [Formula: see text] channels are fitted to reproduce the current data on the associated photoproduction of [Formula: see text] in this energy region. The numerical results are presented for the total and differential cross-sections and are compared with the available experimental data from CLAS and SAPHIR as well as with some of the recent theoretical models.

Hyperon I: Partial-wave amplitudes for K-p scattering

Early data on $K^-$ induced reactions off protons are collected and used in a coupled-channel partial wave analysis (PWA). Data which had been published in the form of Legendre coefficients are included in the PWA. In a {\it primary} fit using 3* and 4* resonances only, we observe some significant discrepancies with the data. In a systematic search for new $\Lambda$ and $\Sigma$ hyperon resonances, additional candidates are found. The significance of the known and of the additional resonances is evaluated. Seventeen resonances listed with 1* or 2* and one resonance listed with 3* in the Review of Particle Properties cannot be confirmed, five new hyperons are suggested. The partial-wave amplitudes deduced in this analysis are compared to those from other analyses.

Hyperon II: Properties of excited hyperons

We report properties of $\Lambda$ and $\Sigma$ hyperon resonances formed in $K^-$ induced reactions. Special emphasis is laid on the analysis of the three-body final states $2\pi^0\Lambda$ and $2\pi^0\Sigma$ and of the quasi-two-body final states $\pi\Lambda(1520)$, $\bar K\Delta(1232)$, $\pi\Sigma(1385)$, $\bar K^*N$, and $\omega\Lambda$. We give pole positions of $\Lambda$ and $\Sigma$ hyperon resonances and transition residues from the $K^-p$ initial to various final states as well as Breit-Wigner masses and widths and decay branching ratios. Twenty resonances and "bumps" reported in the Review of Particle Physics are not required in our fits, evidence for five new resonances is reported. The observed mass spectrum is compared to the spectrum calculated in the Bonn quark model. Three spin doublets, six $\Lambda$ hyperons, are tentatively assigned to the SU(3) singlet system.

Hyperon resonances coupled to pseudoscalar- and vector-baryon channels

We study hyperon resonances by solving coupled channel scattering equations. The coupled systems include pseudoscalar- and vector-baryon channels. The parameters of the model are restricted by making a $\chi^2$-fit to the cross section data on processes: $K^- p \to K^- p$, $K^- p \to \bar K^0 n$, $K^- p \to \eta \Lambda$, $K^- p \to \pi^0 \Lambda$, $K^- p \to \pi^0 \Sigma^0$, $K^- p \to \pi^\pm \Sigma^\mp$. Data on the energy level shift and width of the $1s$ state of the kaonic hydrogen, as well as some cross-section ratios near the threshold are also considered in the fit. Two types of fits are found as a result. In both cases, the properties of $\Lambda(1405)$ are well reproduced. In addition to this, a $\Sigma$ state is also found with mass around 1400 MeV. Cross sections, obtained with one of the two fits, are found to stay close to the data at energies away from the thresholds too, and as a result resonances with higher masses have also been studied.

Observation of Ξ(1620)^{0} and Evidence for Ξ(1690)^{0} in Ξ_{c}^{+}→Ξ^{-}π^{+}π^{+} Decays.

We report the first observation of the double strange baryon Ξ(1620)^{0} in its decay to Ξ^{-}π^{+} via Ξ_{c}^{+}→Ξ^{-}π^{+}π^{+} decays based on a 980 fb^{-1} data sample collected with the Belle detector at the KEKB asymmetric-energy e^{+}e^{-} collider. The mass and width are measured to be 1610.4±6.0(stat)_{-4.2}^{+6.1} (syst) MeV/c^{2} and 59.9±4.8(stat)_{-7.1}^{+2.8}(syst) MeV, respectively. We obtain 4.0σ evidence of the Ξ(1690)^{0} with the same data sample. These results shed light on the structure of hyperon resonances with strangeness S=-2.