arXiv subjects

Carsten Greiner

Publications and source records attributed to Carsten Greiner.

At least 145 records · Page 8Linked to original sources

arXiv2008

QCD Matter Thermalization at RHIC and LHC

Employing the perturbative QCD inspired parton cascade, we investigate kinetic and chemical equilibration of the partonic matter created in central heavy ion collisions at RHIC and LHC energies. Two types of initial conditions are chosen. One is generated by the model of wounded nucleons using the PYTHIA event generator and Glauber geometry. Another is considered as a color glass condensate. We show that kinetic equilibration is almost independent on the chosen initial conditions, whereas there is a sensitive dependence for chemical equilibration. The time scale of thermalization lies between 1 and 1.5 fm/c. The final parton transverse energy obtained from BAMPS calculations is compared with the RHIC data and is estimated for the LHC energy.

hep-ph↗

arXiv2008

Review of the "Bottom-Up" scenario

Thermalization of a longitudinally expanding color glass condensate with Bjorken boost invariant geometry is investigated within parton cascade BAMPS. Our main focus lies on the detailed comparison of thermalization, observed in BAMPS with that suggested in the Bottom-Up scenario. We demonstrate that the tremendous production of soft gluons via $gg \to ggg$, which is shown in the Bottom-Up picture as the dominant process during the early preequilibration, will not occur in heavy ion collisions at RHIC and LHC energies, because the back reaction $ggg\to gg$ hinders the absolute particle multiplication. Moreover, contrary to the Bottom-Up scenario, soft and hard gluons thermalize at the same time. The time scale of thermal equilibration in BAMPS calculations is of order $\as^{-2} (\ln \as)^{-2} Q_s^{-1}$. After this time the gluon system exhibits nearly hydrodynamic behavior. The shear viscosity to entropy density ratio has a weak dependence on $Q_s$ and lies close to the lower bound of the AdS/CFT conjecture.

hep-ph↗

arXiv2008

QCD plasma equilibration, elliptic flow and jet-quenching - phenomena of common origin

Fast thermalization, a subsequent strong buildup of elliptic flow of QCD matter and jet-quenching as found at the Relativistic Heavy Ion Collider (RHIC) are understood as the consequence of perturbative QCD (pQCD) interactions within the 3+1 dimensional parton cascade BAMPS. The main contributions stem from pQCD inspired bremsstrahlung. Comparisons to Au+Au data of the flow parameter v_2 as a function of participation number as well as the gluonic contribution to the nuclear modification factor R_AA for most central collisions are given. Also the shear viscosity to entropy ratio is dynamically extracted, which lies in the range of 0.08-0.15, depending on the chosen coupling constant.

hep-ph↗

arXiv2008

Thermalization of a color glass condensate and review of the "Bottom-Up" scenario

Thermalization of a longitudinally expanding color glass condensate with Bjorken boost invariant geometry is investigated within microscopical parton cascade BAMPS. Our main focus lies on the detailed comparison of thermalization, observed in BAMPS with that suggested in the \BUp scenario. We demonstrate that the tremendous production of soft gluons via $gg \to ggg$, which is shown in the \BUp picture as the dominant process during the early preequilibration, will not occur in heavy ion collisions at RHIC and LHC energies, because the back reaction $ggg\to gg$ hinders the absolute particle multiplication. Moreover, different from the \BUp scenario, soft and hard gluons thermalize at the same time. The time scale of thermal equilibration obtained from BAMPS calculations is of order $\as^{-2} (\ln \as)^{-2} Q_s^{-1}$. After this time the gluon system exhibits nearly hydrodynamical behavior. The shear viscosity to entropy density ratio has weak dependence on $Q_s$ and lies close to the lower bound from the AdS/CFT conjecture.

hep-ph↗

arXiv2008

Perturbative QCD Calculations of Elliptic Flow and Shear Viscosity in Au+Au Collisions at $\sqrt{s_{NN}}=200$ GeV

The elliptic flow $v_2$ and the ratio of the shear viscosity over the entropy density, $η/s$, of gluon matter are calculated from the perturbative QCD (pQCD) based parton cascade Boltzmann approach of multiparton scatterings. For Au+Au collisions at $\sqrt{s}=200$A GeV the gluon plasma generates large $v_2$ values measured at the BNL Relativistic Heavy Ion Collider. Standard pQCD yields $η/s\approx 0.08-0.15$ as small as the lower bound found from the anti-de Sitter/conformal field theory conjecture.

nucl-th↗

arXiv2008

The thermodynamics for a hadronic gas of fireballs with internal color structures and chiral fields

The thermodynamical partition function for a gas of color-singlet bags consisting of fundamental and adjoint particles in both $U(N_c)$ and $SU(N_c)$ group representations is reviewed in detail. The constituent particle species are assumed to satisfy various thermodynamical statistics. The gas of bags is probed to study the phase transition for a nuclear matter in the extreme conditions. These bags are interpreted as the Hagedorn states and they are the highly excited hadronic states which are produced below the phase transition point to the quark-gluon plasma. The hadronic density of states has the Gross-Witten critical point and exhibits a third order phase transition from a hadronic phase dominated by the discrete low-lying hadronic mass spectrum particles to another hadronic phase dominated by the continuous Hagedorn states. The Hagedorn threshold production is found just above the highest known experimental discrete low-lying hadronic mass spectrum. The subsequent Hagedorn phase undergoes a first order deconfinement phase transition to an explosive quark-gluon plasma. The role of the chiral phase transition in the phases of the discrete low-lying mass spectrum and the continuous Hagedorn mass spectrum is also considered. It is found crucial in the phase transition diagram. Alternate scenarios are briefly discussed for the Hagedorn gas undergoes a higher order phase transition through multi-processes of internal color-flavor structure modification.

nucl-th↗

arXiv2008

QCD plasma thermalization, collective flow and extraction of shear viscosity

Fast thermalization and elliptic flow of QCD matter found at the Relativistic Heavy Ion Collider (RHIC) are understood as the consequence of perturbative QCD (pQCD) interactions within a (3+1) dimensional parton cascade. The main contributions stem from pQCD-inspired bremsstrahlung. We extract the shear viscosity to entropy ratio, which is between 0.08 and 0.15.

hep-ph↗

arXiv2008

Shear Viscosity in a Gluon Gas

The relation of the shear viscosity coefficient to the recently introduced transport rate is derived within relativistic kinetic theory. We calculate the shear viscosity over entropy ratio η/s for a gluon gas, which involves elastic gg-> gg perturbative QCD (PQCD) scatterings as well as inelastic gg<->ggg PQCD bremsstrahlung. For α_s=0.3 we find η/s=0.13 and for α_s=0.6, η/s=0.076. The small η/s values, which suggest strongly coupled systems, are due to the gluon bremsstrahlung incorporated.

nucl-th↗

arXiv2007

Transport rates and momentum isotropization of gluon matter in ultrarelativistic heavy-ion collisions

To describe momentum isotropization of gluon matter produced in ultrarelativistic heavy-ion collisions, the transport rate of gluon drift and the transport collision rates of elastic ($gg \leftrightarrow gg$) as well as inelastic ($gg \leftrightarrow ggg$) perturbative quantum chromodynamics- (pQCD) scattering processes are introduced and calculated within the kinetic parton cascade Boltzmann approach of multiparton scatterings (BAMPS), which simulates the space-time evolution of partons. We define isotropization as the development of an anisotropic system as it reaches isotropy. The inverse of the introduced total transport rate gives the correct time scale of the momentum isotropization. The contributions of the various scattering processes to the momentum isotropization can be separated into the transport collision rates. In contrast to the transport cross section, the transport collision rate has an indirect but correctly implemented relationship with the collision-angle distribution. Based on the calculated transport collision rates from BAMPS for central Au+Au collisions at Relativistic Heavy Ion Collider energies, we show that pQCD $gg \leftrightarrow ggg$ bremsstrahlung processes isotropize the momentum five times more efficiently than elastic scatterings. The large efficiency of the bremsstrahlung stems mainly from its large momentum deflection. Due to kinematics, $2\to N$ $(N>2)$ production processes allow more particles to become isotropic in momentum space and thus kinetically equilibrate more quickly than their back reactions or elastic scatterings. We also show that the relaxation time in the relaxation time approximation, which is often used, is strongly momentum dependent and thus cannot serve as a global quantity that describes kinetic equilibration.

hep-ph↗

arXiv2007

The role of the gluonic gg<->ggg interactions in early thermalization in ultrarelativistic heavy-ion collisions

We ``quantify'' the role of elastic as well as inelastic gg<->ggg pQCD processes in kinetic equilibration within a pQCD inspired parton cascade. The contributions of different processes to kinetic equilibration are manifested by the transport collision rates. We find that in a central Au+Au collision at RHIC energy pQCD Bremstrahlung processes are much more efficient for momentum isotropization compared to elastic scatterings. For the parameters chosen the ratio of their transport collision rates amounts to 5:1.

hep-ph↗

arXiv2006

The order, shape and critical point for the quark-gluon plasma phase transition

The order, shape and critical point for the phase transition between the hadronic matter and quark-gluon plasma are considered in a thermodynamical consistent approach. The hadronic phase is taken as Van der Waals gas of all the known hadronic mass spectrum particles $m_H\le 2.0$ GeV as well as Hagedorn bubbles which correspond hadronic states with mass spectrum $m_H> 2.0$ GeV. The density of states for Hagedorn bubbles is derived by calculating the microcanonical ensemble for a bag of quarks and gluons with specific internal color-flavor symmetry. The mixed-grand and microcanonical ensembles are derived for massless and massive flavors. We find Hagedorn bubbles are strongly suppressed in the dilute hadronic matter and they appear just below the line of the phase transition. The order of the phase transition depends on Hagedorn bubble's internal color-flavor structure and the volume fluctuation as well. On the other hand, the highly compressed hadronic matter undergoes a smooth phase transition from the gas of known mass spectrum hadrons to another one dominated by Hagedorn bubbles with specific internal color-flavor structure before the phase transition to quark-gluon plasma takes place at last. The phase transition is found a first order for the intermediate and large chemical potentials. The existence of the tri-critical point depends on the modification of the bubble's internal structure specified by a phenomenological parameter $γ\proptoμ_B$ in the medium.

nucl-th↗

arXiv2006

Dilepton yields from Brown-Rho scaled vector mesons including memory effects

Brown-Rho scaling, which has been strongly discussed after recent NA60 data was presented, is investigated within a nonequilibrium field theoretical description that includes quantum mechanical memory. Dimuon yields are calculated by application of a model for the fireball, and strong modifications are found in the comparison to quasi-equilibrium calculations, which assume instantaneous adjustment of all meson properties to the surrounding medium. In addition, we show results for the situation of very broad excitations.

hep-ph↗

arXiv2006

Interactions of multi-quark states in the chromodielectric model

We investigate 4-quark ($qq\bar{q}\bar{q}$) systems as well as multi-quark states with a large number of quarks and anti-quarks using the chromodielectric model. In the former type of systems the flux distribution and the corresponding energy of such systems for planar and non-planar geometries are studied. From the comparison to the case of two independent $q\bar{q}$-strings we deduce the interaction potential between two strings. We find an attraction between strings and a characteristic string flip if there are two degenerate string combinations between the four particles. The interaction shows no strong Van-der-Waals forces and the long range behavior of the potential is well described by a Yukawa potential, which might be confirmed in future lattice calculations. The multi-quark states develop an inhomogeneous porous structure even for particle densities large compared to nuclear matter constituent quark densities. We present first results of the dependence of the system on the particle density pointing towards a percolation type of transition from a hadronic matter phase to a quark matter phase. The critical energy density is found at $ε_c = 1.2 GeV/fm^3$.

hep-ph↗

arXiv2006

A model of the effect of collisions on QCD plasma instabilities

We study the effect of including a BGK collisional kernel on the collective modes of a QCD plasma which has a hard-particle distribution function which is anisotropic in momentum space. We calculate dispersion relations for both the stable and unstable modes and show that the addition of hard particle collisions slows the rate of growth of QCD plasma unstable modes. We also show that for any anisotropy there is an upper limit on the collisional frequency beyond which no instabilities exist. Estimating a realistic value for the collisional frequency for alpha_s ~ 0.2 - 0.4 we find that for the large-anisotropy case which is relevant for the initial state of matter generated by free streaming in heavy-ion collisions that the collisional frequency is below this critical value.

hep-ph↗

arXiv2005

Thermalization of gluons at RHIC: Dependence on initial conditions

We investigate how thermalization of gluons depends on the initial conditions assumed in ultrarelativistic heavy ion collisions at RHIC. The study is based on simulations employing the pQCD inspired parton cascade solving the Boltzmann equation for gluons. We consider independently produced minijets with $p_T > p_0=1.3 \sim 2.0$ GeV and a color glass condensate as possible initial conditions for the freed gluons. It turns out that full kinetic equilibrium is achieved slightly sooner in denser system and its timescale tends to saturate. Compared with the kinetic equilibration we find a stronger dependence of chemical equilibration on the initial conditions.

hep-ph↗

arXiv2005

Compact Stars for Undergraduates

We report on an undergraduate student project initiated in the summer semester of 2004 with the aim to establish equations of state for white dwarfs and neutron stars for computing mass-radius relations as well as corresponding maximum masses. First, white dwarfs are described by a Fermi gas model of degenerate electrons and neutrons and effects from general relativity are examined. For neutron star matter, the influence of a finite fraction of protons and electrons and of strong nucleon-nucleon interactions are studied. The nucleon--nucleon interactions are introduced within a Hartree-Fock scheme using a Skyrme-type interaction. Finally, masses and radii of neutron stars are computed for given central pressure.

astro-ph↗

arXiv2005

Thermalization of gluons in ultrarelativistic heavy ion collisions by including three-body interactions in a parton cascade

We develop a new 3+1 dimensional Monte Carlo cascade solving the kinetic on-shell Boltzmann equations for partons including the inelastic gg <-> ggg pQCD processes. The back reaction channel is treated -- for the first time -- fully consistently within this scheme. An extended stochastic method is used to solve the collision integral. The frame dependence and convergency are studied for a fixed tube with thermal initial conditions. The detailed numerical analysis shows that the stochastic method is fully covariant and that convergency is achieved more efficiently than within a standard geometrical formulation of the collision term, especially for high gluon interaction rates. The cascade is then applied to simulate parton evolution and to investigate thermalization of gluons for a central Au+Au collision at RHIC energy. For this study the initial conditions are assumed to be generated by independent minijets with p_T > p_0=2 GeV. With that choice it is demonstrated that overall kinetic equilibration is driven mainly by the inelastic processes and is achieved on a scale of 1 fm/c. The further evolution of the expanding gluonic matter in the central region then shows almost an ideal hydrodynamical behavior. In addition, full chemical equilibration of the gluons follows on a longer timescale of about 3 fm/c.

hep-ph↗

arXiv2004

Two- and three-body color flux tubes in the Chromo Dielectric Model

Using the framework of the Chromo Dielectric Model we perform an analysis of color electric flux tubes in meson-like $q\bar{q}$ and baryon-like $qqq$ quark configurations. We discuss the Abelian color structure of the model and point out a symmetry in color space as a remnant of the SU(3) symmetry of QCD. The generic features of the model are discussed by varying the model parameters. We fix these parameters by reproducing the string tension $τ=980 $MeV/fm and the transverse width $ρ=0.35 $fm of the $q\bar{q}$ flux tube obtained in lattice calculations. We use a bag constant $B^{1/4}=(240-260) $MeV, a glueball mass $m_g = (1000-1700) $MeV and a strong coupling constant $C_F α_s = 0.2-0.3$. We show that the asymptotic string profile of an infinitely long flux tube is already reached for $q\bar{q}$ separations $R\ge1.0 $fm. A connection to the Dual Color Superconductor is made by extracting a magnetic current from the model equations and a qualitative agreement between the two descriptions of confinement is shown. In the study of the $qqq$ system we observe a $\mathsfΔ$-like geometry for the color electric fields and a \textsf{Y}-like geometry in the scalar fields both in the energy density distribution and in the corresponding potentials. The resulting total $qqq$ potential is described neither by the $\mathsfΔ$-picture nor by the \textsf{Y}-picture alone.

hep-ph↗