Strange mesons in dense nuclear matter

Experimental data on the production of kaons and antikaons in heavy-ion collisions at relativistic energies are presented and discussed with respect to in-medium effects. The K ^?/K⁺ ratios measured in nucleus-nucleus collisions are 1–2 orders of magnitude larger than in proton-proton collisions. The azimuthal angle distributions of K ⁺ mesons indicate a repulsive kaon-nucleon potential. Microscopic transport calculations consistently explain both the yields and the emission patterns of kaons and antikaons when assuming that their properties are modified in dense nuclear matter. The K ⁺ production excitation functions measured in light and heavy collision systems provide evidence for a soft nuclear equation-of-state.     

K + production in proton and deuteron induced reactions at subthreshold energies

The production ofK ⁺ mesons in proton-nucleus and deuteron-nucleus collisions is analyzed with respect to one-step nucleon-nucleon (NN → N ΛK ⁺),Δ- nucleon (ΔN → N Λ K⁺) and two-step pion-nucleon (πN → K ⁺ Λ) production channels on the basis of experimental ground state momentum distributions and free on-shell production processes. Whereas forK ⁺ production in proton-nucleus reactions the secondary channelπN clearly dominates at subthreshold energies, meson and nucleon induced channels are of similar magnitude in deuteron-nucleus reactions. Contrary to nucleus-nucleus collisions theΔ induced reaction channels are found to be of minor importance. The experimental differentiation of the underlying microscopic reaction channels appears possible via differential proton —K ⁺ coincidence measurements as shown in detail by the microscopic simulations including proton rescattering.     

K +-meson production inpBe interactions atT p=2.9 GeV

The production ofK ⁺ andπ ⁺ mesons and protons inpBe collisions atT _p=2.9 GeV has been studied at the ITEP proton synchrotron. Ejectiles with a momentum ofp=545 MeV/c were observed under an emission angle ?=17°. The detectors which have been developed for the identification of kaons out of a six orders of magnitude more intense background of pions and protons are described. A cross-section ratio d²σ _K ⁺/dΩdp: d² σ _p /dΩdp: d²σ _p /dΩdp of (1±0.34):(85±1):(31±1) has been measured. Normalization with existing pion data yields an invariant differential cross sectionE·d³σ _K ⁺/d³ p=(3.1±1.2) mbGeV^?2c³sr^?1 and a total cross section of σ_tot(pBe)=(3.7±1.5) mb. These cross sections are compared with existing data and theoretical predictions. TheA dependence ofK ⁺ production in the few-GeV range is analyzed.     

Azimuthal Distributions of K + Mesons in Heavy-Ion Collisions

Strange mesons are considered to be sensitive to in-medium modifications. Theory predicts a repulsive K ⁺ N potential and an attractive K ^? N potential in dense matter. A repulsive K ⁺ N potential would repel the K ⁺ mesons from the bulk of the nucleons and therefore cause a preferred out-of-plane emission of K ⁺ mesons at midrapidity and a directed flow opposite to the nucleons at target and projectile rapidity. One of observables to probe in-medium effects is the azimuthal emission pattern of K ⁺ mesons in heavy ion collisions. KaoS collaboration has measured the azimuthal distributions of K ⁺ mesons in Au + Au reactions at 1.5 A GeV and Ni + Ni reactions at 1.93 A GeV. Data show that K ⁺ mesons exhibit a pronounced enhancement at ${\phi = {90}^\circ}$ , i.e. perpendicular to the reaction plane. The data have also been fitted using the first two components of a Fourier series to get the directed flow v ₁ and elliptic flow v ₂. We used the quantum molecular dynamics model based on the covariant kaon dynamics to simulate the Au + Au collisions at 1.5 A GeV and the Ni + Ni collisions at 1.93 A GeV, to analyze the azimuthal distributions of K ⁺ mesons, and to calculate v ₁ and v ₂ of K ⁺ mesons. Calculated results with a repulsive in-medium K ⁺ N potential can reasonably describe the features of KaoS data. This indicates that the azimuthal distribution is one of sensitive probes to extract information on in-medium properties at high densities.     

In-Medium Kaon Potential and Nuclear Equation of State Measured in Nucleus–Nucleus Collisions

The kaon production in heavy ion collisions at intermediate energies provides a sensitive probe to study the in-medium properties and nuclear equation of state of hadrons. Properties of kaons in dense hadronic matter are important for a better understanding of both, the possible restoration of chiral symmetry in dense hadronic matter and the properties of nuclear matter at high densities. We investigated the in-medium kaon potential and nuclear equation of state by transverse mass spectra of K ⁺ mesons in heavy ion collisions. We use quantum molecular dynamics (QMD) models based on covariant kaon dynamics to simulate ${_{28}^{58}Ni +_{28}^{58}Ni}$ collisions at 1.93 A GeV, to analyze the transverse mass spectra of K ⁺. Calculated results with a repulsive in-medium K ⁺N potential can reasonably describe the features of KaoS data. They also shown that the transverse mass spectrum of K ⁺ mesons is a sensitive observable to probe the kaon in-medium potential in dense nuclear matter.     

Features of carbon-carbon interactions at a momentum of 4.2 GeV/c per nucleon that are accompanied by the production of Λ hyperons and K S 0 mesons versus the degree of nuclei collision centrality

Experiemntal data obtained by using the 2-m propane bubble chamber of the Joint Institute for Nuclear Research (JINR, Dubna) are used to study the effect collision centrality on the spectra of Λ hyperons and K _S ⁰ mesons produced in carbon-carbon interactions at 4.2 A GeV/c. The multiplicity of participant protons having momenta in excess of 300 MeV/c is taken to be a measure of collision centrality. The features of pions and protons accompanying strange-particle production are also presented. The experimental data in question are compared with the prediction of a modified version of the FRITIOF model. It is shown that strange particles are predominantly produced in central and semicentral collisions. The average kinematical features of K _S ⁰ mesons are found to be indepedent of collision centrality. At the same time, the average transverse momentum of Λ hyperons and the average value of their emission angle increase slowly with increasing degree of collision centrality. The anisotropy of the angular distributions of both Λ hypersons and K _S/0 mesons in the c.m. frame of nucleon-nucleon collisions decreases with increasing collision centrality. The average transverse momentum of K _S ⁰ mesons is approximately 1.6 times higher than the average transverse momentum of π ^? mesons.     

New method of studying slow strange meson properties in nuclear matter

We suggest a new experimental method to explore the properties of slow strange mesons at normal nuclear matter density. We show that the K⁺ and K^? mesons with extremely small momenta relative to the surrounding medium rest frame can be produced in nucleus-nucleon collisions, and their production cross-sections are experimentally measurable. Experiments on the study of the momentum dependence of meson-nuclear potentials are discussed.     

Strange particles production and absorption in nuclei at high energies

The absorption of the secondary hadrons in nuclear matter is studied as a function of their flavour dependent inelastic cross section on nucleons. In particular the ratiosK ⁺/π⁺ andK ^?/π^? of particles produced in the proton-nucleus high energy interactions are examined and described in the very simple phenomenological model. The enhanced production ofK ⁺ mesons is explained by their relatively small interaction. The presence of the initial particle absorption is clearly demonstrated forx _F>0.5.     

The effect of Lorentz-like force on collective flows of <Emphasis Type="Italic">K</Emphasis><Superscript>+</Superscript> in Au+Au collisions at 1.5 GeV/nucleon

Producing kaon mesons in heavy-ion collisions at beam energies below their threshold energy is an important way to investigate the properties of dense nuclear matter. In this study, based on the newly updated version of the ultrarelativistic quantum molecular dynamics model, we introduce the kaon-nucleon (KN) potential, including both the scalar and vector (also dubbed Lorentz-like) aspects. We revisit the influence of the KN potential on the collective flow of K⁺ mesons produced in Au+Au collisions at E_lab = 1.5 GeV/nucleon and find that the contribution of the newly included Lorentz-like force is very important, particulary for describing the directed flow of K⁺. Finally, the corresponding KaoS data of both directed and elliptic flows can be simultaneously reproduced well.     

Study of multiparticle azimuthal correlations in central CNE, MgMg, CCu, and OPb interactions at energy of 3.7 GeV per nucleon

Azimuthal correlations between protons and between pions have been investigated in central CNe, MgMg, CCu, and OPb collisions at an energy of 3.7 GeV/nucleon. Negative (back-to-back) correlations have been observed for protons in CNe, CCu, and for π ^? mesons in CNe and MgMg collisions. For π^? mesons, positive (side-by-side) azimuthal correlations have been observed for heavy systems of CCu and OPb. The Quark-Gluon String Model satisfactorily describes the experimental results both for protons and π^? mesons.     

Experimental study of the subthreshold production of K +-mesons in proton-nucleus collisions

Energy dependence of the differential cross sections for the production of K ⁺-mesons with a momentum of 1.28 GeV/c (c is the speed of light) by protons incident on Be, Al, Cu, and Ta nuclei was measured for energies both above and below the K ⁺-meson production threshold in pp collisions. Evidence is given for the dominance of the mechanism of direct production in the experiment. The characteristics of momentum distribution are determined for nucleons in the Be and Al nuclei up to 650 MeV/c. The data cannot be described in the model of nuclear spectral function.     

Kaon production in S+nucleus and Pb+Pb collisions at CERN SPS

Charged and neutral kaon production in S-nucleus and Pb-Pb collisions has been measured at CERN in the NA35 and NA49 experiments, close to mid-rapidity. The production ofK ^± in S+S, Ag, Au at 200 GeV/nucleon was measured via their one-prong (kink) topology inside a Time Projection Chamber (TPC). The resulting charged kaon yields are in agreement with other measurements done in the same experiment. TheK ⁺/K ^? ratio was found to be 1.30±0.15 in central S+Au interactions. The production ofK _S ⁰ in Pb+Pb collisions was measured using the NA49’s large (Main) TPCs outside the magnetic field. The resulting yields are compatible with other measurements performed in the same experiment, within the quoted systematic errors. The preliminary rapidity distribution in Pb+Pb scales with the number of participants if compared to S+S collisions. From this we conclude that no further strangeness enhancement (relative to nucleon-nucleon collisions) is observed in going from S+S to Pb+Pb.     

Kaon and pion ratio probes of jet quenching in nuclear collisions

Non-Abelian energy loss in quark gluon plasmas is shown to lead to novel hadron ratio suppression patterns in ultrarelativistic nuclear collisions. Here we investigate pion and kaon production in pp and AA collisions in a perturbative QCD frame, suppression pattern and hadron ratios. The K^?/K⁺ and K⁺/π⁺ ratios are found to be most sensitive to the opacity (density) of the plasma. Experimental data indicate that the fragmentation dominated pQCD region will be reached only at higher p _T; in an intermediate p _Tregion other particle production mechanisms dominate the K/π ratios.     

K + production in proton-nucleus collision

We present a dynamical study ofK ⁺ production in proton-nucleus collisions from 1.2 to 2.5 GeV bombarding energy. The evolution of the proton-nucleus collision is described by a transport equation of the Boltzmann-Uehling-Uhlenbeck type. We incorporate all known sources forK ⁺ production and study their momentum and angular distributions, and the excitation function. We show that at lower energies (E _b<1.5 GeV) theNΔ andNN* channels dominate the kaon yield for light systems. At higher bombarding energies the directNN channel accounts for almost the whole cross section.     

Subthreshold production ofK + mesons and the fragmentation of the target nucleus

The interplay between subthresholdK ⁺-production and the fragmentation of the target nucleus is considered in proton-nucleus interactions at projectile energies between 0.8 and 1 GeV. Inclusive data for the production ofK ⁺ mesons and of fragments are well described in the framework of the modified phase space model. For a coincidence measurement betweenK ⁺ mesons and fragments the model predicts a decrease of the relative yield of intermediate mass fragments by about four orders of magnitude. This strong decrease is a consequence of energy-momentum conservation, because at subthreshold energies a second endothermic process like multifragmentation can hardly proceed simultaneously with theK ⁺ production. AK ⁺ meson is, therefore, expected to be accompanied by a heavy target residue, which is so weakly excited that it may evaporate a few nucleons but cannot decay into fragments. Due to the large energy and momentum transfer in this process the strength of the predicted effect is supposed to be sensitive to the details of the reaction dynamics.The authors would like to thank H.W. Barz, F. Dönau and O. Schult for many valuable comments and discussions.     

Eta and omega meson production in medium energy π+-p and K−p interactions

The production of η(550) and ω(780) mesons is studied in π^±p interactions at 16 GeV/c and K^?p interactions at 10 and 16 GeV/c. Cross sections for exclusive channels are presented, and for the π⁺p data differential cross sections are given for quasi-inclusive production where the η or ω is required to be accompanied by charged particles only. Close similarities are observed between η, ω and also ?⁰(770) production in terms of longitudinal and transverse variables. By a rough estimate, the η, ω and ?⁰ inclusive yields are found to be in the ratio 0.32 : 0.85 : 1, respectively, for 16 GeV/cπ⁺p collisions. For non-peripheral production we estimate this same ratio to be 0.34 : 0.9 : 1.     

Production properties ofD 0,D +,D *+ andD s + in 230 GeV/cπ − andK −-Cu interactions

We have studied the hadronic production of charmed mesons in the NA 32 experiment at CERN. A special trigger together with a high resolution vertex detector consisting of charge coupled devices and silicon microstrip detectors allowed the selection of very clean samples of charmed mesons. We have collected 852 fully reconstructed decays: 60D _s ⁺ →K ⁺ K ^?π⁺, 543D°→K ^?π⁺ andK ^?π⁺π^?π⁺ as well as 249D ⁺→K ^?π⁺π⁺ (or charge conjugate). 147 mesons out of our \({{D^0 } \mathord{\left/ {\vphantom {{D^0 } {\bar D^0 }}} \right. \kern-0em} {\bar D^0 }}\) sample were produced via chargedD ^* state. For all charmed mesons we determine the total production cross-section and study thex _F andp _t ² distributions.