Hyperons In Nuclear Matter Research Articles

Momentum dependence of near-threshold photoproduction of Ξ− hyperons off nuclei and their properties in the nuclear medium

We study the near-threshold inclusive photoproduction of Ξ− hyperons off 12C and 184W target nuclei within a first-collision model relying on the nuclear spectral function and including incoherent Ξ− production in direct elementary γp→K+K+Ξ− and γn→K+K0Ξ− processes. The model takes into account the impact of the nuclear effective scalar K+, K0, Ξ− and their Coulomb potentials on these processes as well as the absorption of final Ξ− hyperons in nuclear matter, the binding of intranuclear nucleons and their Fermi motion. We calculate the absolute differential cross sections and their ratios for the production of Ξ− hyperons off these nuclei at laboratory polar angles ≤ 45∘ by photons with energies of 2.5 and 3.0 GeV, with and without imposing the phase space constraints on the Ξ− emission angle in the respective γp center-of-mass system. We also calculate the momentum dependence of the transparency ratio for Ξ− hyperons for the 184W/12C combination at these photon energies. We show that the Ξ− momentum distributions (absolute and relative) at the adopted initial photon energies possess a definite sensitivity to the considered changes in the scalar Ξ− nuclear mean-field potential at saturation density ρ0 in the low-momentum range of 0.1–0.8 GeV/c. This would permit evaluating the Ξ− potential in this momentum range experimentally. We also demonstrate that the momentum dependence of the transparency ratio for Ξ− hyperons for both photon energies can hardly be used to determine this potential reliably. Therefore, the precise measurements of the differential cross sections (absolute and relative) for inclusive Ξ− hyperon photoproduction on nuclei near threshold in a dedicated experiment at the CEBAF facility will provide valuable information on the Ξ− in-medium properties, which will be supplementary to that inferred from studying of the (K−,K+) reactions at initial momenta of 1.6–1.8 GeV/c.

Hyperon\u2013nucleon interaction within chiral effective field theory revisited

The varLambda N and varSigma N interactions are considered at next-to-leading order in SU(3) chiral effective field theory. Different options for the low-energy constants that determine the strength of the contact interactions are explored. Two variants are analysed in detail which yield equivalent results for varLambda N and varSigma N scattering observables but differ in the strength of the varLambda N rightarrow varSigma N transition potential. The influence of this difference on predictions for light hypernuclei and on the properties of the varLambda and varSigma hyperons in nuclear matter is investigated and discussed. The effect of the variation in the potential strength of the varLambda N-varSigma N coupling (also called varLambda -varSigma conversion) is found to be moderate for the considered ^3_varLambda {{mathrm{H}}} and ^4_varLambda mathrm{He} hypernuclei but sizable in case of the matter properties. Further, the size of three-body forces and their relation to different approaches to hypernuclear interactions is discussed.

In-medium properties of a $ \Xi$ΞN interaction derived from chiral effective field theory

The in-medium properties of a baryon-baryon interaction for the strangeness $S=-2$ sector derived within chiral effective field theory up to next-to-leading order are investigated. The considered $\Xi N$ interaction is in line with available empirical constraints on the $\Lambda\Lambda$ $S$-wave scattering length and on $\Xi N$ elastic and inelastic cross sections. In particular, there are no near-threshold bound states in the $\Xi N$ system. A conventional $G$-matrix calculation for this interaction is performed and reveals that the single-particle potential of the $\Xi$-hyperon in nuclear matter is moderately attractive as suggested by recent experimental evidence for the existence of $\Xi$-hypernuclei.

Foundations of strangeness nuclear physics derived from chiral effective field theory

Dense compact objects like neutron stars or black holes have always been one of Gerry Brown’s favorite research topics. This is closely related to the effects of strangeness in nuclear physics. Here, we review the chiral Effective Field Theory approach to interactions involving nucleons and hyperons, the possible existence of strange dibaryons, the fate of hyperons in nuclear matter and the present status of three-body forces involving hyperons and nucleons.

Hyperons in nuclear matter from SU(3) chiral effective field theory

Brueckner theory is used to investigate the properties of hyperons in nuclear matter. The hyperon-nucleon interaction is taken from chiral effective field theory at next-to-leading order with SU(3) symmetric low-energy constants. Furthermore, the underlying nucleon-nucleon interaction is also derived within chiral effective field theory. We present the single-particle potentials of Lambda and Sigma hyperons in symmetric and asymmetric nuclear matter computed with the continuous choice for intermediate spectra. The results are in good agreement with the empirical information. In particular, our calculation gives a repulsive Sigma-nuclear potential and a weak Lambda-nuclear spin-orbit force.

A study of hyperons in nuclear matter based on chiral effective field theory

The in-medium properties of a hyperon-nucleon potential, derived within chiral effective field theory and fitted to Lambda-N and Sigma-N scattering data, are investigated. Results for the single-particle potentials of the Lambda and Sigma hyperons in nuclear matter are reported, based on a conventional first-order Brueckner calculation. The Sigma-nucleus potential is found to be repulsive, in agreement with phenomenological information. A weak Lambda-nucleus spin-orbit interaction can be achieved by an appropriate adjustment of a low-energy constant corresponding to an antisymmetric Lambda-N -- Sigma-N spin-orbit interaction that arises at next-to-leading order in the effective field theory approach.

Chiral effective field theory description of the hyperon-nucleon interaction at next-to-leading order

In this proceeding results for hyperon-nucleon interactions calculated at next-to-leading order (i.e. one-loop order) in SU(3) chiral effective field theory are presented. The potentials include contributions from one- and two-meson exchange, and four-baryon contact terms provided by the SU(3) chiral Lagrangian. SU(3) flavor symmetry is imposed for the low-energy constants, while explicit SU(3) symmetry breaking is included only through the physical pseudoscalar-meson and baryon masses. Calculations have been performed for hyperon-nucleon scattering cross sections using a regularized Lippmann-Schwinger equation. A good description of the available data is achieved, comparable to modern phenomenological hyperon-nucleon interaction models. These results provide a new basis for studies of hypernuclei or hyperons in nuclear matter.

Hyperons in neutron-star cores and a 2 M⊙pulsar

Recent measurement of mass of PSR J1614-2230 rules out most of existing models of equation of state (EOS) of dense matter with high-density softening due to hyperonization or a phase transition to quark matter or a boson condensate. We look for a solution of an apparent contradiction between the consequences stemming from up-to-date hypernuclear data, indicating appearance of hyperons at 3 nuclear densities and existence of a two-solar-mass neutron star. We consider a non-linear relativistic mean field (RMF) model involving baryon octet coupled to meson fields. An effective lagrangian includes quartic terms in the meson fields. The values of the parameters of the model are obtained by fitting semi-empirical parameters of nuclear matter at the saturation point, as well as potential wells for hyperons in nuclear matter and the strength of the Lambda-Lambda attraction in double-Lambda hypernuclei. We propose a non-linear RMF model which is consistent with up-to-date semiempirical nuclear and hypernuclear data and allows for neutron stars with hyperon cores and M larger than 2 solar masses. The model involves hidden-strangenes scalar and vector mesons, coupled to hyperons only, and quartic terms involving vector meson fields. Our EOS involving hyperons is stiffer than the corresponding nucleonic EOS (with hyperons artificially suppressed) above five nuclear densities. Required stiffening is generated by the quartic terms involving hidden-strangeness vector meson.

Effect of Gluon and Pion Exchanges on Hyperons in Nuclear Matter

A new version of the quark-meson coupling model, in which the effect of gluon and pion exchanges is included self-consistently, is applied to hyperons in a nuclear medium. The hyperfine interaction due to the gluon exchange plays an important role in the in-medium baryon spectra, while the pion-cloud effect is relatively small. At the quark mean-field level, the Λ feels more attractive force than the Σ or Ξ in matter. Subject Index: 211, 214

Magnetic moment of hyperons in nuclear matter by using quark–meson coupling models

We calculate the magnetic moments of hyperons in dense nuclear matter by using relativistic quark models. Hyperons are treated as MIT bags, and the interactions are considered to be mediated by the exchange of scalar and vector mesons which are approximated as mean fields. Model dependence is investigated by using the quark–meson coupling model and the modified quark–meson coupling model; in the former the bag constant is independent of density and in the latter it depends on density. Both models give us the magnitudes of the magnetic moments increasing with density for most octet baryons. But there is a considerable model dependence in the values of the magnetic moments in dense medium. The magnetic moments at the nuclear saturation density calculated by the quark–meson coupling model are only a few percents larger than those in free space, but the magnetic moments from the modified quark–meson coupling model increase more than 10% for most hyperons. The correlations between the bag radius of hyperons and the magnetic moments of hyperons in dense matter are discussed.

Antikaons and hyperons in nuclear matter with saturation

We evaluate the antikaon and hyperon spectral functions in a self-consistent and covariant many-body approach. The computation is based on coupled-channel dynamics derived from the chiral SU(3) Lagrangian. A novel subtraction scheme is developed that avoids kinematical singularities and medium-induced power divergencies all together. Scalar and vector mean fields are used to model nuclear binding and saturation. The effect of the latter is striking for the antikaon spectral function that becomes significantly more narrow at small momenta. Attractive mass shifts of about 30 and 40 MeV are predicted for the Λ ( 1405 ) and Σ ( 1385 ) resonances. Once scalar and vector mean fields for the nucleon are switched on the Λ ( 1520 ) resonance dissolves almost completely in nuclear matter. All together only moderate attraction is predicted for the nuclear antikaon systems at saturation density. However, at larger densities we predict a sizable population of soft antikaon modes that arise from the coupling of the antikaon to a highly collective Λ ( 1115 ) nucleon–hole state. This may lead to the formation of exotic nuclear systems with strangeness and antikaon condensation in compact stars at moderate densities.

Baryon decuplet in the chiral dynamics of Λ hyperons in nuclear matter

We study the long range part of the � -hyperon optical potential in nuclei using quantum many body techniques and flavor-SU(3) chiral Lagrangians as starting point. More precisely, we study the contributions to the � -hyperon ∆

Correlation effects on the nonmesonic weak decay of the Λ hyperon in nuclear matter

The nonmesonic weak decay of a \ensuremath{\Lambda} hyperon is studied in nuclear matter. Special emphasis is placed on a consistent treatment of correlations introduced by the strong interaction on its weak counterpart. The latter is described by the exchange of mesons between the initial $\ensuremath{\Lambda}N$ state and the final NN one. The weak decay is studied in terms of the weak self-energy, which allows a systematic evaluation of short-range and tensor correlation effects that are determined by a realistic hyperon-nucleon interaction. The admixture of $\ensuremath{\Sigma}N$ components through the strong interaction is also included in the calculation of the \ensuremath{\Lambda} decay properties. Calculations for the ratio of the neutron-induced partial width to the corresponding proton-induced one, ${\ensuremath{\Gamma}}_{n}/{\ensuremath{\Gamma}}_{p}$, are discussed in connection with recent experimental results.

Correlation effects onΛpropagation in nuclear matter

The propagation of a $\ensuremath{\Lambda}$ hyperon in nuclear matter is studied within the Green's function formalism. The probability density for adding a $\ensuremath{\Lambda}$ hyperon with momentum $k$ to the correlated nuclear matter ground state is obtained from the complete energy dependence of the real and imaginary parts of the $\ensuremath{\Lambda}$ self-energy. This self-energy incorporates the effects of short-range correlations induced by the hyperon-nucleon interaction and the strong coupling between $\ensuremath{\Lambda}N$ and $\ensuremath{\Sigma}N$ states which is known to be crucial for a correct determination of the $\ensuremath{\Lambda}$ binding energy. The calculated spectral functions and quasi-particle parameters for the $\ensuremath{\Lambda}$ are found to be qualitatively similar to corresponding results for correlated nucleons. In general, the $\ensuremath{\Lambda}$ is less strongly correlated with the nuclear matter environment at ${k}_{F}=1.36\phantom{\rule{0.3em}{0ex}}{\mathrm{fm}}^{\ensuremath{-}1}$ than a nucleon, in agreement with empirical information from finite nuclei.

Single particle potential of a Σ hyperon in nuclear matter

Single particle potential of a Σ hyperon in nuclear matter

On the Binding Energy in Nuclear Matter

The widely used low-order Brueckner (LOB) scheme for calculating the binding energy of the Λ hyperon in nuclear matter, BΛ, is discussed. Two corrections to BΛ are considered: the rearrangement energy and the effect of replacing the LOB spectrum of the single particle Λ energies in the intermediate states of the reaction matrix equation by a continuous spectrum. It is shown that the two corrections approximately cancel.

Isospin dependence of the single-particle potential of theΣhyperon in nuclear matter

The isospin dependence of the real part of the single-particle potential of a $\ensuremath{\Sigma}$ hyperon in nuclear matter, ${V}_{\ensuremath{\Sigma}}$ is investigated. The isospin-dependent part ${V}_{1}$ of ${V}_{\ensuremath{\Sigma}}$ is expressed in terms of an effective $\ensuremath{\Sigma}N$ interaction in nuclear matter, which depends on proton and neutron densities. With suitable approximations numerical results for ${V}_{1}$ are obtained for four models of the Nijmegen baryon-baryon interaction. A comparison with recent ${(K}^{\ensuremath{-}},{\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}})$ experiments favors model F as a realistic representation of the $\ensuremath{\Sigma}N$ interaction.

Lifetimes of Λ hypernuclei up to ΛFe

We have observed that the global behavior of the lifetime of a A hyperon in nuclear matter is almost mass independent. Its lifetime decreases slightly from that of a free Λ and quickly saturates at ∼80% of τ Λ in free space as the mass number increases. We have determined the lifetimes of three Λ hypernuclei as τ( Λ 12 C g.s.) = 231 ± 15 ps, τ( Λ 28 Si g.s.) = 206 ± 12 ps and τ( Λ 56 Fe) = 215 −13 +14 ps. The hypernuclei were produced by the (π +, K +) reaction and identified by the SKS spectrometer at the KEK 12 GeV PS. The lifetimes were obtained by a direct measurement of the production time of a hypernucleus and its decay time with two sets of timing/range counters. These are the first lifetime results of A hypernuclei over this broad mass range with the explicit identification of the produced hypernuclei.

QCD sum rules for hyperons in nuclear matter

Using a finite-density QCD sum-rule approach, we investigate the self-energies of Λ and ∑ hyperons propagating in uniform nuclear matter. We find that the Lorentz vector self-energy of the Λ is substantially smaller than the corresponding nucleon self-energy and the vector self-energy of the ∑ is similar to the nucleon vector self-energy. The Lorentz scalar self-energies, sensitive to the density dependence of certain condensates, tend to cancel the vector self-energies, but not completely.

QCD sum rules for Sigma hyperons in nuclear matter.

Within finite-density QCD sum-rule approach we investigate the self-energies of $\Sigma$ hyperons propagating in nuclear matter from a correlator of $\Sigma$ interpolating fields evaluated in the nuclear matter ground state. We find that the Lorentz vector self-energy of the $\Sigma$ is similar to the nucleon vector self-energy. The magnitude of Lorentz scalar self-energy of the $\Sigma$ is also close to the corresponding value for nucleon; however, this prediction is sensitive to the strangeness content of the nucleon and to the assumed density dependence of certain four-quark condensate. The scalar and vector self-energies tend to cancel, but not completely. The implications for the couplings of $\Sigma$ to the scalar and vector mesons in nuclear matter and for the $\Sigma$ spin-orbit force in a finite nucleus are discussed.