Showing papers on "Strangeness published in 2002"

Chiral unitary approach to S -wave meson baryon scattering in the strangeness S = 0 sector

Abstract: We study the S-wave interaction of mesons with baryons in the strangeness S=0 sector in a coupled channel unitary approach. The basic dynamics is drawn from the lowest order meson baryon chiral Lagrangians. Small modifications inspired by models with explicit vector meson exchange in the t-channel are also considered. In addition the pi pi N channel is included and shown to have an important repercussion in the results, particularly in the isospin 3/2 sector.

Hadrons and Quark–Gluon Plasma

Abstract: Part I. A New Phase of Matter?: 1. Micro-bang: big bang in the laboratory 2. Hadrons 3. Vacuum as a physical medium 4. Statistical properties of hadronic matter Part II. Analysis Tools and Experiments: 5. Nuclei in collision 6. Understanding collision dynamics 7. Entropy and its relevance in heavy ion collisions Part III. Particle Production: 8. Particle spectra 9. Highlights of hadron production Part IV. Hot Hadronic Matter: 10. Relativistic gas 11. First look at hadronic gas 12. Hagedorn gas Part V. QCD, Hadronic Structure and High Temperature: 13. Hadronic structure and quantum chromodynamics 14. Perturbative QCD 15. Lattice quantum chromodynamics 16. Perturbative quark-gluon plasma Part VI. Strangeness: 17. Thermal flavor production in deconfined phase 18. Strangeness background 19. Hadron freeze-out analysis.

Particle ratios, equilibration and the QCD phase boundary

Abstract: We discuss the status of thermal model descriptions of particle ratios in central nucleus–nucleus collisions at ultra-relativistic energy. An alternative to the 'Cleymans–Redlich' interpretation of the freeze-out trajectory is given in terms of the total baryon density. Emphasis is placed on the relation between the chemical equilibration parameters and the QCD phase boundary. Furthermore, we trace the essential difference between thermal model analyses of data from collisions between elementary particles and from heavy ion collisions due to a transition from local strangeness conservation to percolation of strangeness over large volumes, as occurs naturally in a deconfined medium. We also discuss predictions of the thermal model for composite particle production.

Statistical hadronization model and transverse momentum spectra of hadrons in high energy collisions

Abstract: A detailed analysis of transverse momentum spectra of several identified hadrons in high energy collisions within the canonical framework of the statistical model of hadronization is performed. The study of particle momentum spectra requires an extension of the statistical model formalism used to handle particle multiplicities, which is described in detail starting from a microcanonical treatment of single hadronizing clusters. Also, a new treatment of extra strangeness suppression is presented which is based on the enforcement of fixed numbers of \(s\bar{s}\) pairs in the primary hadrons. The considered center-of-mass energies range from \(\simeq 10\) to 30 GeV in hadronic collisions (\(\pi p, pp\) and Kp) and from \(\simeq\) 15 to 35 GeV in \(e^+e^-\) collisions. The effect of the decay chain following hadron generation is accurately and exhaustively taken into account by a newly proposed numerical method. The exact \({\vec p}_{\mathrm{T}}\) conservation at low energy and the increasing hard parton emission at high energy bound the validity of the presently taken approach within a limited center-of-mass energy range. However, within this region, a clear consistency is found between the temperature parameter extracted from the present analysis and that obtained from fits to average hadron multiplicities in the same collision systems. This finding indicates that in the hadronization process the production of different particle species and their momentum spectra are two closely related phenomena governed by one parameter.

Strangeness enhancement and energy dependence in heavy ion collisions

Abstract: The canonical statistical model analysis of strange and multistrange hadron production in central A–A relative to p–p/p–A collisions is presented over the energy range from $s^{1/2}=8.73$ GeV up to $s^{1/2} =130$ GeV. It is shown that the relative enhancement of strange particle yields from p–p/p–A to A–A collisions substantially increases with decreasing collision energy. It is largest at $s^{1/2}= 8.7$ GeV, where the enhancement of $\Omega,\Xi$ and $\Lambda$ is of the order of 100, 20 and 3, respectively. In terms of the model these results are due to the canonical suppression of particle thermal phase space at lower energies, which increases with the strangeness content of the particle and with decreasing size of the collision fireball. The comparison of the model with existing data on the energy dependence of the kaon/pion ratio is also discussed.

Spin, parity, and nature of the xi(1620) resonance.

Abstract: Using a unitary extension of chiral perturbation theory with a lowest-order s-wave SU(3) chiral Lagrangian we study low-energy meson-baryon scattering in the strangeness S=-2 sector. A scattering-matrix pole is found around 1605 MeV which corresponds to an s-wave Xi resonance with J(P)=1/2(-). We identify this resonance with the Xi(1620) state, quoted by the Particle Data Group with I=1/2 but with unknown spin and parity. The addition of the S=-2 state to the recently computed Lambda(1670), Sigma(1620), and N(1535) states completes the octet of J(P)=1/2(-) resonances dynamically generated in this chiral unitary approach.

Continuum limit of quark number susceptibilities

Abstract: We report the continuum extrapolation of quark number susceptibilities in quenched QCD. Deviations from ideal gas behavior at temperature T increase as the lattice spacing is decreased from 1/4T to 1/8T. The continuum extrapolation of the measured susceptibilities is 25% lower than the ideal gas values, and also 15% below the hard thermal loop (HTL) results. The off-diagonal susceptibility is several orders of magnitude smaller than the HTL results. We verify a strong correlation between the lowest screening mass and the susceptibility. We also show that the quark number susceptibilities give a reasonable account of the Wroblewski parameter, which measures the strangeness yield in a heavy-ion collision.

Chiral dynamics of the p wave in K − p and coupled states

Abstract: We perform an evaluation of the p-wave amplitudes of meson-baryon scattering in the strangeness $S=\ensuremath{-}1$ sector starting from the lowest order chiral Lagrangians and introducing explicitly the ${\ensuremath{\Sigma}}^{*}$ field with couplings to the meson-baryon states obtained using SU(6) symmetry. The $N/D$ method of unitarization is used, equivalent, in practice, to the use of the Bethe-Salpeter equation with a cutoff. The procedure leaves no freedom for the p-waves once the s-waves are fixed and thus one obtains genuine predictions for the p-wave scattering amplitudes, which are in good agreement with experimental results for differential cross sections, as well as for the width and partial decay widths of the ${\ensuremath{\Sigma}}^{*}(1385).$

Centrality dependence of thermal parameters in heavy-ion collisions at relativistic energies

Abstract: The centrality dependence of thermal parameters, characterizing the hadron multiplicities, is determined phenomenologically for lead-on-lead collisions at CERN-SPS for a beam energy of 158 AGeV. The strangeness equilibration factor shows a clear, approximately linear, increase with increasing centrality, while the freeze-out temperature and chemical potential remain constant.

Hadrosynthesis at SPS and RHIC and the statistical model

Abstract: An analysis of hadron abundances in heavy ion collisions from SPS to RHIC energy within the statistical–thermal model is presented. Pb–Pb collisions at 40 A GeV are analysed for the first time here. Unlike as stated in similar recent studies, the data analysis rules out a complete strange chemical equilibrium in full phase space. In fact, the use of multiplicities integrated in full phase space or in a limited rapidity window at SPS energy gives rise to different results for the extra-strangeness suppression parameter γS while the extracted values of temperature and baryon-chemical potential do not vary significantly. This behaviour raises the question of whether the observed hadronic strangeness phase space saturation at RHIC within a small mid-rapidity window would hold in a possible full phase space analysis.

Chemical factors in canonical statistical models for relativistic heavy ion collisions

Abstract: We study the effect of enforcing exact conservation of charges in statistical models of particle production for systems as large as those relevant to relativistic heavy ion collisions. By using a numerical method developed for small systems, we have been able to approach the large volume limit keeping the exact canonical treatment of all relevant charges, namely, baryon number, strangeness, and electric charge. Hence, we hereby give the information needed in a hadron gas model whether the canonical treatment is necessary or not in actual cases. Comparison between calculations and experimental particle multiplicities is shown. Also, a discussion on relative strangeness chemical equilibrium is given.

Importance of reaction volume in hadronic collisions: Canonical enhancement

Abstract: We study the canonical flavour enhancement arising from exact conservation of strangeness and charm flavour. Both the theoretical motivation, and the practical consequences are explored. We argue using qualitative theoretical arguments and quantitative evaluation, that this proposal to re-evaluate strangeness signature of quark–gluon plasma is not able to explain the majority of available experimental results.

The strangeness content of the nucleon

Abstract: We evaluate the matrix element of qq in hadron states on a lattice. We find substantial mixing of the connected and disconnected contributions so that the lattice result that the disconnected contribution to the nucleon is large does not imply that the ss content is large. This has implications for dark matter searches.

Intrinsic Polarized Strangeness and Lambda Polarization in Deep-Inelastic Production

Abstract: We propose a model for the longitudinal polarization of Lambda baryons produced in deep-inelastic lepton scattering at any xF, based on static SU(6) quark-diquark wave functions and polarized intrinsic strangeness in the nucleon associated with individual valence quarks. Free parameters of the model are fixed by fitting NOMAD data on the longitudinal polarization of Lambda hyperons in neutrino collisions. Our model correctly reproduces the observed dependences of Lambda polarization on the kinematic variables. Within the context of our model, the NOMAD data imply that the intrinsic strangeness associated with a valence quark has anticorrelated polarization. We also compare our model predictions with results from the HERMES and E665 experiments using charged leptons. Predictions of our model for the COMPASS experiment are also presented.

Evolution of strangeness in an equilibrating and expanding quark-gluon plasma

Abstract: We evaluate the strangeness production from equilibrating and transversely expanding quark gluon plasma which may be created in the wake of relativistic heavy ion collisions. We consider boost invariant longitudinal and cylindrically symmetric transverse expansion of a gluon dominated partonic plasma, which is in local thermal equilibrium. Initial conditions obtained from the self screened parton cascade model are used. We empirically find that the final extent of the partonic equilibration rises almost linearly with the square of the initial energy density. This along with the corresponding variation with the number of participants may help us distinguish between various models of parton production.

Effects of strong color fields on baryon dynamics

Abstract: We calculate the antibaryon-to-baryon ratios, p /p, Λ /Λ, Ξ /Ξ , and Ω /Ω for Au+Au collisions at RHIC ( s NN =200 GeV). The effects of strong color fields associated with an enhanced strangeness and diquark production probability and with an effective decrease of formation times are investigated. Antibaryon-to-baryon ratios increase with the color field strength. The ratios also increase with the strangeness content |S|. The netbaryon number at midrapidity considerably increases with the color field strength while the netproton number remains roughly the same. This shows that the enhanced baryon transport involves a conversion into the hyperon sector (hyperonization) which can be observed in the (Λ− Λ )/(p− p ) ratio.

System size dependence of strangeness production at 158 AGeV

Abstract: Strange particle production in A+A interactions at 158 AGeV is studied by the CERN experiment NA49 as a function of system size and collision geometry. Yields of charged kaons, phi and Lambda are measured and compared to those of pions in central C+C, Si+Si and centrality-selected Pb+Pb reactions. An overall increase of relative strangeness production with the size of the system is observed which does not scale with the number of participants. Arguing that rescattering of secondaries plays a minor role in small systems the observed strangeness enhancement can be related to the space-time density of the primary nucleon-nucleon collisions.

Differential distributions for next-leading-order analyses of charged current neutrino production of charm quarks

Abstract: Experimental analyses of charged current deep inelastic charm production\char22{}as observed through dimuon events in neutrino-iron scattering\char22{}measure the strangeness component of the nucleon sea. A complete analysis requires a Monte Carlo simulation to account for experimental detector acceptance effects; therefore, a fully differential theoretical calculation is necessary to provide complete kinematic information. We investigate the theoretical issues involved in calculating these differential distributions at next-leading order (NLO). Numerical results are presented for typical fixed-target kinematics. We present a corresponding FORTRAN code suitable for experimental NLO analysis.

Intrinsic polarized strangeness and $\Lambda ^0$ polarization in deep-inelastic production

Abstract: We propose a model for the longitudinal polarization of \(\Lambda ^0\) baryons produced in deep-inelastic lepton scattering at any \(x_{\mathrm {F}}\), based on static SU(6) quark-diquark wave functions and polarized intrinsic strangeness in the nucleon associated with individual valence quarks. The free parameters of the model are fixed by fitting NOMAD data on the longitudinal polarization of \(\Lambda ^0\) hyperons in neutrino collisions. Our model correctly reproduces the observed dependences of the \(\Lambda ^0\) polarization on the kinematic variables. Within the context of our model, the NOMAD data imply that the intrinsic strangeness associated with a valence quark has anticorrelated polarization. We also compare our model predictions with results from the HERMES and E665 experiments using charged leptons. Predictions of our model for the COMPASS experiment are also presented.

Strangeness Enhancement in the Parton Model

Abstract: Strangeness enhancement in heavy-ion collisions is studied at the parton level by examining the partition of the new sea quarks generated by gluon conversion into the strange and non-strange sectors. The CTEQ parton distribution functions are used as a baseline for the quiescent sea before gluon conversion. By quark counting simple constraints are placed on the hadron yields in different channels. The experimental values of particle ratios are fitted to determine the strangeness enhancement factor. A quantitative measure of Pauli blocking is determined. Energy dependence between CERN-SPS and relativistic heavy-ion collider energies is well described. No thermal equilibrium or statistical model is assumed.

Strangeness production in microscopic transport models

Abstract: Strangeness production in microscopic transport models for relativistic heavy-ion collisions from SIS to RHIC is reviewed: after a brief introduction into elementary strangeness production processes, the main emphasis is put on strangeness as an indicator of the nuclear equation of state, the excitation function of the K+/π+ ratio and strangeness as a deconfinement indicator.

Strangeness in Au + Au collisions at √sNN = 130 GeV observed with the STAR detector

Abstract: The STAR detector has made a variety of measurements of strange and other hadronic species in Au+Au collisions at s N N = 130 GeV. A comparison of kaon and pion production enables an examination of the systematics of strangeness production with energy by comparing them to lower energy collisions. Anti-baryon to baryon ratios indicate a much reduced net-baryon density and transverse momentum spectra show that a picture of transverse expansion seems appropriate.

Strangeness nucleation in neutron star matter

Abstract: We study the transition from npe-type nuclear matter (consisting of neutrons, protons, and electrons) to matter containing strangeness, using a Walecka-type model predicting a first-order kaon-condensate phase transition. We examine the free energy of droplets of K matter as the density, temperature, and neutrino fraction are varied. Langer nucleation rate theory is then used to approximate the rate at which critical droplets of the new phase are produced by thermal fluctuations, thus giving an estimate of the time required for the new (mixed) phase to appear at various densities and various times in the cooling history of the protoneutron star. We also discuss the famous difficulty of ``simultaneous weak interactions'' which we connect to the literature on nontopological solitons. Finally, we discuss the implications of our results to several phenomenological issues involving neutron star phase transitions.