Hydrodynamic modeling of deconfinement phase transition in nuclear collisions

The (3+1)-dimensional ideal hydrodynamics is used to simulate collisions of gold nuclei with bombarding energies from 1 to 160 GeV per nucleon. The initial state is represented by two cold Lorentz-boosted nuclei. Two equations of state: with and without the deconfinement phase transition are used. We have investigated dynamical trajectories of compressed baryon-rich matter as functions of various thermodynamical variables. The parameters of collective flow and hadronic spectra are calculated. It is shown that presence of the deconfinement phase transition leads to increase of the elliptic flow and to flattening of proton rapidity distributions.Comment: 11 pages, 6 figure

Surface tension implementation for Gensmac 2D

In the present work we describe a method which allows the incorporation of surface tension into the GENSMAC2D code. This is achieved on two scales. First on the scale of a cell, the surface tension effects are incorporated into the free surface boundary conditions through the computation of the capillary pressure. The required curvature is estimated by fitting a least square circle to the free surface using the tracking particles in the cell and in its close neighbors. On a sub-cell scale, short wavelength perturbations are filtered out using a local 4-point stencil which is mass conservative. An efficient implementation is obtained through a dual representation of the cell data, using both a matrix representation, for ease at identifying neighbouring cells, and also a tree data structure, which permits the representation of specific groups of cells with additional information pertaining to that group. The resulting code is shown to be robust, and to produce accurate results when compared with exact solutions of selected fluid dynamic problems involving surface tension

1+1 Dimensional Hydrodynamics for High-energy Heavy-ion Collisions

A 1+1 dimensional hydrodynamical model in the light-cone coordinates is used to describe central heavy-ion collisions at ultrarelativistic bombarding energies. Deviations from Bjorken's scaling are taken into account by choosing finite-size profiles for the initial energy density. The sensitivity of fluid dynamical evolution to the equation of state and the parameters of initial state is investigated. Experimental constraints on the total energy of produced particles are used to reduce the number of model parameters. Spectra of secondary particles are calculated assuming that the transition from the hydrodynamical stage to the collisionless expansion of matter occurs at a certain freeze-out temperature. An important role of resonances in the formation of observed hadronic spectra is demonstrated. The calculated rapidity distributions of pions, kaons and antiprotons in central Au+Au collisions at the c.m. energy 200 GeV per NN pair are compared with experimental data of the BRAHMS Collaboration. Parameters of the initial state are reconstructed for different choices of the equation of state. The best fit of these data is obtained for a soft equation of state and Gaussian-like initial profiles of the energy density, intermediate between the Landau and Bjorken limits.Comment: 43 pages, 27 figure

Thermal photons as a measure for the rapidity dependence of the temperature

The rapidity distribution of thermal photons produced in Pb+Pb collisions at CERN-SPS energies is calculated within scaling and three-fluid hydrodynamics. It is shown that these scenarios lead to very different rapidity spectra. A measurement of the rapidity dependence of photon radiation can give cleaner insight into the reaction dynamics than pion spectra, especially into the rapidity dependence of the temperature.Comment: 3 Figure

The initial state of ultra-relativistic heavy ion collision

A model for energy, pressure and flow velocity distributions at the beginning of ultra-relativistic heavy ion collisions is presented, which can be used as an initial condition for hydrodynamic calculations. Our model takes into account baryon recoil for both target and projectile, arising from the acceleration of partons in an effective field, produced in the collision. The typical field strength (string tension) for RHIC energies is about 5-12 GeV/fm, what allows us to talk about "string ropes". The results show that a QGP forms a tilted disk, such that the direction of the largest pressure gradient stays in the reaction plane, but deviates from both the beam and the usual transverse flow directions. Such initial conditions may lead to the creation of "antiflow" or "third flow component".Comment: 28 pages, 9 figures. The presentation has been changed considerably. Some parts of the model have been reformulated, what led to modifications in several equations: (20-38), Apps. A, B. All the figures have been changed from 100 GeV/nucl initial energy to the achieved RHIC energy of 65 GeV/nucl. The last subplots in the Figs. 3, 4, 5, 6 present E=T^{00} in the laboratory frame now, instead of the energy density in the local rest frame, e, shown in the initial version. We also added the App. C to clarify the transformation from space-time to lightcone coordinates and bac

Neutrons from multiplicity-selected Au-Au collisions at 150, 250, 400, and 650 AMeV

We measured neutron triple-differential cross sections from multiplicity-selected Au-Au collisions at 150, 250, 400, and 650 \AMeV. The reaction plane for each collision was estimated from the summed transverse velocity vector of the charged fragments emitted in the collision. We examined the azimuthal distribution of the triple-differential cross sections as a function of the polar angle and the neutron rapidity. We extracted the average in--plane transverse momentum $\langle P_x\rangle$ and the normalized observable $\langle P_x/P_\perp\rangle$, where $P_\perp$ is the neutron transverse momentum, as a function of the neutron center-of-mass rapidity, and we examined the dependence of these observables on beam energy. These collective flow observables for neutrons, which are consistent with those of protons plus bound nucleons from the Plastic Ball Group, agree with the Boltzmann--Uehling--Uhlenbeck (BUU) calculations with a momentum--dependent interaction. Also, we calculated the polar-angle-integrated maximum azimuthal anisotropy ratio R from the value of $\langle P_x/P_\perp\rangle$.Comment: 20 LaTeX pages. 11 figures to be faxed on request, send email to sender's addres

Effects of Compression and Collective Expansion on Particle Emission from Central Heavy-Ion Reactions

Conditions under which compression occurs and collective expansion develops in energetic reactions of heavy nuclei, are analyzed, together with their effects on emitted light baryons and pions. Within transport simulations, it is shown that shock fronts perpendicular to beam axis form in head-on reactions. The fronts separate hot compressed matter from normal. As impact parameter increases, the angle of inclination of the fronts relative to beam axis decreases, and in-between the fronts a weak tangential discontinuity develops. Hot matter exposed to the vacuum in directions perpendicular to shock motion (and parallel to fronts), starts to expand sideways, early within reactions. Expansion in the direction of shock motion follows after the shocks propagate through nuclei, but due to the delay does not acquire same strength. Expansion affects angular distributions, mean-energy components, shapes of spectra and mean energies of different particles emitted into any one direction, and further particle yields. Both the expansion and a collective motion associated with the weak discontinuity, affect the magnitude of sideward flow within reaction plane. Differences in mean particle energy components in and out of the reaction plane in semicentral collisions, depend sensitively on the relative magnitude of shock speed in normal matter and speed of sound in hot matter.Comment: 71 pages, 33 figures (available on request), report MSUCL-94

Relativistic Nucleus-Nucleus Collisions: from the BEVALAC to RHIC

I briefly describe the initial goals of relativistic nuclear collisions research, focusing on the LBL Bevatron/Bevalac facility in the 1970's. An early concept of high hadronic density fireball formation, and subsequent isentropic decay (preserving information as to the high density stage) led to an outline of physics observables that could determine the nuclear matter equation of state at several times nuclear ground state matter density. With the advent of QCD the goal of locating, and characterizing the hadron-parton deconfinement phase transformation suggested the need for higher $\sqrt{s}$, the research thus moving to the BNL AGS and CERN SPS, finally to RHIC at BNL. A set of physics observables is discussed where present data span the entire $\sqrt{s}$ domain, from Bevalac and SIS at GSI, to top RHIC energy. Referring, selectively, to data concerning bulk hadron production, the overall $\sqrt{s}$ evolution of directed and radial flow observables, and of pion pair Bose-Einstein correlation are discussed. The hadronization process is studied in the grand canonical statistical model. The resulting hadronization points in the plane T vs. $\mu_B$ converge onto the parton-hadron phase boundary predicted by finite $\mu_B$ lattice QCD, from top SPS to RHIC energy. At lower SPS and top AGS energy a steep strangeness maximum occurs at which the Wroblewski parameter $\lambda_s \approx$ 0.6; a possible connection to the QCD critical point is discussed. Finally the unique new RHIC physics is addressed: high $p_T$ hadron suppression and jet "tomography".Comment: 19 pages, 11 figure