Refine
Year of publication
Document Type
- Preprint (100)
- Article (69)
- Conference Proceeding (15)
- Contribution to a Periodical (1)
- Review (1)
Has Fulltext
- yes (186)
Is part of the Bibliography
- no (186)
Keywords
- Kollisionen schwerer Ionen (16)
- heavy ion collisions (14)
- UrQMD (6)
- QGP (5)
- Quark-Gluon-Plasma (5)
- Quark Gluon Plasma (4)
- Drell-Yan (3)
- Hadron (3)
- Heavy-ion collisions (3)
- MEMOs (3)
Institute
ϕ-meson production in In–In collisions at Elab=158A GeV: Evidence for relics of a thermal phase
(2010)
Yields and transverse mass distributions of the ϕ-mesons reconstructed in the ϕ→μ+μ− channel in In+In collisions at Elab=158A GeV are calculated within an integrated Boltzmann+hydrodynamics hybrid approach based on the Ultrarelativistic Quantum Molecular Dynamics (UrQMD) transport model with an intermediate hydrodynamic stage. The analysis is performed for various centralities and a comparison with the corresponding NA60 data in the muon channel is presented. We find that the hybrid model, that embeds an intermediate locally equilibrated phase subsequently mapped into the transport dynamics according to thermal phase-space distributions, gives a good description of the experimental data, both in yield and slope. On the contrary, the pure transport model calculations tend to fail in catching the general properties of the ϕ meson production: not only the yield, but also the slope of the mT spectra, compare poorly with the experimental observations at top SPS energies.
Dilepton production in heavy-ion collisions at top SPS energy is investigated within a coarse-graining approach that combines an underlying microscopic evolution of the nuclear reaction with the application of medium-modified spectral functions. Extracting local energy and baryon density for a grid of small space-time cells and going to each cell’s rest frame enables to determine local temperature and chemical potential by application of an equation of state. This allows for the calculation of thermal dilepton emission. We apply and compare two different spectral functions for the ρ: A hadronic many-body calculation and an approach that uses empirical scattering amplitudes. Quantitatively good agreement of the model calculations with the data from the NA60 collaboration is achieved for both spectral functions, but in detail the hadronic many-body approach leads to a better description, especially of the broadening around the pole mass of the ρ and for the low-mass excess. We further show that the presence of a pion chemical potential significantly influences the dilepton yield.
UrQMD at RHIC energies
(1999)
We use black holes with a negative cosmological constant to investigate aspects of the freeze-out temperature for hadron production in high energy heavy-ion collisions. The two black hole solutions present in the anti-de Sitter geometry have different mass and are compared to the data showing that the small black hole solution is in good agreement. This is a new feature in the literature since the small black hole in general relativity has different thermodynamic behavior from that of the large black hole solution. We find that the inclusion of the cosmological constant (which can be interpreted as the plasma pressure) leads to a lowering of the temperature of the freeze-out curve as a function of the baryochemical potential, improving the description previously suggested by Castorina, Kharzeev, and Satz.
In the present work we study the effect of unparticle modified static potentials on the energy levels of the hydrogen atom. By using Rayleigh–Schrödinger perturbation theory, we obtain the energy shift of the ground state and compare it with experimental data. Bounds on the unparticle energy scale U as a function of the scaling dimension and the coupling constant λ are derived. We show that there exists a parameter region where bounds on U ar are stringent, signaling that unparticles could be tested in atomic physics experiments.
We show that an unambiguous way of determining the universal limiting fragmentation region is to consider the derivative (d 2 n / d eta 2) of the pseudo-rapidity distribution per participant pair. In addition, we find that the transition region between the fragmentation and the central plateau regions exhibits a second kind of universal behavior that is only apparent in d 2 n / d eta 2. The sqrt s dependence of the height of the central plateau (d n / d eta) eta=0 and the total charged particle multiplicity n total critically depend on the behavior of this universal transition curve. Analyzing available RHIC data, we show that (dn/d eta) eta=0 can be bounded by ln 2 s and n total can be bounded by ln 3 s. We also show that the deuteron-gold data from RHIC has the exactly same features as the gold-gold data indicating that these universal behaviors are a feature of the initial state parton-nucleus interactions and not a consequence of final state interactions. Predictions for LHC energy are also given.
The deuteron coalescence parameter 𝐵2 in proton+proton and nucleus+nucleus collisions in the energy range of √s N N = 900–7000 GeV for proton + proton and √s N N = 2–2760 GeV for nucleus + nucleus collisions is analyzed with the Ultrarelativistic Quantum Molecular Dynamics (UrQMD) transport model, supplemented by an event-by-event phase space coalescence model for deuteron and anti-deuteron production. The results are compared to data by the E866, E877, PHENIX, STAR and ALICE experiments. The 𝐵2 values are calculated from the final spectra of protons and deuterons. At lower energies, √s N N ≤ 20 GeV, B2 drops drastically with increasing energy. The calculations confirm that this is due to the increasing freeze-out volume reflected in B2 ∼ 1/V . At higher energies, √s N N ≥ 20 GeV, B2 saturates at a constant level. This qualitative change and the vanishing of the volume suppression is shown to be due to the development of strong radial flow with increasing energy. The flow leads to strong space-momentum correlations which counteract the volume effect.
We suggest to explore an entirely new method to experimentally and theoretically study the phase diagram of strongly interacting matter based on the triple nuclear collisions (TNC).We simulated the TNC using the UrQMD 3.4 model at the beam center of- mass collision energies √SNN = 200 GeV and √SNN = 2.76 TeV. It is found that in the most central and simultaneous TNC the initial baryonic charge density is about 3 times higher than the one achieved in the usual binary nuclear collisions at the same energies. As a consequence, the production of protons and Λ-hyperons is increased by a factor of 2 and 1.5, respectively. Using the MIT Bag model equation we study the evolution of the central cell in TNC and demonstrate that for the top RHIC energy of collision the baryonic chemical potential is 2-2.5 times larger than the one achieved in the binary nuclear collision at the same time of reaction. Based on these estimates, we show that TNC offers an entirely new possibility to study the QCD phase diagram at very high baryonic charge densities.
Trapping black hole remnants
(2005)
Large extra dimensions lower the Planck scale to values soon accessible. The production of TeV mass black holes at the LHC is one of the most exciting predictions. However, the final phases of the black hole's evaporation are still unknown and there are strong indications that a black hole remnant can be left. Since a certain fraction of such objects would be electrically charged, we argue that they can be trapped. In this paper, we examine the occurrence of such charged black hole remnants. These trapped remnants are of high interest, as they could be used to closely investigate the evaporation characteristics. Due to the absence of background from the collision region and the controlled initial state, the signal would be very clear. This would allow to extract information about the late stages of the evaporation process with high precision.
Transverse hadron spectra from proton-proton, proton-nucleus and nucleus-nucleus collisions from 2 AGeV to 21.3 ATeV are investigated within two independent transport approaches (HSD and UrQMD). For central Au+Au (Pb+Pb) collisions at energies above E lab ~ 5 AGeV, the measured K +- transverse mass spectra have a larger inverse slope parameter than expected from the default calculations. The additional pressure - as suggested by lattice QCD calculations at finite quark chemical potential mu q and temperature T - might be generated by strong interactions in the early pre-hadronic/partonic phase of central Au+Au (Pb+Pb) collisions. This is supported by a non-monotonic energy dependence of v2/pT in the present transport model.
We compare multiplicities as well as rapidity and transverse momentum distributions of protons, pions and kaons calculated within presently available transport approaches for heavy ion collisions around 1 AGeV. For this purpose, three reactions have been selected: Au+Au at 1 and 1.48 AGeV and Ni+Ni at 1.93 AGeV.
Based on the microscopic transport model UrQMD in which hadronic and string degrees of freedom are employed, the HBT parameters in the longitudinal co-moving system are investigated for charged pion and kaon, and Λ sources in heavy ion collisions (HICs) at SPS and RHIC energies. In the Cascade mode, RO and the RL at high SPS and RHIC energies do not follow the mT-scaling, however, after considering a soft equation of state with momentum dependence (SM-EoS) for formed baryons and a density-dependent Skyrme-like potential for “pre-formed” particles, the HBT radii of pions and kaons and even those of Λs with large transverse momenta follow the mT-scaling function R=3/mT fairly well.
Based on the UrQMD transport model, the transverse momentum and the rapidity dependence of the Hanbury-Brown-Twiss (HBT) radii R_L, R_O, R_S as well as the cross term R_OL at SPS energies are investigated and compared with the experimental NA49 and CERES data. The rapidity dependence of the R_L, R_O, R_S is weak while the R_OL is significantly increased at large rapidities and small transverse momenta. The HBT "life-time" issue (the phenomenon that the calculated sqrt R_O^2-R_S^2 value is larger than the correspondingly extracted experimental data) is also present at SPS energies.
The pion source as seen through HBT correlations at RHIC energies is investigated within the UrQMD approach. We find that the calculated transverse momentum, centrality, and system size dependence of the Pratt-HBT radii R_L and R_S are reasonably well in line with experimental data. The predicted R_O values in central heavy ion collisions are larger as compared to experimental data. The corresponding quantity sqrt R_O^2-R_S^2 of the pion emission source is somewhat larger than experimental estimates.
The equilibration of hot and dense nuclear matter produced in the central region in central Au+Au collisions at square root s = 200A GeV is studied within the microscopic transport model UrQMD. The pressure here becomes isotropic at t approx 5 fm/c. Within the next 15 fm/c the expansion of the matter proceeds almost isentropically with the entropy per baryon ratio S/A approx 150. During this period the equation of state in the (P, epsilon)-plane has a very simple form, P = 0.15 epsilon. Comparison with the statistical model (SM) of an ideal hadron gas reveals that the time of approx 20 fm/c may be too short to attain the fully equilibrated state. Particularly, the fractions of resonances are overpopulated in contrast to the SM values. The creation of such a long-lived resonance-rich state slows down the relaxation to chemical equilibrium and can be detected experimentally.
The creation of loosely bound objects in heavy ion collisions, e.g. light clusters, near the phase transition temperature () has been a puzzling observation that seems to be at odds with Big Bang nucleosynthesis suggesting that deuterons and other clusters are formed only below a temperature . We solve this puzzle by showing that the light cluster abundancies in heavy ion reactions stay approximately constant from chemical freeze-out to kinetic freeze-out. To this aim we develop an extensive network of coupled reaction rate equations including stable hadrons and hadronic resonances to describe the temporal evolution of the abundancies of light (anti-)(hyper-)nuclei in the late hadronic environment of an ultrarelativistic heavy ion collision. It is demonstrated that the chemical equilibration of the light nuclei occurs on a very short timescale as a consequence of the strong production and dissociation processes. However, because of the partial chemical equilibrium of the stable hadrons, including the nucleon feeding from Δ resonances, the abundancies of the light nuclei stay nearly constant during the evolution and cooling of the hadronic phase. This solves the longstanding contradiction between the thermal fits and the late stage coalescence (and the Big Bang nucleosynthesis) and explains why the observed light cluster yields are compatible with both a high chemical production temperature and a late state emission as modeled by coalescence. We also note in passing that the abundancies of the light clusters in the present approach are in excellent agreement with those measured by ALICE at LHC.
In high multiplicity nucleus-nucleus collisions baryon-antibaryon annihilation and regeneration occur during the final hadronic expansion phase, thus distorting the initial equilibrium multiplicity ratios. We quantify the modifications employing the hybrid UrQMD transport model and apply them to the grand canonical partition functions of the Statistical Hadronization Model(SHM). We analyze minimum bias and central Pb+Pb collision data at SPS and LHC energy. We explain the Pion to Proton ratio puzzle. We also reproduce the deuteron to proton ratio at LHC energy by the SHM, and by UrQMD after attaching a phase space coalescence process. We discuss the resulting (T,μB) diagram.
The physics of EPOS
(2013)
Within a dynamical quark recombination model, we explore various proposed event-by-event observables sensitive to the microscopic structure of the QCD-matter created at RHIC energies. Charge ratio fluctuations, charge transfer fluctuations and baryon-strangeness correlations are computed from a sample of central Au + Au events at the highest RHIC energy available (sNN=200 GeV). We find that for all explored observables, the calculations yield the values predicted for a quark–gluon plasma only at early times of the evolution, whereas the final state approaches the values expected for a hadronic gas. We argue that the recombination-like hadronization process itself is responsible for the disappearance of the predicted deconfinement signals. This might explain why no fluctuation signatures for the transition between quark and hadronic matter was ever observed in the experimental data up to now.