SIS Energies Research Articles

Off-shell transport of broad resonances and dilepton production at SIS energies

The time evolution of vector meson spectral functions is studied within a Boltzmann-Uehling-Uhlenbeck (BUU) type transport model. Applications focus on ρ and ω mesons being important pieces for the interpretation of the dielectron invariant mass spectrum. Since the evolution of the spectral functions is driven by the local density, the in-medium modifications turn out to compete, in this approach, with the known vacuum contributions.

RESULTS ON STRANGENESS PRODUCTION FROM HADES

Recent results concerning the production of K +, K -, ø, [Formula: see text] and Λ particles in the reaction system Ar + KCl at 1.76 AGeV measured by the HADES Collaboration are presented. Transverse momentum distributions have been measured in a large fraction of the phase space. For the first time, at SIS energies, K +, K - and ɸ mesons have been measured independently and the slope parameter for ø mesons could be extracted. A large set of statistics has been collected for the produced [Formula: see text] and Λ in the phase space region around mid-rapidity. The high quality of these data render them very suitable for comparisons with theoretical models.

A versatile method for simulating pp $ \rightarrow$ ppe+e- and dp $ \rightarrow$ pne+e-pspec reactions

We have developed a versatile software package for the simulation of di-electron production in $pp$ and $dp$ collisions at SIS energies. Particular attention has been paid to incorporate different descriptions of the Dalitz decay $\Delta \to N e^+e^-$ via a common interface. In addition, suitable parameterizations for the virtual bremsstrahlung process $NN \to NN e^+e^-$ based on one-boson exchange models have been implemented. Such simulation tools with high flexibility of the framework are important for the interpretation of the di-electron data taken with the HADES spectrometer and the design of forthcoming experiments.

Design of the pluto event generator

We present the design of the simulation package Pluto, aimed at the study of hadronic interactions at SIS and FAIR energies. Its main mission is to offer a modular framework with an object-oriented structure, thereby making additions such as new particles, decays of resonances, new models up to modules for entire changes easily applicable. Overall consistency is ensured by a plugin- and distribution manager. Particular features are the support of a modular structure for physics process descriptions, and the possibility to access the particle stream for on-line modifications. Additional configuration and self-made classes can be attached by the user without re-compiling the package, which makes Pluto extremely configurable.

Transport model study of nuclear stopping in heavy-ion collisions over the energy range from0.09Ato160A GeV

Nuclear stopping in heavy-ion collisions over a beam energy range from SIS and AGS up to SPS is studied in the framework of the modified Ultrarelativistic Quantum Molecular Dynamics transport model, in which mean field potentials of both formed and ``preformed'' hadrons (from string fragmentation) and medium-modified nucleon-nucleon elastic cross sections are considered. It is found that nuclear stopping is influenced by both the stiffness of the equation of state and medium modifications of nucleon-nucleon cross sections at SIS energies. At high SPS energies, a two-bump structure is shown in the experimental rapidity distribution of free protons, which can be understood by considering the preformed hadron potentials.

Collective Flow of [formula omitted] Meson within Covariant Kaon Dynamics

Collective flow of K + mesons in heavy ion collisions at SIS energies is investigated within the covariant kaon dynamics. The theoretical results calculated by using quantum molecular dynamics reasonably reproduce properties of the K + anti-flow. This means that the features of K + collective flow clearly favor the existence of an in-medium repulsive potential for K + meson. It is also shown that the rapidity distribution of K + meson is more sensitive to the nuclear equation of state than the differential directed flow.

Dilepton Production at SIS Energies Studied with HADES

One of the main goals of the HADES experiment is to achieve a detailed understanding of dielectron emission from hadronic systems at moderate bombarding energies. Results obtained on electron pair production in elementary N+ N collisions pave the way to a better understanding of the origin of the pair excess seen in heavy-ion collisions. This puzzling excess, reported first by the former DLS experiment, is now being investigated systematically by HADES.

Catalytic ϕ meson production in heavy-ion collisions

The phi meson production on hyperons πY → ϕY and anti-kaons is argued to be a new efficient source of phi mesons in a nucleus–nucleus collision. These reactions are not suppressed according to the Okubo–Zweig–Izuka rule in contrast to the processes with non-strange particles in the entrance channels, πB and BB with B = N, Δ. A rough estimate of the cross sections within a simple hadronic model shows that the cross sections of πY → ϕY and reactions can exceed that of the πN → ϕN reaction by factors 50 and 60, respectively. In the hadrochemical model for nucleus–nucleus collisions at SIS and lower AGS energies, we calculate the evolution of strange particle populations and the ϕ meson production rate due to the new processes. It is found that the catalytic reactions can be operative if the maximal temperature in nucleus–nucleus collisions is larger than 130 MeV and the collision time is larger than 10 fm. A possible influence of the catalytic reactions on the centrality dependence of the ϕ yield at AGS energies and the ϕ rapidity distributions at SPS energies is discussed.

New strangeness results from HADES

New results concerning the production of K+, K−, ϕ, K0S and Λ particles in the reaction Ar + KCl at 1.76 AGeV measured by the HADES Collaboration are presented. Transverse momentum distributions have been measured in several rapidity slices for all the particles mentioned above in a large fraction of the phase space. For the first time, at SIS energy, K+, K− and ϕ have been measured independently and the slope parameter for ϕ could be extracted. A large statistics has been collected for the produced K0S and Λ in the phase space region around mid-rapity. The high quality of these data render them very suitable for comparisons with theoretical models.

Differential Directed Flow of K+ Meson within Covariant Kaon Dynamics

The collective flow of positive charged kaons in heavy ion reactions at SIS energy is investigated within the frame of covariant kaon dynamics. The theoretical results calculated by using quantum molecular dynamics show that the rapidity distribution of K+ mesons is more sensitive to the nuclear equation of state than the differential directed flow. The contribution of various K+ production reaction channels to the rapidity distribution of the K+ is also analysed in detail. The results indicate that the rapidity distribution of K+ mesons is mainly from the contribution of the N-Δ and N-N channel. This means that the delta resonance state plays a predominantly important role for the K+ subthreshold production.

Compressed baryonic matter—experiments at GSI and FAIR

Experiments on strangeness production in nucleus-nucleus collisions at SIS energies address fundamental aspects of modern nuclear physics: the determination of the nuclear equation-of-state at high baryon densities and the properties of hadrons in dense nuclear matter. Experimental data and theoretical results will be reviewed. The planned “Compressed Baryonic Matter” (CBM) experiment at the future Facility for Antiproton and Ion Research (FAIR) aims at the exploration of the QCD phase diagram at the highest baryon densities. This includes the search for the deconfinement and chiral phase transition and for the QCD critical endpoint. Possible observables and the experimental setup will be discussed.

Dilepton production and off-shell transport dynamics at SIS energies

Dilepton production in nucleus–nucleus collisions at 1–2 A GeV as well as in elementary pp and pd reactions is studied within the microscopic HSD transport approach which includes the off-shell dynamics of vector mesons explicitly. The study addresses additionally the production of π 0 and η mesons since their Dalitz decays provide a sizeable contribution to the dilepton invariant mass spectra up to about 0.5 GeV. Our transport results agree with the TAPS experimental data on π and η multiplicities in C 12 + C 12 collisions from 0.8 to 2 A GeV. We find that the ‘DLS-puzzle’—which stands for a theoretical underestimation of the e + e − yield in the mass range from 0.2 to 0.5 GeV in C 12 + C 12 and Ca 40 + Ca 40 collisions—may be solved when incorporating a stronger bremsstrahlung contribution in line with recent OBE calculations. Moreover, the HSD results with ‘enhanced’ bremsstrahlung cross sections agree very well with the HADES experimental data for the dilepton mass spectra for C 12 + C 12 at 1 and 2 A GeV, especially when including a collisional broadening in the vector-meson spectral functions. Detailed predictions for dilepton spectra from pp and p n / p d reactions at 1.25 GeV, 2.2 GeV and 3.5 GeV are presented which will allow to verify/falsify the larger bremsstrahlung contributions from the experimental side in the near future.

Probing dense matter with strangeness production in nuclear collisions

Experiments on strangeness production in nucleus–nucleus collisions at SIS energies address fundamental questions of modern nuclear physics: the determination of the nuclear equation-of-state at high baryon densities and the properties of hadrons in dense nuclear matter. From the yields of K+ mesons measured in heavy-ion collisions a value for the nuclear compressibility of κ≈200MeV is extracted for nuclear densities around twice saturation density using different microscopic transport models. Both the yield of K+ mesons and their anisotropic azimuthal angular distribution (elliptic flow) exhibit strong evidence for a repulsive K+N potential. The yields of K+ and K− mesons measured in proton–nucleus collisions can be reproduced by transport calculations assuming at saturation density a repulsive K+N potential of U=25MeV and an attractive K−N potential of U=−80±20MeV.

Freeze-out volume of hot dense fireball

A thermodynamically consistent excluded volume model is proposed to account for the particle multiplicities obtained from lowest SIS energies to the highest RHIC energies. The chemical freeze-out volumes lying in a slice of one unit of rapidity for pions and kaons are separately inferred from this analysis and the results are compared with the corresponding thermal freeze-out volumes obtained from the Hanbury–Brown Twiss (HBT) pion interferometry. Furthermore, we extract the variations of freeze-out number densities for pions and nucleons with the center-of-mass energy in our model and compare them with the HBT data.

STRANGE PARTICLES AND NEUTRON STARS — EXPERIMENTS AT GSI

Experiments on strangeness production in nucleus-nucleus collisions at SIS energies address fundamental aspects of modern nuclear physics: the determination of the nuclear equation-of-state at high baryon densities and the properties of hadrons in dense nuclear matter. Experimental data and theoretical results will be reviewed. Future experiments at the FAIR accelerator aim at the exploration of the QCD phase diagram at highest baryon densities.

FLOW OF STRANGE PARTICLES FROM FOPI AT SIS

Study of strange particles produced at (sub)threshold energies in nucleus-nucleus collisions can deliver insights into fundamental questions about the in-medium properties of hadrons in dense baryonic matter. Many theoretical calculations for the production and propagation of strangeness at SIS energies lead to various predictions concerning the existence and the magnitude of in-medium effects for strange particles in nuclear matter. FOPI measured the flow of strange particles in Ni + Ni collisions at 1.93A GeV and the results are compared with transport models which are favorable to in-medium effect.

Influence of In-Medium Properties of Vector Mesons on Dilepton Production in Heavy Ion Collisions at SIS Energies

Dileptons represent a unique probe for nuclear matter under extreme conditions reached in heavy ion collisions. They allow the study of meson properties, like mass and decay width, at various density and temperature regimes. Up to now, in the Tubingen model for dilepton production, modification of meson properties in nuclear medium has been accounted for by allowing a density dependence of the mass (Brown-Rho scaling) together with an ad hoc dependence of the meson decay widths on the same variable. We use the extended vector meson dominance (eVMD) model to extract meson properties in nuclear matter by computing the in-medium meson spectral functions. Dilepton spectra for C+C at 1.0 and 2.0 AGeV are calculated and compared with previous results.

Kaon Production from 1 to 40 AGeV

Kaon production is studied within the Giessen Boltzmann-Uehling-Uhlenbeck (GiBUU) model. Results are compared with experiment and with other models. The influence of the kaon potential on the kaon azimuthal distributions at SIS energies is considered. We also discuss the role of the many-body collisions at high-density phase of reaction.

Dilepton production at SIS energies

In this short review the results of detailed studies for dilepton production from p+A andA+A reactions at SIS energies are presented. The calculations are based on a semi-classical BUU transport model that includes the off-shell propagation of vector mesons and evaluates the width of the vector mesons dynamically. Different scenarios of in-medium modifications of vector mesons, such as collisional broadening and dropping vector meson masses, are investigated and the possibilities for an experimental observation of in-medium effects in p+A reactions at 1–4 GeV are discussed for a variety of nuclear targets.

Statistical production of antikaon nuclear bound states in heavy ion collisions

Recently it was conjectured that the strongly attractive antikaon–nucleon potential can result in the formation of antikaon nuclear bound states. We discuss the formation of such states as possible residues in heavy ion collisions. In this context, we calculate the excitation functions of single- and double- K − clusters in terms of the statistical thermal model. We show that, if such objects indeed exist, then, in heavy ion collisions, the single- K − clusters are most abundantly produced at present SIS energies, while the double- K − clusters show a pronounced maximum yield in the energy domain of the future accelerator at GSI. This is a direct consequence of: (i) the baryonic dominance in low energy heavy ion collisions and the large baryonic content of the antikaonic bound states; (ii) the strong energy dependence of strangeness production at low energies. The production yields of double-strange clusters is compared with that of double strange baryons. It is shown that at SIS energy there is a linear scaling relation of the Ξ −/ K + with K +/ p yields ratio.