Sigma Hyperons Research Articles

Two-Dimensional and Three-Dimensional Analyses of Sigma Precipitates and Porosity in a Superaustenitic Stainless Steel

X-ray micro-tomography revealed not only the true three-dimensional (3-D) distribution, interconnectivity, and morphology of coarse sigma precipitates in an AL-6XN superaustenitic stainless steel, but also the presence of an internal void network associated with these precipitates. The voids were present within the bulk of the undeformed material, indicating that they formed during alloy fabrication. The presence of these voids in association with the brittle sigma particles has important implications for the mechanical behavior of these alloys.

Effect of strong magnetic fields on neutral pion condensation in neutron star matter

We explore within the mean-field theory with TM1 parametrization the possibility of the π0 condensation under strong external magnetic fields in neutron star matter that includes sigma particles. At the magnetic field of B = 1018 G, π0 does not condense. At B = 6 × 1018 G, π0 condensation together with a partial stiffening of the matter equation of state occurs in the baryon number densities below 1 fm−3. It is anticipated that the average magnetic field in the stellar interior does not exceed O(1018) G in a neutron star as massive as two times a solar mass.

Signal of the pion string at high-energy collisions

We study the possible signals of a pion string associated with the QCD chiral phase transition in LHC Pb-Pb collision at energy $\sqrt{s}=5.5\phantom{\rule{0.3em}{0ex}}\text{TeV}$. In terms of the Kibble-Zurek mechanism we discuss the production and evolution of the pion string. The pion string is not topologically stable; it decays into neutral pions and sigma mesons that in turn decay into pions. Our results show that all the neutral pions from the pion string are distributed at the low momentum and the ratio of neutral to charged pions from the pion string violates the isospin symmetry. For the momentum spectra of the total pions, the signal from the sigma particle decay from the pion string will be affected by the large decay width of the sigma significantly.

The quark–antiquark asymmetry of the strange sea of the nucleon

The strange sea of the proton is generally assumed to have quark–antiquark symmetry. However, it has been known for some time that non-perturbative processes involving the meson cloud of the proton may break this symmetry. Recently this has been of interest as it affects the analysis of the so-called ‘NuTeV anomaly’, and could explain the large discrepancy between the NuTeV measurement of sin2θW and the currently accepted value. In this Letter we re-examine strange–anti-strange asymmetry using the meson cloud model. We calculate contributions to the strange sea arising from fluctuations in the proton wavefunction to states containing either Lambda or Sigma hyperons together with either Kaons or pseudovector K∗ mesons. We find that we should not ignore fluctuations involving K∗ mesons in this picture. The strange sea asymmetry is found to be small, and is unlikely to affect the analysis of the Llewellyn–Smith cross section ratios or the Paschos–Wolfenstein relationship.

Resonating-group study of baryon-baryon interactions for the complete baryon octet:NNinteraction

We upgrade a SU_6 quark-model description for the nucleon-nucleon and hyperon-nucleon interactions by improving the effective meson-exchange potentials acting between quarks. For the scalar- and vector-meson exchanges, the momentum-dependent higher-order term is incorporated to reduce the attractive effect of the central interaction at higher energies. The single-particle potentials of the nucleon and Lambda, predicted by the G-matrix calculation, now have proper repulsive behavior in the momentum region q_1=5 - 20 fm^-1. A moderate contribution of the spin-orbit interaction from the scalar-meson exchange is also included. As to the vector mesons, a dominant contribution is the quadratic spin-orbit force generated from the rho-meson exchange. The nucleon-nucleon phase shifts at the non-relativistic energies up to T_lab=350 MeV are greatly improved especially for the 3E states. The low-energy observables of the nucleon-nucleon and the hyperon-nucleon interactions are also reexamined. The isospin symmetry breaking and the Coulomb effect are properly incorporated in the particle basis. The essential feature of the Lambda N - Sigma N coupling is qualitatively similar to that obtained from the previous models. The nuclear saturation properties and the single-particle potentials of the nucleon, Lambda and Sigma are reexamined through the G-matrix calculation. The single-particle potential of the Sigma hyperon is weakly repulsive in symmetric nuclear matter. The single-particle spin-orbit strength for the Lambda particle is very small, in comparison with that of the nucleons, due to the strong antisymmetric spin-orbit force generated from the Fermi-Breit interaction.

Hyperonic matter in neutron star

We discuss appearance and role of hyperons in neutron star matter with recent compiled nuclear equation of state from heavy-ion collisions. We also discuss different hyperon couplings from analysis of various experimental data on hypernuclei. We find that the sigma baryons do not appear in the neutron star matter, when it experiences a strong repulsive potential. However, the change in maximum mass of the neutron stars is insignificant against the different hyperon couplings.

Event-by-event fluctuations in heavy ion collisions and the QCD critical point

The event-by-event fluctuations of suitably chosen observables in heavy ion collisions at SPS, RHIC and LHC can tell us about the thermodynamic properties of the hadronic system at freeze-out. By studying these fluctuations as a function of varying control parameters, it is possible to learn much about the phase diagram of QCD. As a timely example, we stress the methods by which present experiments at the CERN SPS can locate the second-order critical endpoint of the first-order transition between quark-gluon plasma and hadron matter. Those event-by-event signatures which are characteristic of freeze-out in the vicinity of the critical point will exhibit nonmonotonic dependence on control parameters. We focus on observables constructed from the multiplicity and transverse momenta of charged pions. We first consider how the event-by-event fluctuations of such observables are affected by Bose-Einstein correlations, by resonances which decay after freeze-out and by fluctuations in the transverse flow velocity. We compare our thermodynamic predictions for such noncritical event-by-event fluctuations with NA49 data, finding broad agreement. We then focus on effects due to thermal contact between the observed pions and a heat bath with a given (possibly singular) specific heat, and due to the direct coupling between the critical fluctuations of the sigma field and the observed pions. We also discuss the effect of the pions produced in the decay of sigma particles just above threshold after freeze-out on the inclusive pion spectrum and on multiplicity fluctuations. We estimate the size of these nonmonotonic effects which appear near the critical point, including restrictions imposed by finite size and finite time, and conclude that they should be easily observable.

Hyperonic nuclear matter in Brueckner theory

We determine in an extended Brueckner-Hartree-Fock formalism self-consistent single-particle potentials of nucleons, lambda, and sigma hyperons for a system consisting of symmetric nuclear matter and lambda hyperons of uniform densities ${\ensuremath{\rho}}_{N}$ and ${\ensuremath{\rho}}_{\ensuremath{\Lambda}},$ respectively. The binding energy per baryon of this system is discussed and its maximum strangeness content preserving binding is evaluated. The results are used to introduce a hyperonic symmetry energy term in a generalized mass formula for multistrange hypernuclei.

Further Analysis on bcy Particle Properties

In a previous work, we have reanalyzed the I=0 S-wave \pi\pi-scattering phase-shift through a new method of Interfering Breit-Wigner amplitudes, and have shown the existence of a light scalar, \sigma particle, accompanied by a negative "repulsive core"-type background phase-shift, whose origin may have some correspondence to a ``compensating'' \pi\pi contact interaction. In this work we make further analysis on the phase shift under KKbar threshold from the same standpoint, to determine precise values of \sigma mass and width, so far as present experimental data are concerned.

Isospin violation in Delta S=1 weak transitions-a re-examination

The violation of the Delta I=1/2 rule and the effects of the isospin violation in Delta S=1 weak transitions are analysed for the K to 2 pi , K to 3 pi , angle , Xi and Sigma hyperon decays.

The optics of secondary polarized-proton beams

Abstract Polarized protons can be produced by the parity-violating decay of either Lambda or Sigma hyperons. The preservation of the polarization as the protons are transmitted to the final focus dictates the optics of the beam line. The equations of motion are solved here, including higher-order terms which are represented as sets of commutators. A beam of polarized protons produced by Lambda hyperons has been built and operated at Fermilab. The design and performance are described. A polarized proton beam using Sigma particles has also been designed. Differences from and similarities to the Lambda-produced beam are also described.

Precipitation effects in austenitic stainless weld metals

Creep‐rupture specimen of similar welded joints of X6 CrNi 18 11 (1.4948/AISI 304) and X6 CrNiMo 1713 (1.4919/AISI 316) show only low elongation after longtime testing. The reason for this loss of ductility was found by metallographic investigations.The weld metal of five joints had ferrite contents from 3 to 7.5%, due to a primary ferritic solidification. During creep testing in the temperature range from 500 to 800°C carbide precipitation takes place at the austenite‐ferrite grain boundaries, because delta ferrite is not in a state of equilibrium at these temperatures. After carbon has been used up, the remaining delta ferrite changes into sigma phase, if its alloying element content is high enough. In the upper temperature range, coagulation of sigma phase is dominating. At these large particles grain boundary migration is hindered. During the grain boundary sliding sigma phase particles break and initiate creep cracks. In fully austenitic weld metal sigma precipitation starts at austenite – austenite grain boundaries. During creep testing, void formation starts at sigma particles. Growing of voids leads to grain separations.

Effect of strain rate on sigma formation in ferrite-austenite stainless steel at high temperatures

Sigma precipitation in an Fe-19Cr-5Ni-2.7Mo ferrite -austenite duplex stainless steel has been investigated during and after uniaxial compression at 900° C. At this temperature ferrite and austenite phases are present in equivolume proportions (1:1 ). It was found that sigma precipitation was enhanced by hot working and the volume proportion of sigma decreased as the straining rate increased. Sigma nucleated at ferrite-austenite interfaces and grew into ferrite grains by the eutectoid reaction α → σ + γ. Transmission electron microscopy revealed that large sigma particles enhanced recrystallization of the austenite phase in duplex structures at high temperature.

On the Σ-hypernuclear states in (K-, π±) reactions on12C

The binding energies BSigma of Sigma hyperons in hypernuclei produced in (K-, pi +or-) reactions on 12C are analysed, with the result that B( Sigma +)>B( Sigma 0)>B( Sigma -). If confirmed by further measurements, the result suggests that charge independence of the Sigma N interaction is broken.

Hadron-nucleus scattering lengths derived from exotic atom data

Hadron-nucleus S-wave scattering lengths for kaons, antiprotons and sigma hyperons have been derived from measurements of strong interaction effects in exotic atoms. The results are compared with the predictions of a simple square-well model.

Kaon scattering and charged Sigma hyperon production in K-p interactions below 300 MeV/c

In an experiment on K--proton interactions in a hydrogen bubble chamber, the cross sections and angular distributions have been studied for elastic and charge-exchange scattering, and charged sigma hyperon production in the momentum range 90-300 MeV/c. Good agreement is found with previous cross sections except for the charge-exchange channel where the new values are 60% larger. The production distributions of the charged sigma hyperons and the polarisation of the Sigma + hyperon clearly indicate that P waves are present down to about 150 MeV/c, much lower than previously reported.

Hypernuclei with sigma particles

The production of Σ particles in the (K −, π −) on 9Be at 720 MeV/ c has been studied at the CERN proton synchrotron (PS). The missing-mass spectrum shows structures in the Σ region, which have been interpreted as belonging to hypernuclei with sigma particles. An upper limit of 8 MeV for the width of these states has been estimated.

Goldstone mode of an extended σmodel

The spontaneous breakdown of symmetry of the sigma model involving a sigma particle within the (1,1) representation of the SU(2)*SU(2) group is investigated. It is shown that spontaneous breakdown occurs for all squared mass terms, mu 2, in the Lagrangian, unlike the case of the sigma model in the (1/2,1/2) representation, in which spontaneous breakdown occurs only for the case mu 2<0. Further, different but equivalent methods of obtaining the nonlinear realization for pions in the framework of such extended sigma models are demonstrated. Finally, it is shown that the results obtained can be generalized to all (1/2N,1/2N) sigma models of the SU(2)*SU(2) chiral group.

Inclusive production of charged and neutral sigma hyperons in K −p interactions at 14.3 GeV/ c

Inclusive production of Σ +, Σ − and Σ 0 hyperons in K −p interactions at 14.3 GeV/ c has been studied and compared to Λ production. Cross sections are presented as a function of longitudinal and transverse momenta and compared to the pp → Σ + + anything data.

Formula omitted] from 594 to 820 MeV/c

Data on K −p interactions yielding charged sigma hyperons are presented. The cross sections and angular distributions are explained by Λ (1670) and Λ(1690) resonances. The Σ(1660) found in production experiments shows little or no coupling to the KN system.