SIS Energies Research Articles

Collective flow at SIS energies within a hadronic transport approach: Influence of light nuclei formation and equation of state

Collective flow observables are known to be a sensitive tool to gain insights on the equation of state of nuclear matter from heavy-ion collision observations. Towards more quantitative constraints one has to carefully assess other influences on the collective behavior. In this work a hadronic transport approach smash (simulating many accelerated strongly interacting hadrons) is applied to study the first four anisotropic flow coefficients in $\text{Au}+\text{Au}$ collisions at ${E}_{\mathrm{lab}}=1.23A$ GeV in the context of the recently measured data by the HADES Collaboration. In particular, the formation of light nuclei is important in this energy regime. Two different approaches are contrasted to each other: A clustering algorithm inspired by coalescence as well as microscopic formation of deuterons via explicit cross sections. The sensitivity of directed and elliptic flow observables to the strength of the Skyrme mean field is explored. In addition, it is demonstrated that the rapidity-odd ${v}_{3}$ coefficient is practically zero in this energy regime and the ratio of ${v}_{4}/{v}_{2}^{2}$ is close to the value of 0.5 expected from hydrodynamic behavior. This study establishes the current understanding of collective behavior within the smash approach and lays the ground for future more quantitative constraints on the equation of state of nuclear matter within improved mean-field calculations.

Studies of hyperon production in HADES at FAIR-Phase0 — the case of an inclusive Σ(1385)+ measurement in pp reaction

In this proceedings first results of an inclusive measurement of the Σ(1385) + resonance produced in the proton-proton reaction at the beam kinetic energy of 3.5 GeV are presented. Mass of the Σ(1385)+ resonance extracted from a fit with the Breit-Wigner function is found to be M0 = 1380.02 MeV with a fixed width of Γ = 40MeV resulting from the previous exclusive measurements done by HADES. The presented analysis is a first trial to obtain the inclusive cross section for the production of the first-excited state of the Σ hyperon in elementary collisions at SIS energy range. The inclusive cross section is important for studies of the hyperon electromagnetic decays, which are one of the main fields of interest of the HADES experimental program during the FAIR-Phase0.

Strangeness Production in Nucleus-Nucleus Collisions at SIS Energies

Simulating Many Accelerated Strongly-interacting Hadrons (SMASH) is a new hadronic transport approach designed to describe the non-equilibrium evolution of heavy-ion collisions. The production of strange particles in such systems is enhanced compared to elementary reactions (Blume and Markert 2011), providing an interesting signal to study. Two different strangeness production mechanisms are discussed: one based on resonances and another using forced canonical thermalization. Comparisons to experimental data from elementary collisions are shown.

Medium modification of strange hadronic resonances at SIS, RHIC and LHC energies

The properties of strange pseudoscalar and vectors mesons as well as strange baryon resonances in dense matter are reviewed. Some open questions on the properties of strange hadrons in medium are addressed, such as the experimental signatures of inmedium effects coming from the hadronic phase on the final observables in heavy-ion collisions for the experimental conditions at SIS, RHIC and LHC energies.

Dilepton and $ \phi$ meson production in elementary and nuclear collisions at the NICA fixed-target experiment

We argue that the NICA fixed target experiment will be able to provide very important new experimental data on dilepton and $\phi$ meson production in the basically undiscovered energy domain between the SIS and SPS energies. Experimental information about elementary cross sections in this energy region is an essential ingredient of models of nuclear collisions in the same energy range.

Thermal dileptons from coarse-grained transport as fireball probes at SIS energies

Utilizing a coarse-graining method to convert hadronic transport simulations of Au+Au collisions at SIS energies into local temperature, baryon and pion densities, we compute the pertinent radiation of thermal dileptons based on an in-medium $\rho$ spectral function that describes available spectra at ultrarelativistic collision energies. In particular, we analyze how far the resulting yields and slopes of the invariant-mass spectra can probe the lifetime and temperatures of the fireball. We find that dilepton radiation sets in after the initial overlap phase of the colliding nuclei of about 7 fm/c, and lasts for about 13 fm/c. This duration closely coincides with the development of the transverse collectivity of the baryons, thus establishing a direct correlation between hadronic collective effects and thermal EM radiation, and supporting a near local equilibration of the system. This fireball "lifetime" is substantially smaller than the typical 20-30 fm/c that naive considerations of the density evolution alone would suggest. We furthermore find that the total dilepton yield radiated into the invariant-mass window of $M=0.3-0.7$ GeV/$c^{2}$, normalized to the number of charged pions, follows a relation to the lifetime found earlier in the (ultra-) relativistic regime of heavy-ion collisions, and thus corroborates the versatility of this tool. The spectral slopes of the invariant-mass spectra above the $\phi$ mass provide a thermometer of the hottest phases of the collision, and agree well with the maximal temperatures extracted from the coarse-grained hadron spectra.

Rapidity distribution of protons from the potential version of UrQMD model and the traditional coalescence afterburner

Rapidity distributions of both E895 proton data at AGS energies and NA49 net proton data at SPS energies can be described reasonably well with a potential version of the UrQMD in which mean-field potentials for both pre-formed hadrons and confined baryons are considered, with the help of a traditional coalescence afterburner in which one parameter set for both relative distance $R_0$ and relative momentum $P_0$, (3.8 fm, 0.3 GeV$/$c), is used. Because of the large cancellation between the expansion in $R_0$ and the shrinkage in $P_0$ through the Lorentz transformation, the relativistic effect in clusters has little effect on the rapidity distribution of free (net) protons. Using a Woods-Saxon-like function instead of a pure logarithmic function as seen by FOPI collaboration at SIS energies, one can fit well both the data at SIS energies and the UrQMD calculation results at AGS and SPS energies. Further, it is found that for central Au+Au or Pb+Pb collisions at top SIS, SPS and RHIC energies, the proton fractions in clusters are about 33$\%$, 10$\%$, and 0.7$\%$, respectively.

Strange hadron production at SIS energies: an update from HADES

We present and discuss recent experimental activities of the HADES collaboration on open and hidden strangeness production close or below the elementary NN threshold. Special emphasis is put on the feed-down from ϕ mesons to antikaons, the presence of the Ξ- excess in cold nuclear matter and the comparison of statistical model rates to elementary p+p data. The implications for the interpretation of heavy-ion data are discussed as well.

Pion and eta production in elementary and heavy-ion collisions at SIS energies

A main emphasis of the HADES program is on the dielectron signal from the early phase of heavy-ion collisions. We discuss the data with respect to the emission of charged hadrons, specifically the production of π + and π − mesons, which are related to neutral pions representing a dominant contribution to the dielectron yield. In addition, inclusive pion production obtained with the photon-conversion method is disscused. Preliminary results from Au+Au at 1.23 AGeV are presented.

Dilepton production and reaction dynamics in heavy-ion collisions at SIS energies from coarse-grained transport simulations

Dilepton invariant-mass spectra for heavy-ion collisions at SIS 18 and BEVALAC energies are calculated using a coarse-grained time evolution from the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) model. The coarse-graining of the microscopic simulations enables to calculate thermal dilepton emission rates by application of in-medium spectral functions from equilibrium quantum-field theoretical calculations. The results show that extremely high baryon chemical potentials dominate the evolution of the created hot and dense fireball. Consequently, a significant modification of the $\rho$ spectral shape becomes visible in the dilepton invariant-mass spectrum, resulting in an enhancement in the low-mass region $M_{ee} = 200$ to 600 MeV/$c^{2}$. This enhancement, mainly caused by baryonic effects on the $\rho$ spectral shape, can fully describe the experimentally observed excess above the hadronic cocktail contributions in Ar+KCl ($E_{\mathrm{lab}}=1.76$ $A$GeV) reactions as measured by the HADES collaboration and also gives a good explanation of the older DLS Ca+Ca ($E_{\mathrm{lab}}=1.04$ $A$GeV) data. For the larger Au+Au ($E_{\mathrm{lab}}=1.23$ $A$GeV) system, we predict an even stronger excess from our calculations. A systematic comparison of the results for different system sizes from C+C to Au+Au shows that the thermal dilepton yield increases stronger ($\propto A^{4/3}$) than the hadronic background contributions, which scale with $A$, due to its sensitivity on the time evolution of the reaction. We stress that the findings of the present work are consistent with our previous coarse-graining results for the NA60 measurements at top SPS energy. We argue that it is possible to describe the dilepton results from SIS 18 up to SPS energies by considering the modifications of the $\rho$ spectral function inside a hot and dense medium within the same model.

Dilepton Production in Transport-based Approaches

We investigate dilepton production in transport-based approaches and show that the baryon couplings of the ρ meson represent the most important ingredient for understanding the measured dilepton spectra. At SIS energies, the baryon resonances naturally play a major role and affect already the vacuum spectra via Dalitz-like contributions, which can be captured well in transport simulations. Recent pion-beam measurements at GSI will help to constrain the properties of the involved resonances further.

Dilepton Production in Transport Calculations and Coarse-Grained Dynamics

We present transport calculations with the Ultra-relativistic Quantum Molecular Dynamics approach (UrQMD) for dilepton spectra at SIS energies and simulations for NA60 within a coarse-graining approach. While the transport model obtains a good agreement with experiment for elementary reactions, in heavy-ion collisions an excess in the invariant mass spectra is observed which cannot be described by the model. As the pure transport calculations do not include explicit in-medium effects and are limited to hadronic degrees of freedom, we present an alternative approach that uses coarse-grained output from transport calculations to determine thermal dilepton emission rates. For this we apply the medium-modified ρ spectral function by Eletsky et al. In a first exemplary comparison to data from the NA60 experiment we find that the coarse-graining approach gives reasonable results.

ROLE OF BARYONIC RESONANCES IN VIRTUAL PHOTON EMISSION IN ELEMENTARY AND HEAVY-ION COLLISIONS

The HADES experiment aims to explore strongly interacting baryon dominated matter at moderate temperatures using rare and penetrating probes. Special emphasis is put on the region of moderate beam energies where comparatively long-lived states of compressed matter are created. In the energy domain of 1–2 GeV per nucleon, HADES has measured hadrons and di-electron signals in C + C , Ar + KCl , p+p, d+p and p+ Nb collisions. For the first time the electron pairs were reconstructed from quasi-free n+p sub-reactions by detecting the proton spectator from the deuteron breakup. For the first time di-electrons radiated from cold nuclear matter in a kinematic regime, where strong medium effects are expected, have been detected. An experimentally constrained N + N reference spectrum was established. A direct comparison of the N + N reference spectrum with the e+e- invariant mass distribution measured in the heavier system Ar + KCl at 1.76 GeV/u shows an excess yield above the reference. The observation may be interpreted as the onset of an actual medium effect. We argue, that couplings between the ρ meson and the baryonic resonances are of fundamental importance in understanding dilepton spectra at SIS energies.

Comment on “Investigation of Hadron Multiplicity and Hadron Yield Ratios in Heavy-Ion Collisions”

Oliinychenko, Bugaev, and Sorin [arXiv:1204.0103 [hep-ph]] considered the role of conservation laws in discussing possible weaknesses of thermal models which are utilized in describing the hadron multiplicities measured in central nucleus-nucleus collisions. They argued to analyze the criteria for chemical freeze-out and to conclude that none of them was robust. Based on this, they suggested a new chemical freeze-out criterion. They assigned to the entropy per hadron the ad hoc value 7.18 and supposed to remain unchanged over the whole range of the baryo-chemical potentials. Due to the unawareness of the recent literature, the constant entropy per hadron has been discussed in Refs. [Fizika B 18, 141 (2009), Europhys. Lett. 75, 420 (2006), Phys. Rev. C 85, 014908 (2012) and nucl-ph/1306.3291]. Furthermore, it has been shown that the constant entropy per hadron is equivalent to the constant entropy normalized to the cubic temperature, an earlier criterion for the chemical freeze-out introduced in Refs. [Europhys. Lett. 75, 420 (2006), Nucl. Phys. A 764, 387 (2006)]. In this comment, we list out the ignored literature, compare between the entropy-number density ratio and two criteria of averaged energy per averaged particle number and constant entropy per cubic temperature. All these criteria are confronted to the experimental results. The physics of constant entropy per number density is elaborated. It is concluded that this ratio cannot remain constant, especially at large chemical potentials related to the AGS and SIS energies.

Dilepton production at SIS energies with the GiBUU transport model

We present dilepton spectra from nucleus-nucleus collisions at SIS energies, which were simulated with the GiBUU transport model in a resonance-model approach. These spectra are compared to the data published by the HADES collaboration. We argue that the interpretation of dilepton spectra at SIS energies critically depends on the couplings between the ρ meson and the baryonic resonances.

Dilepton production at SIS energies with the UrQMD model

The production of lepton pairs in elementary and nucleus-nucleus collisions at SIS energies is studied within the Ultra-relativistic Quantum Molecular Dynamics transport approach. In comparison to previous calculations, we now included an improved treatment of the Δ1232 resonance. The resulting invariant mass and transverse momentum spectra are compared to available HADES data. In general, our results give a good description of the data. With regard to the observed overestimation of the ρ contribution, the different production channels and the respective production cross-sections are investigated. A problem remains the lack of experimental data for cross-sections and branching ratios of intermediate baryonic resonances, which are crucial for the ρ production. However, recent HADES cross-section measurements close to the ρ threshold might help to further constrain the model parameters.

Erratum to: Dilepton production in proton-induced reactions at SIS energies with the GiBUU transport model

We present dilepton spectra from p + p , d + p and p + Nb reactions at SIS energies, which were simulated with the GiBUU transport model in a resonance model approach. These spectra are compared to the data published by the HADES and DLS Collaborations. It is shown that the ρ spectral function includes non-trivial effects already in elementary reactions, due to production via baryon resonances, which can yield large contributions to the dilepton spectrum. Dilepton spectra from nuclear reactions in the energy range of the HADES experiment are thus found to be sensitive also to properties of nucleon resonances in the nuclear medium.

Dilepton production in proton-induced reactions at SIS energies with the GiBUU transport model

We present dilepton spectra from p + p , d + p and

Propagation of spectral functions and dilepton production at SIS energies

The time evolution of vector meson spectral functions is studied within a 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 inmedium modifications turn out to compete, in this approach, with the known vacuum contributions.

Collective Flow of Λ Hyperons within Covariant Kaon Dynamics

The collective flow of Λ hyperons produced in association with positively charged kaon mesons in nuclear reactions at SIS energies is studied using the quantum molecular dynamics (QMD) model within covariant kaon dynamics. Our calculation indicates that both the directed and differential directed flows of Λs are almost in agreement with the experimental data. This suggest that the covariant kaon dynamics based on the chiral mean field approximation can not only explain the collective flow of kaon mesons, but also give reasonable results for the collective flow of Λ hyperons at SIS energies. The final-state interaction of Λ hyperons with dense nuclear matter enhances their directed flow and improves the agreement of their differential directed flow with the experimental data. The influence of the interaction on the Λ collective flow is more appreciable at large rapidity or transverse momentum region.