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.
Quenched and unquenched quark models predict very different patterns for the spectrum of the low excited hyperon states. Evidence is accumulating for the existence of some new hyperon resonances, such as a $Σ^*$ of spin-parity $J^P=1/2^-$ around 1400 MeV instead of 1620 MeV as listed in PDG, a new $Σ(1540)3/2^-$ resonance, a new narrow $Λ(1670)3/2^-$ resonance and a new $Λ(1680)3/2^+$ resonance. All these new hyperon resonances fit in the predicted pattern of the unquenched quark models very well. It is extremely important to check and establish the spectrum of these low excited hyperon states…
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 re…
We have investigated the role of hyperon resonances in the kaon photoproduction process, $γp\to K^+Λ$, by using a covariant isobar model. To this end, new experimental data are used in the fitting process, whereas the old SAPHIR 1998 data are also used for comparison. The result indicates that the $Λ(1600)P_{01}$ and $Λ(1810)P_{01}$ hyperon resonances can significantly reduce the $χ^2$ and, simultaneously, can increase the hadronic form factor cut-off in the background terms. This finding is different from the result of the previous studies, which showed that the $Λ(1800)S_{01}$ was important …
Radiative decays of decuplet hyperons and octet hyperon charge radii are evaluated in a chiral constituent quark model emphasizing the role of exchange currents. Exchange currents largely cancel for the M1 decay amplitudes, while they dominate the E2 amplitude. Due to the pseudoscalar meson cloud the charge radii of Sigma^- and Xi^- are almost as large as the proton radius, in agreement with recent experimental results from SELEX. Strangeness suppression is weakened by exchange currents for several observables.
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 = 1/2^+/-, 3/2^+/- 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 ph…
In this work we extend our formalism to study meson-baryon interactions by including $s$- and $u$-channel diagrams for pseudoscalar-baryon systems. We study the coupled systems with strangeness $-1$ and focus on studying the isospin-1 resonance(s), especially in the energy region around 1400 MeV. By constraining the model parameters to fit the cross section data available on several processes involving relevant channels, we find resonances in the isoscalar as well as the isovector sector in the energy region around 1400 MeV.
The complete renormalization of the weak Lagrangian to chiral order q^2 in heavy baryon chiral perturbation theory is performed using heat kernel techniques. The results are compared with divergences appearing in the calculation of Feynman graphs for the nonleptonic hyperon decay Lambda -> p pi^- and an estimate for the size of the counterterm contributions to the s-wave amplitudes in nonleptonic hyperon decays is given.
We calculate the radiative decay widths of decuplet hyperons in a chiral constituent quark model including electromagnetic exchange currents between quarks. Exchange currents contribute significantly to the E2 transition amplitude, while they largely cancel for the M1 transition amplitude. Strangeness suppression of the radiative hyperon decays is found to be weakened by exchange currents. Differences and similarities between our results and other recent model predictions are discussed.
We compute the leading contribution to the E2/M1 mixing ratio of the decay $Δ\rightarrow Nγ$ in heavy baryon chiral perturbation theory. We find the mixing ratio to be $4\% \ltap |δ_{\rm E2/M1}| \ltap 11\%$, much larger than estimates based on the quark model and other hadronic models. We also compute the mixing ratio for the radiative decay of the hyperon resonances. The decays $Σ^{*+}\rightarrowΣ^+γ$ and $Ξ^{*-}\rightarrowΞ^-γ$ provide a particularly sensitive probe of deviations from heavy baryon spin-flavour SU(6).
Hyperon non-leptonic weak decay amplitudes are studied in the chiral perturbation theory. The weak interaction vertices caused by the four quark operators are substituted by the products of the hadronic currents and by the phenomenologically introduced weak Hamiltonian of hadron operators. Our study suggests the improvement of the theoretical prediction for the weak decay amplitudes.
A short overview of motivations and successes of two-body exchange currents between constituent quarks for electromagnetic hadron observables like charge radii, magnetic and quadrupole moments is given. We then predict and analyze exchange current effects on the radiative decay widths of decuplet hyperons, which are to be measured soon. In our chiral constituent quark model, exchange currents dominate the E2 transition amplitude, while they largely cancel for the M1 transition amplitude. Strangeness suppression of the radiative hyperon decays is weakened by exchange currents. The SU(F)_3 flavo…
The $K^- d \to πYN$reaction is theoretically studied so as to investigate the $K^{-}$-induced production of the hyperon resonances $Σ(1385)$ and $Λ(1405)$. For this purpose we take into account the p-wave amplitudes for the meson-baryon two-body scatterings as well as the s-wave amplitudes. Due to the fact that the hyperon resonances are selectively produced from the $\bar{K} N$ channel in this reaction, the $Λ(1405)$ peak appears at 1420 MeV, which implies that $Λ(1405)$ and $Σ(1385)$ could be separately seen in the missing mass spectrum of the emitted nucleon in the $K^{-} d \to n X$ reactio…
We investigate the importance of final state interactions in weak nonleptonic hyperon decays within a relativistic chiral unitary approach based on coupled channels. The effective potentials for meson-baryon scattering are derived from a chiral effective Lagrangian and iterated in a Bethe-Salpeter equation, which generates the low lying baryon resonances dynamically. The inclusion of final state interactions decreases the discrepancy between theory and experiment for both s and p waves. Our study indicates that contributions from higher order terms of the weak effective Lagrangian may play an …
Hyperon non-leptonic weak decay amplitudes are studied in the chiral perturbation theory. We employ the low energy effective weak Hamiltonian which contains the perturbative QCD correction. To include the non-perturbative QCD effect, quark currents of the effective Hamiltonian are substituted with hadronic currents which are color singlet and are derived by the chiral perturbation theory. We find that the amplitudes caused by the product of hadronic currents are small. It reproduce the small amplitudes of $ΔI=3/2$, which are derived by the strong interaction correction.
A partial wave analysis is presented of data on photoproduction of hyperons including single and double polarization observables. The large spin transfer probability reported by the CLAS collaboration can be successfully described with an isobar partial wave analysis.
The importance of resonances for the radiative hyperon decays is examined in the framework of chiral perturbation theory. Low lying baryon resonances are included into the effective theory and tree contributions to these decays are calculated. We find significant contributions to both the parity-conserving and parity-violating decay amplitudes and a large negative value for the asymmetry parameter in polarized Sigma^+ -> p gamma is found, in agreement with the experimental result alpha(p Sigma^+) = -0.76 +/- 0.08.
We examine the importance of resonances for the non-leptonic hyperon decays in the framework of chiral perturbation theory. Lower lying resonances are included into the effective theory. Integrating out the heavy degrees of freedom in the resonance saturation scheme generates higher order counterterms in the effective Lagrangian, providing an estimate of the pertinent coupling constants. A fit to the eight independent decay amplitudes that are not related by isospin symmetry is performed and reasonable agreement for both s- and p- waves is achieved.
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 $χ^2$-fit to the cross section data on processes: $K^- p \to K^- p$, $K^- p \to \bar K^0 n$, $K^- p \to ηΛ$, $K^- p \to π^0 Λ$, $K^- p \to π^0 Σ^0$, $K^- p \to π^\pm Σ^\mp$. 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 case…
We compute the strangeness-conserving double beta decay of $Σ^-$ hyperons, which is the only hadronic system that can undergo such decays. We consider both, the lepton number conserving %$Σ^-\go Σ^++2e^-+2 \barν$ $Σ^-\go Σ^+e^-e^-\barν\barν$ ($ββ_{Σ_{2ν}}$) and the lepton number violating $Σ^-\go Σ^+e^-e^-$ ($ββ_{Σ_{0ν}}$) modes. The branching ratios of these $ββ$ decays are suppressed at the level of $10^{-30}$ considering a light neutrino scenario in the case of the $ββ_{Σ_{0ν}}$ channel. The dynamical origin of such low rates and their possible enhancements are briefly discussed. Given its …
The electromagnetic decay processes of excited hyperon states are a very sensitive probe of the structure of hyperons. We will argue that the recent measurements of electromagnetic decay rates indicate that the wave functions of the hyperon ground states should contain sizeable components of excited quark states (configuration mixing). Flavor-SU(3) is a broken symmetry and it appears that the hyperon wave functions should preferably be written in a $uds$-basis, where only the light $u$ and $d$ quarks are symmetrized, and not in the usual SU(6)-basis.
The study of neutrinoless double beta decays of nuclei and hyperons require the calculation of hadronic matrix elements of local four-quark operators that change the total charge by two units ΔQ=2 . Using a low energy effective Lagrangian that induces these transitions, we compute these hadronic matrix elements in the framework of the MIT bag model. As an illustrative example we evaluate the amplitude and transition rate of Σ- -> p e- e-, a decay process that violates lepton number by two units (ΔL=2). The relevant matrix element is evaluated without assuming the usual factorization approximat…
We report on a study of the sequential hyperon decay $Σ^0\rightarrowΛγ;Λ\rightarrow pπ^- $ and its corresponding anti-hyperon decay. We derive a multi-dimentional and model-independent formalism for the case when the hyperons are produced in the reaction $e^+e^-\rightarrowΣ^0\barΣ^0$. Cross-section distributions are calculated using the folding technique. We also study sequential decays of single-tagged hyperons.
The recent data taken by the CLAS Collaboration at the Thomas Jefferson National Accelerator Facility for the reaction of $γp \to K^+ K^+ Ξ^-$ are reanalyzed within a relativistic meson-exchange model of hadronic interactions. The present model is an extension of the one developed in an earlier work by Nakayama, Oh, and Haberzettl [Phys. Rev. C 74, 035205 (2006)]. In particular, the role of the spin-5/2 and -7/2 hyperon resonances, which were not included in the previous model, is investigated in the present study. It is shown that the contribution of the $Σ(2030)$ hyperon having spin-7/2 and …
The hyperon decays with 2 negative charged leptons in the final states can be of interest as possible tests for the Majorana netrinos. The study of these decays are complementary to the double beta decays of heavy nuclei.
We give an personal overview of some of the unsolved problems related to hyperon decays. We cover nonleptonic decays, radiative decays and magnetic moments. Some of the theoretical issues are also touched upon.
We study the |Delta I|=3/2 amplitudes of hyperon non-leptonic decays of the form B->B'+pi in the context of chiral perturbation theory. The lowest-order predictions are determined in terms of only one unknown parameter and are consistent within errors with current data. We investigate the theoretical uncertainty of these predictions by calculating the leading non-analytic corrections. We also present an estimate for the size of the S-wave Lambda and Cascade decays which vanish at leading order. We find that the corrections to the lowest-order predictions are within the expectations of naive po…
We calculate the contribution to the polarization of $Λ$ hyperons in relativistic nuclear collisions at high energy from the decays of $Σ^*(1385)$ and $Σ^0$, which are the predominant sources of $Λ$ production besides the primary component, as a function of the $Λ$ momentum. Particularly, we estimate the longitudinal component of the mean spin vector as a function of the azimuthal angle in the transverse plane, assuming that primary $Σ^*$ and $Σ^0$ polarization follow the predictions of local thermodynamic equilibrium in a relativistic fluid. Provided that the rapidity dependence around midrap…
The $\bar{K} + N \to K + Ξ$ reaction is studied for center-of-momentum energies ranging from threshold to 3 GeV in an effective Lagrangian approach that includes the hyperon $s$- and $u$-channel contributions as well as a phenomenological contact amplitude. The latter accounts for the rescattering term in the scattering equation and possible short-range dynamics not included explicitly in the model. Existing data are well reproduced and three above-the-threshold resonances were found to be required to describe the data, namely, the $Λ(1890)$, $Σ(2030)$, and $Σ(2250)$. For the latter resonance …
Using the recent measurement of the $Ξ^0 \to Λγ$ asymmetry as an input, we reanalyse nonleptonic and weak radiative hyperon decays in a single symmetry-based framework. In this framework the old S:P problem of nonleptonic decays is automatically resolved when the most important features of weak radiative decays are taken into account. Experimental data require that symmetry between the two types of hyperon decays be imposed at the level of currents, not fields. Previously established connections between hyperon decays and nuclear parity violation imply that the conflict, originally suggested b…
The CP properties in hyperon decays are briefly reviewed. We discuss the general phenomenology and define CP odd observables in hyperon decays. With these observables, we discuss the predictions of some models and their observational potential.
We consider all hyperon data relevant to spin and flavor structure of hyperons. In addition to masses and magnetic moments considered as static properties in Hyperon99 we include also relevant data from hyperon decays and spin structure determined from deep inelastic scattering. Any theoretical model for the hyperons with parameters to be determined from experiment should use input from all these data. Of particular interest are new data from $Ξ^o$ decay and the polarixation of $Λ's$ produced in Z^o decays and deep inelastic scattering.
We report our recent effort for the extraction of resonance parameters (complex pole mass and residues etc.) associated with Lambda* and Sigma* hyperons. This was accomplished via a comprehensive partial-wave analysis of the data for K^- p --> barK N, pi Sigma, pi Lambda, eta Lambda, K Xi reactions from the thresholds up to W=2.1 GeV within a dynamical coupled-channels approach. The results suggest a possible existence of new narrow J^P=3/2^+ Λresonance with pole mass 1671^{+2}_{-8} -i(5^{+11}_{-2}) MeV, located close to the eta Lambda threshold. This resonance is found to be responsible for r…
Strong-flavor and parity a priori mixing in hadrons are shown to describe well the experimental evidence on weak radiative decays of hyperons. An independent determination of the a priori mixing angles is performed. The values obtained for them are seen to have a universality-like property, when compared to their values in non-leptonic decays of hyperons.
We show that the so-called parity-conserving amplitudes predicted in the a priori mixing scheme for non-leptonic and weak radiative decays of hyperons vanish in the strong-flavor symmetry limit.
We construct a new equation of state for the baryonic matter under an intense magnetic field within the framework of covariant density functional theory. The composition of matter includes hyperons as well as $ Δ$-resonances. The extension of the nucleonic functional to the hypernuclear sector is constrained by the experimental data on $Λ$ and $Ξ$-hypernuclei. We find that the equation of state stiffens with the inclusion of the magnetic field, which increases the maximum mass of neutron star compared to the non-magnetic case. In addition, the strangeness fraction in the matter is enhanced. Se…
We discuss the perspectives of measuring direct CP violation parameters in hyperon decays by measurements with proton and/or antiproton beams, to be carried out at various facilities, and in particular we propose new experiments at the Tevatron and at the LHC.
This note emphasizes that due to isospin violation the two central states in the octet mix, and this mixing can be measured in semileptonic decays, in particular with an accurate determination of the sigma plus semileptonic branching ratio.
We show that a large CP asymmetry in hyperon decays can be naturally realized in the framework of SUSY models. The possibility is implemented by the hermiticities of Yukawa and A matrices. And also, the observed values of $ε$ and $ε'$ are explained.
We investigate production and decay of the $Λ_c^+ $ hyperon. The production considered is through the $e^+e^-$ annihilation channel, $e^+e^-\rightarrowΛ_c^+ \barΛ_c^-$, with summation over the $\barΛ_c^- $ anti-hyperon spin directions. It is in this situation that the $Λ_c^+ $ decay chain is identified. Two kinds of sequential decays are studied. The first one is the doubly weak decay $B_1\rightarrow B_2 M_2$, followed by $B_2 \rightarrow B_3 M_3$. The other one is the mixed weak-electromagnetic decay $B_1\rightarrow B_2 M_2$, followed by $B_2 \rightarrow B_3 γ$. In both schemes $B$ denotes ba…
The radiative decays of hyperons are studied in the framework of the bound state soliton model. Detailed predictions for the total decay widths and the E2/M1 ratios corresponding to decuplet--to--octet electromagnetic transitions are presented in relation to future planned experiments at CEBAF and Fermilab. The results are compared to those obtained in quark based models.
I review the estimate of the CP violating asymmetry $A(Λ^0_-)$ within the standard model. I then review the estimate of the upper bound on this asymmetry that arises from measurements of CP violation in kaon decays.
A priori mixings of eigenstates in physical states are quantum mechanical effects well known in several realms of physics. The possibility that such effects are also present in particle physics, in the form of flavor and parity mixings, is studied. Applications to weak radiative and non-leptonic decays of hyperons are discussed.
We consider the gluon dipole penguin operator contributions to $ε'/ε$ and CP violation in hyperon decays. It has been proposed by Bertolini et al. that the contribution to $ε'/ε$ may be significant. We show that there is a cancellation in the leading order contribution and this contribution is actually suppressed by a factor of order O($m_π^2, m_K^2)/Λ^2$. We find that the same operator also contributes to CP violation in hyperon decays where it is not suppressed. The gluon dipole penguin operator can enhance CP violation in hyperon decays by as much as 25\%.
I shall present a pedagogical discussion of hyperon semileptonic decays, covering some of the historical background, the basics notions of hyperon semileptonic decays, deeply inelastic scattering and the CKM matrix, and the description of SU(2) and SU(3) breaking. I shall also present a prediction for a process under current experimental study.
The discrepancies found between S-wave and P-wave fits for hyperon decays are reinvestigated using the heavy baryon chiral Lagrangian formalism. The agreement is found to improve through the inclusion of previously omitted diagrams. The S-waves are unaffected by this, but the P-wave predictions are modified. A correlated fit to the chiral parameters is performed and the results discussed.
In this talk I discuss CP violation in hyperon decays in Left-Right symmetric models. I show that the asymmetry in polarization in $Λ\to pπ^-$ can be as large as $6\times 10^{-4}$ in these models, which is an order of magnitude larger than the Standard Model prediction.
We explore the strangeness-changing decays of the lightest hyperons into another baryon plus missing energy within and beyond the standard model (SM). In the SM these processes arise from the loop-induced quark transition $s\to dν\barν$ and their branching fractions are estimated to be less than $10^{-11}$. In the presence of new physics (NP) the rates of these hyperon decays with missing energy could increase significantly with respect to the SM expectations because of modifications to the SM process or contributions from additional modes with new invisible particles. Adopting a model-indepen…
Weak radiative hyperon decays are discussed in the diquark-level approach. It is pointed out that in the general diquark formalism one may reproduce the experimentally suggested pattern of asymmetries, while maintaining Hara's theorem in the SU(3) limit. At present, however, no detailed quark-based model of parity-violating diquark-photon coupling exists that would have the necessary properties.
It is shown that an alternative form of the parity-nonconserving (PNC) transition electromagnetic current resolves partly a puzzle with the Hara theorem. New formulation of it has allowed PNC weak radiative hyperon transitions of the charged hyperons $Σ^{+} \Rightarrow p + γ$ and $Ξ^{-} \Rightarrow Σ^{-} + γ$ revealing hitherto unseen transition toroid dipole moment.
We study the |Delta I|=3/2 amplitudes of the octet-hyperon decays B -> B' + pion and of the decays Omega^- -> Cascade + pion in the context of heavy-baryon chiral perturbation theory. For the octet-hyperon decays, we investigate the theoretical uncertainty of the lowest-order predictions by calculating the leading non-analytic corrections. We find that these corrections are within the expectations of naive power counting and, therefore, that this picture can be tested more accurately with improved measurements. For the Omega- decays, we obtain at leading order two operators responsible for the…
Weak radiative hyperon decays are considered in the framework of a quark model. The phenomenological model includes 1-quark transitions with the effective $sdγ$-vertex and 2-quark ones with the W-exchange $s+u\to u+d+γ$ and turns out to describe well the data and gives predictions for asymmetry of the decays $Λ\to n+γ$ and $Ξ^-\to Σ^- +γ$.
The chiral quark model gives a reasonably good description of many low-energy observables by incorporating the effective degrees carried by the constituent quarks and Goldstone bosons. We calculate the decuplet to octet transition magnetic moments and the decay widths of several excited hyperons using this model. The various radiative decay widths from the chiral quark roughly agree with experimental data including the recent JLAB measurement.
The radiative decays of the $(3/2)^+$ baryons are studied in the three flavor generalization of the Skyrme model. The kaon fields are treated in the slow rotator approach which properly accounts for the observed deviations from the $U$-spin relations for the hyperon magnetic moments. This makes possible a critical discussion of the $U$-spin selection rules for the radiative hyperon decays. The variation of the decay widths with strangeness is studied and a comparison with other treatments of the $SU(3)$ Skyrme model is performed in order to analyze the effects of flavor symmetry breaking.
