Baryon Rapidity Distributions Research Articles

Landau and Eckart frames for relativistic fluids in nuclear collisions

The quark matter created in relativistic nuclear collisions is interpreted as a nearly-perfect fluid. The recent efforts to explore its finite-density properties in the beam energy scan programs motivate one to revisit the issue of the local rest frame fixing in off-equilibrium hydrodynamics. I first investigate full second-order relativistic hydrodynamics in the Landau and the Eckart frames. Then numerical hydrodynamic simulations are performed to elucidate the effect of frame choice on flow observables in relativistic nuclear collisions. The results indicate that the flow can differ in the Landau and the Eckart frames but charged particle and net baryon rapidity distributions are mostly frame independent when off-equilibrium kinetic freeze-out is considered.

Dissipative hydrodynamic effects on the quark-gluon plasma at finite baryon density

The quark-gluon plasma behaves as a relativistic viscous fluid in high-energy heavy ion collisions. I develop a causal dissipative hydrodynamic model at finite baryon density for RHIC and LHC to estimate the net baryon rapidity distribution. The net baryon number is found to be carried to forward rapidity by the flow, effectively enhancing the transparency of the collisions. This suggests that the energy available for the production of a hot medium could be larger than that naively implied from experimental data. Also the distribution can be sensitive to baryon dissipation as much as to shear and bulk viscosity.

Event-by-event analysis of baryon-strangeness correlations: Pinning down the critical temperature and volume of quark-gluon-plasma formation

The recently proposed baryon-strangeness correlation (C_BS) is studied with a string-hadronic transport model (UrQMD) for various energies from E_lab=4 AGeV to \sqrt s=200 AGeV. It is shown that rescattering among secondaries can not mimic the predicted correlation pattern expected for a Quark-Gluon-Plasma. However, we find a strong increase of the C_BS correlation function with decreasing collision energy both for pp and Au+Au/Pb+Pb reactions. For Au+Au reactions at the top RHIC energy (\sqrt s=200 AGeV), the C_BS correlation is constant for all centralities and compatible with the pp result. With increasing width of the rapidity window, C_BS follows roughly the shape of the baryon rapidity distribution. We suggest to study the energy and centrality dependence of C_BS which allow to gain information on the onset of the deconfinement transition in temperature and volume.

Monte Carlo model for multiparticle production at ultrarelativistic energies.

The Monte Carlo parton string model for multiparticle production in hadron-hadron, hadron-nucleus, and nucleus-nucleus collisions at high energies is described. An adequate choice of the parameters in the model gives the possibility of recovering the main results of the dual parton model, with the advantage of treating both hadron and nuclear interactions on the same footing, reducing them to interactions between partons. Also the possibility of considering both soft and hard parton interactions is introduced. Comparison to the available experimental data on nucleon and nuclear collisions, together with predictions for mean multiplicities, net baryon rapidity distributions, and the temporal evolution of meson densities for heavy ion collisions at the CERN Super Proton Synchrotron (194A) GeV) and the BNL Relativistic Heavy Ion Collider (200A GeV) are presented. Furthermore, predictions for charm production at these energies are given.

Collective global dynamics in Au+Au collisions at the BNL AGS.

Signatures of collective effects are studied in the quark gluon string model and in the fluid dynamical model for Au+Au collisions at 11.6A GeV/c. In the fluid dynamical model the dependence of observables on the quark-gluon plasma (QGP) formation in the equation of state is pointed out although the maximal total amount of pure QGP formed is only about 8 ${\mathrm{fm}}^{3}$ in these reactions. In contrast to the baryon rapidity distribution, the in-plane transverse flow and especially the squeeze-out effect are particularly sensitive to the EOS. In the QGSM the lifetime and extent of baryon density in strings are studied. The QGSM picture is very similar to the one obtained in the fluid dynamical model with a pure hadronic EOS.

Viscosity and the equation of state in high energy heavy-ion reactions.

Viscous hydrodynamic calculations of high energy heavy-ion collisions (Nb-Nb and Au-Au) from 200 to 800 MeV/nucleon are presented. The resulting baryon rapidity distributions, the in-plane transverse momentum transfer (bounce-off), and the azimuthal dependence of the midrapidity particles (off-plane squeeze out) compare well with Plastic Ball data. We find that the considered observables are sensitive both to the nuclear equation of state and to the nuclear shear viscosity [eta]. Transverse momentum distributions indicate a high shear viscosity ([eta][approx]60 MeV/fm[sup 2] [ital c]) in the compression zone, in agreement with nuclear matter estimates. The bulk viscosity [zeta] influences only the entropy production during the expansion stage; collective observables like flow and [ital dN]/[ital dY] do not depend strongly on [zeta]. The recently observed off-plane ([phi]=90[degree]) squeeze-out, which is found in the [ital triple]-[ital differential] rapidity distribution, exhibits the strongest sensitivity to the nuclear equation of state. It is demonstrated that for very central collisions, [ital b]=1 fm, the squeeze-out is visible even in the double-differential cross section. This is experimentally accessible by studying azimuthally symmetric events, as confirmed recently by data of the European 4[pi] detector collaboration at Gesellchaft fuer Schwerionforschung Darmstadt.

HYDRODYNAMIC SIMULATIONS OF 16O+208Pb COLLISIONS AT 200 GeV/N

We investigate collisions of 16 O with 208 Pb at a bombarding energy of 200 GeV /N in the lab frame. Our basis is an ideal, one-fluid 3-D hydrodynamics model. Several equations of state are used to study the qualitative relation between final baryon rapidity distributions and the stiffness of the EOS. We find that 75% of the baryonic matter lies within one unit of the initial target rapidity, in qualitative agreement with recent experimental data. In addition, our calculation is able to reproduce the integrated value of the dET/dy distribution that is observed experimentally.

Baryon production in the central region of ultrarelativistic heavy-ion collisions.

We introduce a new string model (venus 2) for ultrarelativistic heavy-ion collisions, based on color exchange between quarks and also between antiquarks. These two mechanisms affect the rapidity distribution of baryons differently: Quark color exchange shifts baryons towards central rapidity, antiquark color exchange moves baryons back towards large (c.m.) rapidities. We obtain almost flat baryon rapidity distributions for symmetric A+A collisions. Rapidity distributions for proton-nucleus collisions show a forward plateau rather than a forward peak, in agreement with data.

Target fragmentation in proton-nucleus and16O-nucleus reactions at 60 and 200 GeV/nucleon

Target remnants withZ<3 from proton-nucleus and16O-nucleus reactions at 60 and 200 GeV/nucleon were measured in the angular range from 30° to 160° (−1.7<η<1.3) employing the Plastic Ball detector. The excitation energy of the target spectator matter in central oxygen-induced collisions is found to be high enough to allow for complete disintegration of the target nucleus into fragments withZ<3. The average longitudinal momentum transfer per proton to the target in central collisions is considerably higher in the case of16O-induced reactions (≈300 MeV/c) than in proton-induced reactions (≈130 MeV/c). The baryon rapidity distributions are roughly in agreement with one-fluid hydrodynamical calculations at 60 GeV/nucleon16O+Au but are in disagreement at 200 GeV/nucleon, indicating the higher degree of transparency at the higher bombarding energy. Both, the transverse momenta of target spectators and the entropy produced in the target fragmentation region are compared to those attained in head-on collisions of two heavy nuclei at Bevalac energies. They are found to be comparable or do even exceed the values for the participant matter at beam energies of about 1–2 GeV/nucleon.

Baryon distribution in relativistic heavy-ion collisions

In order to determine whether a pure quark-gluon plasma with no net baryon density can be formed in the central rapidity region in relativistic heavy-ion collisions, we estimate the baryon distribution by using a Glauber-type multiple-collision model in which the nucleons of one nucleus degrade in energy as they make collisions with nucleons in the other nucleus. As a test of this model, we study first nucleon-nucleus collisions at 100 GeV/c and compare the theoretical results with the experimental data of Barton et.al The results are then generalized to study the baryon distribution in nucleus-nucleus collisions. It is found that in the head-on collision of two heavy nuclei ($A\ensuremath{\gtrsim}100$), the baryon rapidity distributions have broad peaks and extend well into the central rapidity region. The energy density of the baryon in the central rapidity region is about 5-6% of the total energy density at a center-of-mass energy of 30 GeV per nucleon and decreases to about 2-3% at a center-of-mass energy of 100 GeV per nucleon. The stopping power for a baryon in nuclear matter is extracted.

Baryon rapidity distribution and stopping power of high-energy colliding nuclei

A very simple estimate is made of the baryon rapidity distribution to be expected from central collisions of high energy nuclei. This estimate is based upon straight-line trajectories and some average rapidity loss per nucleon-nucleon collision. It is found that U + U will become transparent at a beam energy of about 500 GeV/nucleon, or 15 + 15 GeV/nucleon for colliding beams.