Search

The increase in strangeness production with charged particle multiplicity, as seen by the ALICE collaboration at CERN in p-p, p-Pb and Pb-Pb collisions, is investigated in the hadron resonance gas model taking into account interactions among hadrons using S-matrix corrections based on known phase shift analyses. Strangeness conservation is taken into account in the framework of the canonical strangeness ensemble. A very good description is obtained for the variation of the strangeness content in the final state as a function of the number of charged hadrons in the mid-rapidity region using the same fixed temperature value as obtained in the most central Pb-Pb collisions. It is shown that the number of charged hadrons is linearly proportional to the volume of the system. For small multiplicities the canonical ensemble with local strangeness conservation restricted to mid-rapidity leads to a stronger suppression of (multi-)strange baryons than seen in the data. This is compensated by introducing a global conservation of strangeness in the whole phase-space which is parameterized by the canonical correlation volume larger than the fireball volume at the mid-rapidity. The results on comparing the hadron resonance gas model with and without S-matrix corrections, are presented in detail. It is shown that the interactions introduced by the phase shift analysis via the S-matrix formalism are essential for a better description of the yields data., Comment: 15 pages, 10 figures

We present a systematic study of the strange and multistrange particle production in Pb $+$ Pb collisions at $\sqrt{{s}_{NN}}=2.76$ TeV based on paciae model simulations. Two different mechanisms, namely the single string structure variations and multiple string interactions, are implemented in the simulations. These modifications give rise to an enhancement of the string tension value involved in the Lund string fragmentation framework and generate more strange particles in the hadronic final state. By comparing the simulation results with the ALICE experimental data, it turns out that the inclusion of the variable effective string tension in the paciae model results in an improved agreement between theory and experiment on the strangeness production in Pb $+$ Pb collisions.

We present STAR measurements of strange hadron (KS0, Λ, Λ¯, Ξ−, Ξ¯+, Ω−, Ω¯+, and ϕ) production at midrapidity (|y

Strange particles and hyperfragments in collisions of antiprotons and protons on nuclei have been investigated systematically within a microscopic transport model. The hyperons are produced from the annihilation in antibaryon-baryon collisions and strangeness exchange process in antiproton induced reactions. A coalescence approach is used for constructing the primary hyperfragments in phase space and the statistical model is modified for describing the decay of hyperfragments via evaporating hyperon, neutron, charged particles etc, in which the shell effect, binding energy and root-mean-square radii are taken into account. It is found that the influence of the hyperon-nucleon interaction on the free $\Lambda$ and $\Xi^{-}$ production is negligible. However, the large hyperfragment yields are obvious with the attractive potential. The production of double strangeness hyperfragments are reduced below 1$\mu b$ in comparison to the yields of $\Lambda$-hyperfragments with the cross sections of 0.05-0.1 mb in the antiproton induced reactions on $^{63}$Cu at the incident momenta of 1-5 GeV/c. The light hyperfragments are formed in the dynamical fragmentation process. The energy dependence of hyperfragment formation is weak once the incident energy above the threshold energy for the hyperon production., Comment: 10 pages, 9 figures

The capabilities of the new version of the Li\`ege intranuclear cascade model (labeled $\mathrm{INCL}++6$ from now on) are presented in detail. This new version of INCL is able to handle strange particles, such as kaons and the $\mathrm{\ensuremath{\Lambda}}$ and $\mathrm{\ensuremath{\Sigma}}$ hyperons, and the associated reactions and also allows extending nucleon-nucleon collisions up to about $15\text{--}20$ GeV incident energy. Compared to the previous version, new observables can be studied, e.g., kaon, hyperon, and hypernuclei production cross sections (with the use of a suitable de-excitation code) as well as aspects of kaon-induced spallation reactions. The main purpose of this paper is to present the specific ingredients of the new INCL version and its new features, notably the newly implemented variance reduction scheme. We also compare, for some illustrative strangeness production cases, theoretical results calculated using this version of INCL with experimental data.

Author(s): Steinberg, V; Staudenmaier, J; Oliinychenko, D; Li, F; Erkiner, O; Elfner, H | Abstract: Production of strange hadrons in elementary and heavy-ion reactions is studied with the hadronic transport approach called simulating many accelerated strongly interacting hadrons. The poorly known branching ratios of the relevant hadronic resonances are constrained from the known elementary hadronic cross sections and from invariant mass spectra of dileptons. The constrained model is employed as a baseline to compare to heavy-ion-collision experiments at low energies [Ekin=(1-2)AGeV] and to predict some of the upcoming pion-beam results at the High-Acceptance Di-Electron Spectrometer, which are expected to be sensitive to the resonance properties. The employed vacuum-resonance approach proves to be viable for small systems at these energies, but for large systems additional in-medium effects might be required.

Background: Statistical models are successfully used to describe particle multiplicities in (ultra)relativistic heavy ion collisions. Transport models usually lack the ability to describe special aspects of the results of these experiments, as the fast equilibration and some multiplicity ratios. Purpose: An alternative, unorthodox picture of the dynamics of heavy ion collisions is developed using the concept of Hagedorn states. Method: A prescription of the bootstrap of Hagedorn states respecting the conserved quantum numbers baryon number B, strangeness S, isospin I is implemented into the GiBUU transport model. Results: Using a strangeness saturation suppression factor suitable for nucleon-nucleon collisions, recent experimental data for the strangeness production by the HADES collaboration in Au + Au and Ar + KCl is reasonably well described. The experimentally observed exponential slopes of the energy distributions are nicely reproduced. Conclusions: A dynamical model using Hagedorn resonance states, supplemented by a strangeness saturation suppression factor, is able to explain essential features (multiplicities, exponential slope) of experimental data for strangeness production in nucleus-nucleus collisions close to threshold.

Photoproduction of ${\mathrm{\ensuremath{\Lambda}}}^{*}(1520)$ resonance of spin-parity ${\frac{3}{2}}^{\ensuremath{-}}$ off the proton target is investigated within the Regge framework where the reggeization of the $t$-channel meson exchange is applied for the $K(494)+{K}^{*}(892)+{K}_{2}^{*}(1430)$ exchanges in the Born amplitude. The present model is based on two basic ingredients; one is the minimal gauge prescription for the convergence of the reaction and the other is the role of ${K}_{2}^{*}$ crucial to be consistent with high energy data. Cross sections for total, differential, and photon polarization asymmetry are reproduced without fit parameters and compared with existing data. The LAMP2 and LEPS measurements of the angular distribution of ${K}^{\ensuremath{-}}$ in the ${\mathrm{\ensuremath{\Lambda}}}^{*}(1520)\ensuremath{\rightarrow}{K}^{\ensuremath{-}}p$ decay are investigated and found to be dominated by the decay of ${\mathrm{\ensuremath{\Lambda}}}^{*}$ with helicity $\ifmmode\pm\else\textpm\fi{}3/2$ based on the analysis of the density matrix elements. Detailed discussion on the density matrix elements is given to clarify the analysis of the observable. The reaction mechanism is featured by the dominance of the contact term with $K$ and ${K}_{2}^{*}$ exchanges following in the low energy region. The ${K}^{*}$ exchange appears in a minor role. At high energies beyond ${E}_{\ensuremath{\gamma}}\ensuremath{\approx}5$ GeV the role of ${K}_{2}^{*}$ exchange leads over other exchanges in the reaction process.

The freeze-out conditions in proton-proton collisions at $\sqrt{{s}_{\mathrm{NN}}}=200$, 900, and 7000 GeV have been extracted by fits to the mean hadron yields at midrapidity within the framework of the statistical model of an ideal gas of hadrons and resonances in the grand canonical ensemble. The variation of the extracted freeze-out thermal parameters and the goodness of the fits with $\sqrt{{s}_{NN}}$ are discussed. We find the extracted temperature and baryon chemical potential of the freeze-out surface to be similar in $p+p$ and heavy-ion collisions. On the other hand, the thermal behavior of the strange hadrons is qualitatively different in $p+p$ as compared to $A+A$ collisions. We find an additional parameter accounting for nonequilibrium strangeness production is essential for describing the $p+p$ data. This is in contrast to $A+A$ where the nonequilibrium framework could be successfully replaced by a sequential and complete equilibrium model with an early freeze-out of the strange hadrons.

We study the effect of the chiral symmetry restoration (CSR) on heavy-ion collisions observables in the energy range $\sqrt{s_{NN}}$=3-20 GeV within the Parton-Hadron-String Dynamics (PHSD) transport approach. The PHSD includes the deconfinement phase transition as well as essential aspects of CSR in the dense and hot hadronic medium, which are incorporated in the Schwinger mechanism for the hadronic particle production. We adopt different parametrizations of the nuclear equation of state from the non-linear $\sigma-\omega$ model, which enter in the computation of the quark scalar density for the CSR mechanism, in order to estimate the uncertainty in our calculations., Comment: 22 pages, 19 figures

Azimuthal emission patterns of $K^\pm$ mesons have been measured in Ni + Ni collisions with the FOPI spectrometer at a beam kinetic energy of 1.91 A GeV. The transverse momentum $p_{T}$ integrated directed and elliptic flow of $K^{+}$ and $K^{-}$ mesons as well as the centrality dependence of $p_{T}$ - differential directed flow of $K^{+}$ mesons are compared to the predictions of HSD and IQMD transport models. The data exhibits different propagation patterns of $K^{+}$ and $K^{-}$ mesons in the compressed and heated nuclear medium and favor the existence of a kaon-nucleon in-medium potential, repulsive for $K^{+}$ mesons and attractive for $K^{-}$ mesons.

Spectra of KS0 mesons and Λ hyperons were measured in p + C interactions at 31 GeV/c with the large acceptance NA61/SHINE spectrometer at the CERN SPS. The data were collected with an isotropic graphite target with a thickness of 4% of a nuclear interaction length. Interaction cross sections, charged pion spectra, and charged kaon spectra were previously measured using the same data set. Results on KS0 and Λ production in p + C interactions serve as a reference for the understanding of the enhancement of strangeness production in nucleus-nucleus collisions. Moreover, they provide important input for the improvement of neutrino flux predictions for the T2K long baseline neutrino oscillation experiment in Japan. Inclusive production cross sections for KS0 and Λ are presented as a function of laboratory momentum in intervals of the laboratory polar angle covering the range from 0 up to 240 mrad. The results are compared with predictions of several hadron production models. The KS0 mean multiplicity in production processes 〈nKS0〉 and the inclusive cross section for KS0 production σKS0 were measured and amount to 0.127 ± 0.005 (stat) ± 0.022 (sys) and 29.0 ± 1.6 (stat) ± 5.0 (sys) mb, respectively. © 2014 American Physical Society.

The weak production of Lambdas in nucleon-nucleon scattering is studied in a meson-exchange framework. The weak transition operator for the $NN \to N \Lambda$ reaction is identical to a previously developed weak strangeness-changing transition potential $\Lambda N \to NN$ that describes the nonmesonic decay of hypernuclei. The initial $NN$ and final $YN$ state interaction has been included by using realistic baryon-baryon forces that describe the available elastic scattering data. The total and differential cross sections as well as the parity-violating asymmetry are studied for the reaction $pn \to p\Lambda$. These observables are found to be sensitive to the choice of the strong interaction potential and the structure of the weak transition potential.

We study the propagation of charm quarks, produced from the initial fusion of partons, in an equilibrating quark-gluon plasma which may be formed in the wake of relativistic collisions of gold nuclei. Initial conditions are taken from a self-screened parton cascade model and the chemical equilibration is assumed to proceed via gluon multiplication and quark production. The energy loss suffered by the charm quarks is obtained by evaluating the drag force generated by the scattering with quarks and gluons in the medium. We find that the charm quarks may lose only about 10% of their initial energy in conditions likely to be attained at the Relativistic Heavy Ion Collider, while they may lose up to 40% of their energy while propagating in the plasma created at the Large Hadron Collider. We discuss the implications for signals of the quark-gluon plasma.

We have used the event generator LUCIAE to analyze the data of singly strange particle production in pp, pA, and AA reactions at 200A GeV and compared them with the corresponding experimental data and theoretical results of HIJING, VENUS, and RQMD. The results indicate that for reproducing the NA35 data of AA reactions it requires a somewhat larger s quark suppression factor (s=0.3) and shorter formation time ({tau}=1 fm/c) of produced particles than for reproducing the experimental data of pp and pA (s=0.2 and {tau}=1.5 fm/c). This might be understood intuitively from the relationship between the effective string tension and the violence of collision. However, the NA36 data of the negative multiplicity dependence of {Lambda} in S + Pb reaction at 200 A GeV could not be well reproduced by LUCIAE, VENUS, or RQMD, and the NA36 data prefers model parameters of s=0.2 and {tau}=1.5 fm/c especially, this seems hard to understand and needs further studies. From comparing the NA35 data of the rapidity distribution of {Lambda} in S + Ag reactions to the corresponding results of LUCIAE one knows that the effect of varying the s quark suppression factor from 0.2 to 0.3 is smaller than the effect ofmore » rescattering with respect to strangeness production. {copyright} {ital 1997} {ital The American Physical Society}« less

We show that all central rapidity hadron yields measured in Pb--Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV are well described by the chemical non-equilibrium statistical hadronization model (SHM), where the chemically equilibrated QGP source breaks up directly into hadrons. SHM parameters are obtained as a function of centrality of colliding ions, and we compare CERN Large Hadron Collider (LHC) with Brookhaven National Laboratory Relativistic Heavy Ion Collider (RHIC) results. We predict yields of unobserved hadrons and address anti-matter production. The physical properties of the quark--gluon plasma fireball particle source show universality of hadronization conditions at LHC and RHIC.

Associated strangeness production has been studied in the --> --> p pi(-) reaction at the CERN antiproton facility LEAR using the experimental setup of PS185. Results from two high-statist ...

We examine the implications of the constraints arising due to strangeness conservation on the strangeness production in various existing thermal hadron-gas models. The dependence of strangeness chemical potential {mu}{sub {ital S}} on the baryon chemical potential {mu}{sub {ital B}} and temperature {ital T} is investigated. The incorporation of finite-size, hard-core, repulsive interactions in the thermodynamically consistent description of hot and dense hadron gas alters the results obtained for pointlike particles. We compare results in two extreme alternative cases: (1) {ital K} and {ital K}{sup {asterisk}} mesons are treated as point particles and they can penetrate all volumes occupied by baryons and antibaryons and (2) the volume occupied by the baryons and antibaryons is not accessible to them. We find that the results indeed depend on the assumptions made. Moreover, the anomalous results obtained for the ratios {bar {Xi}}/{Xi} and {bar {Lambda}}/{Lambda} rule out the second possibility. {copyright} {ital 1996 The American Physical Society.}

We study the hadron yield correlation in the combination models in high energy heavy ion collisions. We derive the relationship between the average yields of different hadrons produced in the combination of a system consisting of equal number of quarks and antiquarks. We present the results for the directly produced hadrons as well as those for the final hadrons including the strong and electromagnetic decay contributions. We also study the net quark influence by considering the case when the number of quarks is larger than that of the antiquarks. We make comparison with the data wherever possible.

Measurements of charged pion and kaon production are presented in centrality selected Pb+Pb collisions at 40A GeV and 158A GeV beam energy as well as in semicentral C+C and Si+Si interactions at 40A GeV. Transverse mass spectra, rapidity spectra, and total yields are determined as a function of centrality. The system-size and centrality dependence of relative strangeness production in nucleus-nucleus collisions at 40A GeV and 158A GeV beam energy are derived from the data presented here and from published data for C+C and Si+Si collisions at 158A GeV beam energy. At both energies a steep increase with centrality is observed for small systems followed by a weak rise or even saturation for higher centralities. This behavior is compared to calculations using transport models (ultra-relativistic quantum molecular dynamics and hadron-string dynamics), a percolation model, and the core-corona approach.

We study the strangeness production in antiproton-nucleus collisions at the beam momenta from 200 MeV/c to 15 GeV/c within the Giessen Botzmann-Uehling-Uhlenbeck (GiBUU) transport model. The GiBUU model contains a very detailed description of the underlying antinucleon-nucleon cross sections, in-particular, of the strangeness production channels. We compare our calculations with the bubble chamber data on \Lambda- and K^0_S-production in antiproton-nucleus collisions and with the earlier intranuclear cascade calculations. The contributions of the various partial channels to the hyperon production are reported and systematic differences with experiment are discussed. The possible formation of the bound \Lambda- and \Lambda\Lambda-nucleus systems is also considered. Finally, the results on the \Xi-hyperon production are presented in view of the forthcoming experiments with antiproton beams at FAIR., Comment: 44 pages, 15 figures, (new Figs. 5,9,11,12), accepted in Phys. Rev. C

Strangeness production at the CERN Super Proton Synchrotron energies is studied within the quark-gluon string model. The observed shape of rapidity and transverse mass distributions are reproduced fairly well for both peripheral and central heavy ion collisions. However, for central collisions the model underpredicts strange particle abundances by a factor of about 2:2:4 for ${\mathit{K}}_{\mathit{S}}^{0}$, \ensuremath{\Lambda}, and \ensuremath{\Lambda}\ifmmode\bar\else\textasciimacron\fi{}, respectively. This discrepancy can be considered as a possible manifestation of collective string-string interactions similar to the formation of a color rope. Model predictions for coming experiments with the Pb beam at CERN are given.

Within the framework of the improved isospin dependent quantum molecular dynamics (ImIQMD) model, the dynamics of strangeness (K$^{0,+}$, $\Lambda$ and $\Sigma^{-,0,+}$) production in heavy-ion collisions near threshold energies is investigated systematically, in which the strange particles are considered to be mainly produced by the inelastic collisions of baryon-baryon and pion-baryon. The collisions in the region of supra-saturation densities of the dense baryonic matter formed in heavy-ion collisions dominate the yields of strangeness production. The total multiplicities as functions of incident energies and collision centralities are calculated with the Skyrme parameter SLy6. The excitation function of strangeness production is analyzed and also compared with the KaoS data for the K$^{+}$ production in the reactions $^{12}$C+$^{12}$C and $^{197}$Au+$^{197}$Au., Comment: 4 pages, 4 figures

Identified charged-particle spectra of pi(+/-), K(+/-), p, and (p) over bar at midrapidity (vertical bar y vertical bar < 0.1) measured by the dE/dx method in the STAR (solenoidal tracker at the BNL Relativistic Heavy Ion Collider) time projection chamber are reported for pp and d + Au collisions at root s(NN) = 200 GeV and for Au + Au collisions at 62.4, 130, and 200 GeV. Average transverse momenta, total particle production, particle yield ratios, strangeness, and baryon production rates are investigated as a function of the collision system and centrality. The transverse momentum spectra are found to be flatter for heavy particles than for light particles in all collision systems; the effect is more prominent for more central collisions. The extracted average transverse momentum of each particle species follows a trend determined by the total charged-particle multiplicity density. The Bjorken energy density estimate is at least several GeV/fm(3) for a formation time less than 1 fm/c. A significantly larger net-baryon density and a stronger increase of the net-baryon density with centrality are found in Au + Au collisions at 62.4 GeV than at the two higher energies. Antibaryon production relative to total particle multiplicity is found to be constant over centrality, but increases with the collision energy. Strangeness production relative to total particle multiplicity is similar at the three measured RHIC energies. Relative strangeness production increases quickly with centrality in peripheral Au + Au collisions, to a value about 50% above the pp value, and remains rather constant in more central collisions. Bulk freeze-out properties are extracted from thermal equilibrium model and hydrodynamics-motivated blast-wave model fits to the data. Resonance decays are found to have little effect on the extracted kinetic freeze-out parameters because of the transverse momentum range of our measurements. The extracted chemical freeze-out temperature is constant, independent of collision system or centrality; its value is close to the predicted phase-transition temperature, suggesting that chemical freeze-out happens in the vicinity of hadronization and the chemical freeze-out temperature is universal despite the vastly different initial conditions in the collision systems. The extracted kinetic freeze-out temperature, while similar to the chemical freeze-out temperature in pp, d + Au, and peripheral Au + Au collisions, drops significantly with centrality in Au + Au collisions, whereas the extracted transverse radial flow velocity increases rapidly with centrality. There appears to be a prolonged period of particle elastic scatterings from chemical to kinetic freeze-out in central Au + Au collisions. The bulk properties extracted at chemical and kinetic freeze-out are observed to evolve smoothly over the measured energy range, collision systems, and collision centralities.

We discuss the open charm production in the peripheral reactions $\overline{p}p\ensuremath{\rightarrow}{\overline{Y}}_{c}{Y}_{c}$ and $\overline{p}p\ensuremath{\rightarrow}{M}_{c}{\overline{M}}_{c}$, where ${Y}_{c}$ and ${M}_{c}$ stand for ${\ensuremath{\Lambda}}_{c}^{+},{\ensuremath{\Sigma}}_{c}^{+}$ and $D,{D}^{*}$, respectively, at $\sqrt{s}\ensuremath{\lesssim}15$ GeV, which corresponds to the energy range of FAIR. Our consideration is based on the topological decomposition of the planar quark and diquark diagrams, which allows us to estimate consistently meson and baryon exchange trajectories and energy scale parameters as well. The spin dependance is determined by the effective interaction of the lowest exchanged resonance. Unknown parameters are fixed by an independent analysis of open strangeness production in $\overline{p}p\ensuremath{\rightarrow}\overline{Y}Y$ and $\overline{p}p\ensuremath{\rightarrow}\overline{K}K$ reactions and of SU(4) symmetry. We present the corresponding cross sections and longitudinal double-spin asymmetries for exclusive binary reactions with open charm mesons and baryons in the final state. The polarization observables have a nontrivial $t$ and $s$ dependence that is sensitive to details of the open charm production mechanism.

First measurement of subthreshold {sigma}(1385) production is presented. Experimental data are presented for Al+Al reactions at 1.9A GeV measured with the FOPI detector at SIS/GSI. The {sigma}(1385)/{lambda} ratio is found to be in good agreement with the transport and statistical model predictions. The results allow for a better understanding of subthreshold strangeness production and strangeness exchange reaction which is the dominant process for K{sup -} production below and close-to threshold.

We study, in the dynamically evolving quark-gluon plasma (QGP) fireball formed in relativistic heavy ion collisions at the BNL Relativistic Heavy Ion Collider (RHIC) and CERN Large Hadron Collider (LHC), the growth of strangeness yield toward and beyond the chemical equilibrium. We account for the contribution of the direct strangeness production and evaluate the thermal-QCD strangeness production mechanisms. The specific yield of strangeness per entropy, $s/S$, is the primary target variable. We explore the effect of collision impact parameter, i.e., fireball size, on kinetic strangeness chemical equilibration in QGP. Insights gained in studying the RHIC data with regard to the dynamics of the fireball are applied to the study of strangeness production at the LHC. We use these results and consider the strange hadron relative particle yields at RHIC and LHC in a systematic fashion. We consider both the dependence on $s/S$ and the direct dependence on the participant number.

Strange particle ratios and ${p}_{\ensuremath{\perp}}$ spectra are calculated in a thermal model with single freeze-out, previously used successfully to describe nonstrange particle production at RHIC. The model and the recently released data for $\ensuremath{\varphi},$ $\ensuremath{\Lambda},$ $\overline{\ensuremath{\Lambda}},$ and ${K}^{*}{(892)}^{0}$ are in very satisfactory agreement, showing that the thermal approach can be used to describe the strangeness production at RHIC.

The strangeness content of hadronic matter is studied in a string-flip model that reproduces various aspects of the QCD-inspired phenomenology, such as quark clustering at low density and color deconfinement at high density, while avoiding long range van der Waals forces. Hadronic matter is modeled in terms of its quark constituents by taking into account its internal flavor (u,d,s) and color (red, blue, green) degrees of freedom. Variational Monte-Carlo simulations in three spatial dimensions are performed for the ground-state energy of the system. The onset of the transition to strange matter is found to be influenced by weak, yet not negligible, clustering correlations. The phase diagram of the system displays an interesting structure containing both continuous and discontinuous phase transitions. Strange matter is found to be absolutely stable in the model., 14 pages, 1 table, 8 eps figures, revtex. Submitted to Phys. Rev. C, Presented at INPC2001 Berkeley, Ca. july 29-Aug 3

Using a relativistic transport model for heavy ion collisions at energies that are below the threshold for kaon and antikaon production in nucleon-nucleon collisions, we study how their abundances approach the canonical equilibrium during the collisions. We find that kaons are far from chemical equilibrium at the initial and high density stage, and they approach equilibrium only during the expansion stage of the collisions when their production rate is small and becomes comparable to their annihilation rate. In contrast, antikaons approach chemical equilibrium much earlier but eventually fall out of equilibrium again as a result of their large annihilation cross sections in nuclear matter., 4 pages, Revtex, 3 figs incl

A systematic study is performed of fully integrated particle multiplicities in central Au-Au and Pb-Pb collisions at beam momenta of $1.7A \mathrm{GeV}/c, 11.6A \mathrm{GeV}/c$ (Au-Au), and $158A \mathrm{GeV}/c$ (Pb-Pb) by using a statistical-thermal model. The close similarity of the colliding systems makes it possible to study heavy ion collisions under definite initial conditions over a range of center-of-mass energies covering more than 1 order of magnitude. In order to further study the behavior of strangeness production, an updated study of Si-Au collisions at $14.6A \mathrm{GeV}$ is also presented. The data analysis has been performed with two completely independent numerical algorithms giving closely consistent results. We conclude that a thermal model description of particle multiplicities, with additional strangeness suppression, is possible for each energy. The degree of chemical equilibrium of strange particles and the relative production of strange quarks with respect to u and d quarks are higher than in ${e}^{+}{e}^{\ensuremath{-}},$ $pp,$ and $p\overline{p}$ collisions at comparable and even at lower energies. The behavior of strangeness production as a function of center-of-mass energy and colliding system is presented and discussed. The average energy per hadron in the comoving frame is close to 1 GeV per hadron despite the fact that the energy increases more than tenfold.

We investigate the pp --> p Lambda K+ and pp --> p Sigma0 K+ reactions at beam energies near their thresholds within an effective Lagrangian model, where the strangeness production proceeds via the excitation of N*(1650), N*(1710), and N*(1720) baryonic resonances. It is found that the $N^*$(1650) resonance dominates both these reactions at near threshold energies. The contributions from this resonance together with the final state interaction among the outgoing particles are able to explain the observed beam energy dependence of the ratio of the cross sections of the two reactions in the near threshold region.

The measured K{sup +}/{pi}{sup +} ratios from heavy-ion reactions are compared with the K{sup +}/{pi}{sup +} ratios from p+p reactions over the energy range 2-160 A GeV. The K/{pi} enhancement in heavy-ion reactions is largest at the lower energies, consistent with strangeness production in secondary scattering becoming relatively more important than initial collisions near the kaon production threshold. The enhancement decreases steadily from 4 to 160 A GeV, suggesting that the same enhancement mechanism of hadronic rescattering and decay of strings may be applicable over this full energy range. Based on existing data, the midrapidity K{sup +}/{pi}{sup +} ratio is predicted to be 0.25{+-}0.02 for the forthcoming Pb+Pb reactions at 40 A GeV/c. (c) 2000 The American Physical Society.

In this Reply we argue that the factor of 4 enhancement of the local midrapidity densities of {Lambda} and the strong suppression of {Lambda} in the projectile fragmentation region in {ital p}+S relative to {ital p}+{ital p} reactions provide strong evidence of nonequilibrium dynamics in strangeness production. Second, we show that the dramatic changes in the {Lambda} rapidity distributions in {ital pA} cannot be attributed to nuclear baryon stopping power. {copyright} {ital 1996 The American Physical Society.}