SIS Energies Research Articles

Kaon production in heavy-ion collisions and kaon condensation in neutron stars

The recent past witnesses the growing interdependence between the physics of hadrons, the physics of relativistic heavy-ion collisions, and the physics of compact objects in astrophysics. A notable example is the kaon which plays a special roles in all the three fields. In this contribution, we present a transport model study of kaon production and flow in heavy-ion collisions at SIS energies. We also discuss the effects of the kaon in-medium properties extracted from heavy-ion data on neutron star properties.

Influence of impact parameter on thermal description of relativistic heavy ion collisions at(1–2)A GeV

Attention is drawn to the role played by the size of the system in the thermodynamic analysis of particle yields in relativistic heavy ion collisions at SIS energies. This manifests itself in the non-linear dependence of K+ and K- yields in $AA$ collisions at 1 -- 2 A.GeV on the number of participants. It is shown that this dependence can be quantitatively well described in terms of a thermal model with a canonical strangeness conservation. The measured particle multiplicity ratios (pi+/p, pi-/pi+, d/p, K+/pi+ and K+/K- but not eta/pi0) in central Au-Au and Ni-Ni collisions at 0.8 -- 2.0 A.GeV are also explained in the context of a thermal model with a common freeze-out temperature and chemical potential. Including the concept of collective flow a consistent picture of particle energy distributions is derived with the flow velocity being strongly impact-parameter dependent.

Chiral Kaon dynamics in heavy ion collisions

The influence of the chiral mean field on the collective motion of kaons in relativistic heavy ion reactions at SIS energies is investigated. We consider three types of collective motion, i.e. the transverse flow, the out-of-plane flow (squeeze-out) and the radial flow, The kaon dynamics is thereby described with a relativistic mean field as it originates form chiral lagrangiens. For the K mesons inside the nuclear medium we adopt a covariant quasi-particle picture including scalar and vector fields and compare this to a treatment with a static potential like force. The comparison to the available data ( K +) measured by FOPI and KaoS strongly favor the existence of an in-medium potential. However, using full covariant dynamics makes it more difficult to describe the data which might indicate that the mean field level is not sufficient for a reliable description of the kaon dynamics.

Lambda collective flow in heavy-ion reactions

Collective flow of lambda hyperons in heavy-ion reactions at SIS energies is investigated. It is found that a Λ mean field constructed on the basis of the quark model leads to a good description of the experimental data of the in-plane transverse flow of Λ's. The attractive mean field can also give rise to an additional “virtual” Λ radial flow directed inwards, which is reflected by a “concave” structure of the transverse mass spectrum of the Λ hyperons emitted at midrapidity. The Λ radial flow is found to exhibit a strong mass dependence: The flow is visible in the Ni + Ni system, but is strongly reduced in the system of Au on Au.

Strangeness production and propagation in relativistic heavy ion collisions at SIS energies

Strangeness production at incident energies close to the elementary production threshold is considered a tool to extract information about the properties of the hot and dense state of nuclear matter produced as a transient state in heavy ion collisions. Production yields and azimuthal anisotropies of strange particles (K +,K −,π and Φ) are measured with the FOPI detector at SIS. The distributions are examined with respect to their significance of possible in-medium modifications.

Strange Mesons as a probe for dense nuclear matter

The production and propagation of kaons and antikaons has been studied in symmetric nucleus-nucleus collisions in the SIS energy range. The ratio of the excitation functions of K + production in Au+Au and C+C collisions increases with decreasing beam energy. This effect was predicted for a soft nuclear equation-of-state. In non-central Au+Au collisions, the K+ mesons are preferentially emitted perpendicular to the reaction plane, The K −/K + ratio from A+A collisions at beam energies which are equivalent with respect to the threshold is found to be about two orders of magnitude larger than the corresponding ratio from proton-proton collisions. Both effects are considered to be experimental signatures for a modification of kaon properties in the dense nuclear medium.

Strangeness production in the HSD transport approach from SIS to SPS energies

We study systematically the production of strangeness in nuclear reactions from SIS to SPS energies within the covariant hadronic transport approach HSD. Whereas the proton and pion rapidity distributions as well as pion transverse momentum spectra are well described in the hadronic transport model from 2–200 A GeV, the K + and K − spectra are noticeably underestimated at AGS energies while the K + spectra match well at SIS and SPS energies with the experimental data. We conclude that the failure of the hadronic model at AGS energies points towards a non-hadronic phase during the collision of heavy systems around 10 A GeV.

Baryon flow at SIS energies and beyond

We calculate the baryon flow 〈 P x A(y) 〉 in the energy range from 0.25 to ⩽ 4.0A GeV in a relativistic transport model for Ni + Ni and Au + Au collisions employing various models for the baryon self-energies. We find that to describe the flow data of the FOPI Collaboration above 1A GeV the strength of the vector potential has to be reduced at high relative momentum or at high density. The latter phenomenon leads to a softening of the nuclear equation of state at high density.

Radial flow of kaon mesons in heavy ion reactions

This work investigates the collective motion of kaons in heavy ion reactions at SIS energies (about 1--2 GeV/nucleon). A radial collective flow of ${K}^{+}$ mesons is predicted to exist in central Au + Au collisions, which manifests in a characteristic ``shoulder-arm'' shape of the transverse mass spectrum of the midrapidity ${K}^{+}$ mesons. The ${K}^{+}$ radial flow arises from the repulsive ${K}^{+}$ mean field in nuclear matter. In spite of a strong reabsorption and rescattering the attractive ${K}^{\ensuremath{-}}$ mean field leads as well to a collective radial flow of ${K}^{\ensuremath{-}}$ mesons. The ${K}^{\ensuremath{-}}$ radial flow, however, is different from that of ${K}^{+}$ mesons and can be observed by a characteristic ``concave'' structure of the transverse mass spectrum of the ${K}^{\ensuremath{-}}$ mesons emitted at midrapidity. The kaon radial flows can therefore serve as a novel tool for the investigation of kaon properties in dense nuclear matter.

Do extreme conditions of nuclear matter influence particle properties? summary of the HADES project

Heavy ion collisions at beam energies of 1–2 GeV/u (SIS at GSI, Darmstadt) offer a unique possibility to investigate the properties of hot, compressed nuclear matter. Baryon densities of 2–3ρ 0 and temperature up to 100 MeV can be reached during a collision phase. Due to a high density of the phase new particles are produced in multiple baryon-baryon collisions. Objects decaying into electron-positron pairs play an important role in this context. In contrast to hadrons, dileptons carry away undistorted by final-state interactions information about the hot and dense phase. The two-body decays of the vector mesons ρ, ω and ϕ are of special interest since width and masses of the resonances are accessible in the dielectron mass spectra. A High Acceptance DiElectron Spectrometer (HADES) has been proposed for the SIS accelerator at GSI in Darmstadt and currently is constructed by the HADES Collaboration in many places of Europe. A Ring Imaging Cerenkov Counter (RICH), blind for hadrons, together with Multiplicity Electron Trigger Array (META) containing Shower detector and Time-of-Flight wall will serve for the electron identification. The momentum of dileptons will be determined in 2 sets of Mini-Drift Chambers (MDC) placed in front of and behind a six-fold super-conducting coil. An expected invariant mass resolution in the ρ/ω region less than 1% (σ) assures a clear separation of the ω and ϕ from ρ. For the difficult case of the 1 GeV/u Au+Au collision we expect a 10:1 signal-to-background ratio at the ρ/ϕ region. The performance of the spectrometer was studied by detailed simulations using GEANT. Results of simulations and tests of prototypes assure that with its high counting rate capability and large geometrical acceptance HADES will be able to measuree + e − pairs for the heaviest systems at the highest SIS energies. First results are expected in 1999.

Coulomb effects on particle spectra in relativistic nuclear collisions

Coulomb effects on ${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}$ and ${K}^{\ifmmode\pm\else\textpm\fi{}}$ spectra in relativistic nuclear collisions are investigated. At collision energies around 1 GeV the ratio of ${\ensuremath{\pi}}^{\ensuremath{-}}$ to ${\ensuremath{\pi}}^{+}$ is enhanced several times at low transverse momenta but less at ultrarelativistic energies. We describe the ratios at SIS, AGS, and SPS energies with simple analytic models as well as more elaborate numerical models incorporating the expansion dynamics. The Coulomb effect depends on the properties of the source after the violent collision phase and provides information on source sizes, freeze-out times, and expansion velocities. Comparison with results from HBT analyses are made. Predictions for ${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}$ and ${K}^{\ifmmode\pm\else\textpm\fi{}}$ at RHIC and LHC energies are given.

Meson mT-scaling in heavy-ion collisions at SIS energies

We perform systematical studies on transverse-mass spectra of $\pi^0$, $\eta$, $\omega$, $\phi$, $K^+$ and $K^-$ mesons for C + C, Ni + Ni, Au + Au collisions at SIS energies within the HSD transport approach. We find that the $m_T$-spectra sensitively reflect the in-medium properties of the mesons. The 'bare mass' scenario leads to a general scaling behaviour for the meson $m_T$-spectra when including a mass shift due to the associate strangeness production threshold for kaons and antikaons whereas a 'dropping' mass scheme violates this $m_T$-scaling. The relative slope parameters, however, provide valuable information about the average meson potentials in the nuclear medium.

Nucleus-nucleus collisions in the SIS energy regime

Nucleus-nucleus collisions in the SIS energy regime

Kaons in dense matter, kaon production in heavy-ion collisions, and kaon condensation in neutron stars

The recent past witnesses the growing interdependence between the physics of hadrons, the physics of relativistic heavy-ion collisions, and the physics of compact objects in astrophysics. A notable example is the kaon which plays special roles in all the three fields. In this paper, we first review the various theoretical investigations of kaon properties in nuclear medium, focusing on possible uncertainties in each model. We then present a detailed transport model study of kaon production in heavy-ion collisions at SIS energies. We shall discuss especially the elementary kaon and antikaon production cross sections in hadron-hadron interactions, that represent one of the most serious uncertainties in the transport model study of particle production in heavy-ion collisions. The main purpose of such a study is to constrain kaon in-medium properties from the heavy-ion data. This can provide useful guidances for the development of theoretical models of the kaon in medium. In the last part of the paper, we apply the kaon in-medium properties extracted from heavy-ion data to the study of neutron star properties. Based on a conventional equation of state of nuclear matter that can be considered as one of the best constrained by available experimental data on finite nuclei, we find that the maximum mass of neutron stars is about 2 M ⊙, which is reduced to about 1.5 M ⊙ once kaon condensation as constrained by heavy-ion data is introduced.

Phi meson production in heavy-ion collisions at SIS energies

Phi meson production in heavy-ion collisions at SIS/GSI energies (∼ 2 GeV/nucleon) is studied in the relativistic transport model. We include contributions from baryon-baryon, pion-baryon, and kaon-antikaon collisions. The cross sections for the first two processes are obtained in a one-boson-exchange model, while that for the last process is taken to be of Breit-Wigner form through the phi meson resonance. The dominant contribution to phi meson production in heavy-ion collisions at these energies is found to come from secondary pion-nucleon collisions. Effects due to medium modifications of kaon masses, which are needed to account for the observed enhancement of antikaon yield, are also studied and are found to reduce the phi meson yield by about a factor of two, mainly because of an increased phi decay width as a result of dropping kaon-antikaon masses. In this case, the ø K − ratio is about 4%, which is a factor of 2–3 below preliminary experimental data from the FOPI collaboration at GSI. Including also the reduction of the phi meson mass in medium increases this ratio to about 8%, which is then in reasonable agreement with the data.

Analysis of kaon production at SIS energies

We analyse the production and propagation of pions and kaons in heavy-ion reactions from 0.8–1.8 A GeV within a coupled channel transport approach including the kaon production channels BB → K + YN, πB → K + Y, BB → NNK K , πB → NK K , K + B → K + B, and ππ → K K . Assuming the hyperon self-energy to be 2 3 of the nucleon self-energy we find that all inclusive experimental K + spectra at SIS energies can be reproduced reasonably well without any self-energies for the kaons although a slightly repulsive kaon potential cannot be excluded by the present data on kaon spectra and flow.

Kaon versus antikaon production at SIS energies

We analyse the production and propagation of kaons and antikaons in Ni + Ni reactions from 0.8–1.85 GeV/u within a coupled channel transport approach including the channels BB → K + YN, πB → K + Y, BB → NNK K , πB → NK K , K + B → K + B, KB → KB , YN → KNN , ππ → K K as well as πY → KN and KN → πY for the antikaon absorption. Whereas the experimental K + spectra can be reproduced without introducing any self-energies for the mesons in Ni + Ni collisions from 0.8 to 1.8 GeV/u, the K − yield is underestimated by a factor of 5–7 at 1.66 and 1.85 GeV/u. However, introducing density dependent antikaon masses in line with effective chiral Lagrangians, the antikaon spectra can be reasonably well described.

Pion-production in heavy-ion collisions at SIS energies

Pion-production in heavy-ion collisions at SIS energies

Particle production and flow at SIS energies

An overview is given over recent measurement of flow and particle production in the energy range from 0.1 to 2 AGeV. Excitation functions for the directed sideward and the azimuthally symmetric transverse flow are presented and show the importance of flow phenomena in this incident energy regime. Rapidity density distributions are indicative of a system size dependence of the stopping process. The role of strange particles as a probe for the hot and dense phase of hadronic matter is discussed with respect to the production and propagation. The spectra of Kaons indicate an equilibration with the surrounding baryons during the expansion while their directed flow pattern is different from that of the nucleons.

Pion-production in heavy-ion collisions at SIS energies

We investigate the production of pions in heavyion collisions in the energy range of 1–2 GeV/A. The dynamics of the nucleus-nucleus collisions is described by a set of coupled transport equations of the Boltzmann-Uehling-Uhlenbeck type for baryons and mesons. Besides theN(938) and theΔ(1232) we also take into account nucleon resonances up to masses of 1.95 GeV/c2 as well asπ-,η- andρ-mesons. We study in detail the influence of the higher baryonic resonances and the 2π-production channels (NN→NNππ) on the pion spectra in comparison toπ − data fromAr+KCl collisions at 1.8 GeV/A andπ 0-data forAu+Au at 1.0GeV/A. We, furthermore, present a detailed comparison of differential pion angular distributions with the BEVALAC data forAr+KCl at 1.8 GeV/A. The general agreement obtained indicates that the overall reaction dynamics is well described by our novel transport approach.