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Global and local polarization of Λ hyperons in Au+Au collisions at 200 GeV from STAR
New results on global polarization of Λ hyperons in Au+Au collisions at √ s NN = 200 GeV reveal non-zero signal on the order of a few tenths of a percent. Compared with lower energy results, the current measurement shows that the polarization decreases at higher collision energy, the trend being well reproduced by theoretical calculations. The polarization is found to be larger in more peripheral collisions, as well as in the in-plane than in the out-of-plane direction. The signal seems to depend on the asymmetry between positive and negative charge in each event, which may indicate an influence of the axial current induced by the magnetic field. In addition, the first measurement of a local polarization along the beam direction was performed. The results show a quadrupole modulation relative to the second-order event plane, as expected from the elliptic flow.
Hyperon Photoproduction from Polarized H and D: towards a complete N* experiment
New complete experiments in pseudoscalar meson photo-production are being pursued at several laboratories. Here the designation of complete refers to measurements of most if not all of the possible reaction observables, of which there are 16 involving spins of the beam, target and recoil baryon. Hyperon production to Λ or Σ{sup +} final states affords attractive opportunities, since their weak decays provide an efficient self-analysis of their polarization. When the beam and target are also polarized, the resulting triple polarization measurements determine the full suite of observables with a single target orientation. This has been a focus at Jefferson Lab in the recently completed g9/FROST and g14/HDice experiments now under analysis. Multipole analyses of γp->K{sup +}Λ have been carried out with a large though incomplete set of recently published polarization data, and the uniqueness of the extracted amplitudes has been studied. Experiments with realistically achievable uncertainties require a significantly greater number of spin asymmetries than the in-principle minimum needed for a mathematical solution of the amplitude.
Photoproduction of Λ and Σ 0 hyperons using linearly polarized photons
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Global hyperon polarization at local thermodynamic equilibrium with vorticity, magnetic field, and feed-down
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Λ hyperon polarization in relativistic heavy ion collisions from a chiral kinetic approach
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Hyperon matter at low densities
It was reported recently that hyperons can be present inside PSRJ1614-2230 compact star. This can be realized only if the strength of the ω-hyperons and φ-hyperons coupling of conventional hyperons coupling constant on the extended relativistic mean field (ERMF) model increase by a factor of 1.5 to 3. In the present work, the mass and radius relation of the neutron star that is calculated by using BSR28 parameter set of ERMF model augmented with maximal coupling strength of the ω-hyperons and φ-hyperons (X=1), is compared to the mass and radius relation of the neutron star that is predicted by the same RMF parameter set but by assuming that hyperons do not exist in the matter (No. Hyp) as well as those by assuming the hyperons coupling constant fulfilled the conventional SU(6) and SU(3) symmetry. The consequences of implementing X=1 prescription are also discussed. The potential depths of hyperons in symmetric nuclear matter (SNM), pure neutron matter (PNM) and pure lambda matter (PLM) based on this parameter set are also calculated by using the X=1, SU (6) and SU (3) prescriptions. The results are compared to those obtained from microscopic models, quark meson coupling model (χ QMM) and the QCD sum rule for finite density (QCD SM) result.
NEW HYPERON EQUATIONS OF STATE FOR SUPERNOVAE AND NEUTRON STARS IN DENSITY-DEPENDENT HADRON FIELD THEORY
We develop new hyperon equation of state (EoS) tables for core-collapse supernova simulations and neutron stars. These EoS tables are based on a density-dependent relativistic hadron field theory where baryon-baryon interaction is mediated by mesons, using the parameter set DD2 for nucleons. Furthermore, light and heavy nuclei along with interacting nucleons are treated in the nuclear statistical equilibrium model of Hempel and Schaffner-Bielich which includes excluded volume effects. Of all possible hyperons, we consider only the contribution of Λs. We have developed two variants of hyperonic EoS tables: in the npΛφ case the repulsive hyperon-hyperon interaction mediated by the strange φ meson is taken into account, and in the npΛ case it is not. The EoS tables for the two cases encompass a wide range of densities (10{sup –12} to ∼1 fm{sup –3}), temperatures (0.1 to 158.48 MeV), and proton fractions (0.01 to 0.60). The effects of Λ hyperons on thermodynamic quantities such as free energy per baryon, pressure, or entropy per baryon are investigated and found to be significant at higher densities. The cold, β-equilibrated EoS (with the crust included self-consistently) results in a 2.1 M {sub ☉} maximum mass neutron star for the npΛφ case, whereas that for the npΛ case is 1.95 M {sub ☉}. The npΛφ EoS represents the first supernova EoS table involving hyperons that is directly compatible with the recently measured 2 M {sub ☉} neutron stars.
Hyperons in neutron star matter within relativistic mean-field models
Since the discovery of neutron stars with masses around 2M{sub ⊙} the composition of matter in the central part of these massive stars has been intensively discussed. Within this paper we will (re)investigate the question of the appearance of hyperons. To that end we will perform parameter study within relativistic mean field models. We will show that it is possible to obtain high mass neutron stars with a substantial amount of hyperons and a spinodal instability at the onset of hyperons. The results depend strongly on the interaction in the hyperon-hyperon channels, on which only very little information is available from terrestrial experiments up to now.
LARGE-MASS NEUTRON STARS WITH HYPERONIZATION
Within a density-dependent relativistic mean-field model and using in-medium meson-hadron coupling constants and meson masses, we explore the effects of in-medium hyperon interactions on the properties of neutron stars. We found that hyperonic constituents in large-mass neutron stars cannot be simply ruled out, while the recently measured mass of the millisecond pulsar J1614-2230 can significantly constrain in-medium hyperon interactions. In addition, we discuss the effects of nuclear symmetry energy on hyperonization in neutron stars.
Appearance of hyperons in neutron stars within LOCV method
Chemical potentials of nucleons in asymmetric nuclear matter are calculated in a completely self-consistent manner, using the lowest order constrained variational (LOCV) method. Employing different nucleon–nucleon potentials, threshold density of free hyperons formation and the maximum mass of hypernuclear star are determined. Furthermore, the particles fraction in beta stable and charge neutral matter is calculated as well. The effects of three-body force (TBF) on the onset of hyperons, particle composition and maximum mass of the star are discussed. The results show that, for all different nucleon–nucleon interactions considered, onset of hyperons formation sets in at about 2–3 times of the nuclear saturation density. In particular, it is shown that by considering the TBF, the density of appearance of hyperons shifts to a lower value. The maximum masses of hypernuclear star are obtained about 1.3068M{sub ⊙} and 1.1032M{sub ⊙} with and without TBF respectively. Although these results are much below the ones required by observations of astrophysics, they are in agreement with the previous works based on Brueckner–Hartree–Fock (BHF) approach as well as other non-relativistic potential models.
Hyperon global polarization in isobar Ru+Ru and Zr+Zr collisions at $\sqrt{s_{NN}}$ = 200Ge V
The polarization of Λ, $\overline{Λ}$, $Ξ$ – , and $\overline{Ξ}$ + hyperons along the angular momentum of the system has been measured in isobar collisions of Ru+Ru and Zr+Zr at $\sqrt{s_{NN}}$ = 200 GeV with the STAR detector at RHIC. The polarization dependence on collision centrality exhibits an increasing trend in more peripheral collisions. Λ and $\overline{Λ}$ polarization dependence on the transverse momentum and pseudorapidity have been investigated, but no significant dependence was observed. The polarizations of Λ and $\overline{Λ}$ are found to be consistent with each other, indicating little contribution of the spin-magnetic coupling to the measured polarization. Comparison to previously measured polarization in Au+Au collisions show no obvious system size dependence. The results are qualitatively consistent with hydrodynamic calculations including contributions from shear-induced polarization and thermal vorticity. For the first time in heavy-ion collisions, the dependence of the global polarization on the hyperon’s emission azimuthal angle relative to the second-order event plane has been measured, indicating stronger polarization for the in-plane emitted hyperons at the level of 2.4 σ significance in 20–50 % centrality. The $Ξ$ hyperon polarization measurements via polarization transfer analysis yield finite positive values with 2.9 σ significance in 20–50 % centrality, slightly larger compared to the inclusive Λ polarization.
Simulations of stellar collapses to black holes: Influence of hyperons
Stellar core collapse to a black hole is an event where extremely high temperatures and densities are reached. It is then expected that additional particles to nucleons and electrons appear, as hyperons. We present numerical simulations of the collapse of massive stars to black holes, with an equation of state based on the Lattimer-Swesty one, but allowing for the presence of Λ hyperons. A feature of these equations of state is that its cold and beta-equilibrium version allows for neutrons stars with almost two solar masses, marginally compatible with recent observations. Spherically symmetric simulations, under some conditions, may show the effect of a phase transition to hyperonic matter before the appearance of the black hole. We show axisymmetric simulations, too, where this phase transition would be smoothed, but where gravitational waves show clear imprint of the presence of hyperons.
Hyperon production from neutrino-nucleon reaction
The neutrino induced hyperon production processes ν¯ e/μ + p → e + /μ + + π + Λ/Σ may provide a unique clean place for studying low energy πΛ/Σ interaction and hyperon resonances below KN threshold. The production rates for some neutrino induced hyperon production processes are estimated with theoretical models. Lastly, suggestions are made for the study of hyperon production from neutrino–nucleon reaction at present and future neutrino facilities.
Absorption of {Lambda}(1520) hyperons in photon-nucleus collisions
In the framework of the nuclear spectral function approach for incoherent primary photon-nucleon and secondary pion-nucleon production processes we study the inclusive {Lambda}(1520)-hyperon production in the interaction of 2-GeV photons with nuclei. In particular, the A and momentum dependences of the absolute and relative {Lambda}(1520)-hyperon yields are investigated in two scenarios for its in-medium width. Our model calculations show that the pion-nucleon production channel contributes appreciably to the {Lambda}(1520) creation at intermediate momenta both in light and heavy nuclei in the chosen kinematics and, hence, has to be taken into consideration on close examination of the dependences of the {Lambda}(1520)-hyperon yields on the target mass number with the aim to get information on its width in the medium. They also demonstrate that the A and momentum dependences of the absolute and relative {Lambda}(1520)-hyperon production cross sections at incident energy of interest are markedly sensitive to the {Lambda}(1520) in-medium width, which means that these observables may be an important tool to determine the above width.