arXiv subjects

M. Bleicher

Publications and source records attributed to M. Bleicher.

At least 127 records · Page 7Linked to original sources

arXiv2001

Microscopic coloured quark-dynamics in the soft non-perturbative regime -- Description of hadron formation in relativistic S+Au collisions at CERN

The quark-molecular-dynamics model is used to study microscopically the dynamics of the coloured quark phase and the subsequent hadron formation in relativistic S+Au collisions at the CERN-SPS. Particle spectra and hadron ratios are compared to both data and the results of hadronic transport calculations. The non-equilibrium dynamics of hadronization and the loss of correlation among quarks are studied.

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arXiv2001

Event-by-event fluctuations and the QGP

We discuss the physics underlying event-by-event fluctuations in relativistic heavy ion collisions. We will emphasize how the fluctuations of particle ratios can be utilized to explore the properties of the matter created in these collisions. In particular, we will argue that the fluctutions of the ratio of positively over negatively charged particles may serve as a unique signature for the Quark Gluon Plasma.

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arXiv2001

Current Status of Quark Gluon Plasma Signals

Compelling evidence for the creation of a new form of matter has been claimed to be found in Pb+Pb collisions at SPS. We discuss the uniqueness of often proposed experimental signatures for quark matter formation in relativistic heavy ion collisions. It is demonstrated that so far none of the proposed signals like Jψmeson production/suppression, strangeness enhancement, dileptons, and directed flow unambigiously show that a phase of deconfined matter has been formed in SPS Pb+Pb collisions. We emphasize the need for systematic future measurements to search for simultaneous irregularities in the excitation functions of several observables in order to come close to pinning the properties of hot, dense QCD matter from data.

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arXiv2000

Local equilibrium in heavy-ion collisions: Microscopic analysis of a central cell versus infinite matter

We study the local equilibrium in the central $V = 125$ fm$^3$ cell in heavy-ion collisions at energies from 10.7 AGeV (AGS) to 160 AGeV (SPS) calculated in the microscopic transport model. In the present paper the hadron yields and energy spectra in the cell are compared with those of infinite nuclear matter, as calculated within the same model. The agreement between the spectra in the two systems is established for times $t \geq 10$ fm/$c$ in the central cell. The cell results do not deviate noticeably from the infinite matter calculations with rising incident energy, in contrast to the apparent discrepancy with predictions of the statistical model (SM) of an ideal hadron gas. The entropy of this state is found to be very close to the maximum entropy, while hadron abundances and energy spectra differ significantly from those of the SM.

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arXiv2000

Bremsstrahlung from a Microscopic Model of Relativistic Heavy Ion Collisions

We compute bremsstrahlung arising from the acceleration of individual charged baryons and mesons during the time evolution of high-energy Au+Au collisions at the Relativistic Heavy Ion Collider using a microscopic transport model. We elucidate the connection between bremsstrahlung and charge stopping by colliding artificial pure proton on pure neutron nuclei. From the intensity of low energy bremsstrahlung, the time scale and the degree of stopping could be accurately extracted without measuring any hadronic observables.

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arXiv2000

Enhanced Strange Particle Yields - Signal of a Phase of Massless Particles ?

The yields of strange particles are calculated with the UrQMD model for p,Pb(158 AGeV)Pb collisions and compared to experimental data. The yields are enhanced in central collisions if compared to proton induced or peripheral Pb+Pb collisions. The enhancement is due to secondary interactions. Nevertheless, only a reduction of the quark masses or equivalently an increase of the string tension provides an adequate description of the large observed enhancement factors (WA97 and NA49). Furthermore, the yields of unstable strange resonances as the Lambda_star(1520) resonance or the phi meson are considerably affected by hadronic rescattering of the decay products.

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arXiv2000

Transition to resonance-rich matter in heavy ion collisions at RHIC energies

The equilibration of hot and dense nuclear matter produced in the central region in central Au+Au collisions at $\sqrt{s}=200$ AGeV 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 $(P,ε)$-plane has a very simple form, $P=0.15 ε$. 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.

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arXiv2000

Equation of state of resonance-rich matter in the central cell in heavy-ion collisions at $\sqrt{s}$=200 AGeV

The equilibration of hot and dense nuclear matter produced in the central cell of central Au+Au collisions at RHIC ($\sqrt{s}=200$ AGeV) energies is studied within a microscopic transport model. The pressure in the cell becomes isotropic at $t\approx 5$ fm/$c$ after beginning of the collision. Within the next 15 fm/$c$ the expansion of matter in the cell proceeds almost isentropically with the entropy per baryon ratio $S/A \approx 150$, and the equation of state in the $(P,ε)$ plane has a very simple form, $P=0.15ε$. Comparison with the statistical model of an ideal hadron gas indicates that the time $t \approx 20$ fm/c may be too short to reach the fully equilibrated state. Particularly, the creation of long-lived resonance-rich matter in the cell decelerates the relaxation to chemical equilibrium. This resonance-abundant state can be detected experimentally after the thermal freeze-out of particles.

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arXiv2000

Strangeness enhancement from strong color fields at RHIC

In ultra-relativistic heavy ion collisions, early stage multiple scatterings may lead to an increase of the color electric field strength. Consequently, particle production - especially heavy quark (and di-quark) production - is greatly enhanced according to the Schwinger mechanism. We test this idea via the Ultra-relativistic Quantum Molecular Dynamics model (UrQMD) for Au+Au collisions at the full RHIC energy ($\sqrt{s} = 200$ AGeV). Relative to p+p collisions, a factor of 60, 20 and 7 enhancement respectively, for $Ω$ ($sss$), $Ξ$ ($ss$), and $Λ$, $Σ$ ($s$) is predicted for a model with increased color electric field strength.

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arXiv2000

Enhanced Event-by-Event Fluctuations in Pion Multiplicity as a Signal of Disoriented Chiral Condensates at RHIC

The factorial moments of the pion multiplicity distributions are calculated with HIJING and UrQMD and found to be independent of the pT range included, in contrast to recent simulations with the linear sigma model which leads to large enhancements for pions with transverse kinetic energies below 200 MeV. This supports the use of the ratio of the factorial moments of low and high pT pions as a signal of ``new'' physics at low momentum scales, such as the formation of disoriented chiral condensates.

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arXiv2000

Event-by-event fluctuations of the charged particle ratio from non-equilibrium transport theory

The event by event fluctuations of the ratio of positively to negatively charged hadrons are predicted within the UrQMD model. Corrections for finite acceptance and finite net charge are derived. These corrections are relevant to compare experimental data and transport model results to previous predictions. The calculated fluctuations at RHIC and SPS energies are shown to be compatible with a hadron gas. Thus, deviating by a factor of 3 from the predictions for a thermalized quark-gluon plasma.

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arXiv2000

Anisotropic flow in ultra-relativistic heavy ion collisions

A sign reversal of the directed flow parameter $v_1$ in the central rapidity region in Au+Au collisions at $\sqrt s =200$~AGeV is predicted. This anti-flow is shown to be linked to the expansion of the hot matter created. In line with this observation the predicted elliptic flow parameter $v_2$ of various particle species is predicted and linked to the mean free path of these particles.

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arXiv2000

Baryonic contributions to the dilepton spectra in relativistic heavy ion collisions

We investigate the baryonic contributions to the dilepton yield in high energy heavy ion collisions within the context of a transport model. The relative contribution of the baryonic and mesonic sources are examined. It is observed that most dominant among the baryonic channels is the decay of N*(1520) and mostly confined in the region below the rho peak. In a transport theory implementation we find the baryonic contribution to the lepton pair yield to be small.

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arXiv2000

Enhanced antiproton production in Pb(160 AGeV)+Pb reactions: evidence for quark gluon matter?

The centrality dependence of the antiproton per participant ratio is studied in Pb(160 AGeV)+Pb reactions. Antiproton production in collisions of heavy nuclei at the CERN/SPS seems considerably enhanced as compared to conventional hadronic physics, given by the antiproton production rates in $pp$ and antiproton annihilation in $\bar{p}p$ reactions. This enhancement is consistent with the observation of strong in-medium effects in other hadronic observables and may be an indication of partial restoration of chiral symmetry.

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arXiv2000

Dilepton Production and Chiral Symmetry

We discuss how dilepton production is related to chiral symmetry and its restoration. We then analyse presently available data by the CERES collaboration in this context. We find that the present data do not support any conclusions concerning the restoration of chiral symmetry. We finally provide a prediction for the dilepton spectrum for the just completed low energy (40 GeV) run at the SPS.

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arXiv1999

Global observables and secondary interactions in central Au+Au reactions at $\sqrt{s}=200$ AGeV

The Ultra-relativistic Quantum Molecular Dynamics model (UrQMD) is used to study global observables in central reactions of Au+Au at $\sqrt{s}=200$ AGeV (RHIC). Strong stopping governed by massive particle production is predicted if secondary interactions are taken into account. The underlying string dynamics and the early hadronic decoupling implies only small transverse expansion rates. However, rescattering with mesons is found to act as a source of pressure leading to additional flow of baryons and kaons, while cooling down pions.

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arXiv1999

Last Call for RHIC Predictions

This paper contains the individual contributions of all speakers of the session on 'Last Call for RHIC Predictions' at Quark Matter 99, and a summary by the convenor.

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arXiv1999

Relativistic Hadron-Hadron Collisions in the Ultra-Relativistic Quantum Molecular Dynamics Model (UrQMD)

Hadron-hadron collisions at high energies are investigated in the Ultra-relativistic-Quantum-Molecular-Dynamics approach (UrQMD). This microscopic transport model is designed to study pp, pA and A+A collisions. It describes the phenomenology of hadronic interactions at low and intermediate energies ($\sqrt s <5$ GeV) in terms of interactions between known hadrons and their resonances. At high energies, $\sqrt s >5$ GeV, the excitation of color strings and their subsequent fragmentation into hadrons dominates the multiple production of particles in the UrQMD model. The model shows a fair overall agreement with a large body of experimental h-h data over a wide range of h-h center-of-mass energies. Hadronic reaction data with higher precision would be useful to support the use of the UrQMD model for relativistic heavy ion collisions.

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