Ultrarelativistic Nucleus-nucleus Collisions Research Articles

Flavor production in Pb(160A GeV) on Pb collisions: Effect of color ropes and hadronic rescattering.

Collective interactions in the preequilibrium quark matter and hadronic resonance gas stage of ultrarelativistic nucleus-nucleus collisions are studied in the framework of the transport theoretical approach RQMD. The paper reviews string fusion into color ropes and hadronic rescattering which serve as models for these interactions. Hadron production in central Pb(160A GeV) on Pb collisions has been calculated. The changes of the final flavor composition are more pronounced than in previous RQMD studies of light ion induced reactions at 200A GeV. The ratio of created quark pairs ss\ifmmode\bar\else\textasciimacron\fi{}/(u\ifmmode \bar{u}\else \={u}\fi{}+dd\ifmmode\bar\else\textasciimacron\fi{}) is enhanced by a factor of 2.4 in comparison to pp results. Color rope formation increases the initially produced antibaryons to 3 times the value in the ``NN mode,'' but only one quarter of the produced antibaryons survives because of subsequent strong absorption. The differences in the final particle composition for Pb on Pb collisions compared to S-induced reactions are attributed to the hadronic resonance gas stage which is baryon-richer and lasts longer.

Dilepton Production in the Central Rapidity Region

Generalized Boltzmann approximation of the phase-space distribution function of quarks in the quark phase and of pions in the hadron phase, adopted by previous authors, to a complete expression, the dilepton production in Bjorken's (1+1) dimensional scaling expansion system is studied. We find that the dilepton production is heightened, and the shape of the characteristic dilepton distribution for the quark matter formation in ultrarelativistic nucleus-nucleus collisions is significantly changed.

Mass dependence of hadron distributions in ultrarelativistic nucleus-nucleus collisions

(Anti-) baryon and kaon distributions in nucleus-nucleus reactions at 200 AGeV are studied in the framework of the transport model RQMD. Production mechanisms for strangeness and baryon pairs are tested by comparing their projectile and target mass dependence to available experimental data. RQMD contains two collective production processes, fusion of overlapping strings into highly charged chromoelectric ropes and hadronic rescattering. It turns out that both rope formation and hadronic rescattering are of importance for creation-and annihilation-of strangeness and antibaryons.

Quark exchange model for charmonium dissociation in hot hadronic matter.

A diagrammatic approach to quark exchange processes in meson-meson scattering is applied to the case of inelastic reactions of the type (QQ\ifmmode\bar\else\textasciimacron\fi{})+(qq\ifmmode\bar\else\textasciimacron\fi{})\ensuremath{\rightarrow}(Qq\ifmmode\bar\else\textasciimacron\fi{})+(qQ\ifmmode\bar\else\textasciimacron\fi{}), where Q and q refer to heavy and light quarks, respectively. This string-flip process is discussed as a microscopic mechanism for charmonium dissociation (absorption) in hadronic matter. The cross section for the reaction J/\ensuremath{\psi}+\ensuremath{\pi}\ensuremath{\rightarrow}D+D\ifmmode\bar\else\textasciimacron\fi{} is calculated using a potential model, which is fitted to the meson mass spectrum. The temperature dependence of the relaxation time for the J/\ensuremath{\psi} distribution in a homogeneous thermal pion gas is obtained. The use of charmonium for the diagnostics of the state of hot hadronic matter produced in ultrarelativistic nucleus-nucleus collisions is discussed.

A Study of Collision Frequency and Particle Density in Ultrarelativistic Nuclear Collisions by an Event Generator URASiMA

Local collision frequencies of produced hadrons in ultrarelativistic nucleus-nucleus collisions are studied by an event generator URASiMA. From these frequencies, local number densities of produced hadrons are derived. It turns out that thermalized high density local system is achievable in CERN-SPS energy region although its size is not always large. Time evolution and nuclear mass number dependence of these quantities, frequency and density, are also investigated.

Two-body correlations in pionic systems

In close analogy to fermionic many-body theory the truncation of the bosonic BBGKY density matrix hierarchy on the two-body level leads to a coupled set of nonlinear equations of motion for the one-body density matrix and the two-body correlation function. These equations provide a nonperturbative description of the nonequilibrium time evolution of particle number conserving bosonic many-body systems including the dynamical resummation of parquet-like diagrams. Within this framework we study the properties of a pionic system as a function of temperature and density with focus on two-body quantities. For each temperature we find a related pion density for which the relative strength of the two-body correlation function assumes a maximum and the pionic system is far from the mean-field limit. Since these correlated phases up to $T$=200 MeV only appear at rather low pion density, the hot and dense pion gas as generated in ultrarelativistic nucleus-nucleus collisions should be well described within mean-field theory; i.e. the HBT analysis of pion sources from $\pi-\pi$ correlations should remain valid even in the case of strongly interacting pions.

The lead ion accelerating facility and the relative experimental program at CERN SPS fixed target

The status of the lead beam construction and commissioning is reviewed. A very wide experimental program with heavy nuclei was approved at the CERN SPS for a dedicated study of ultrarelativistic nucleus-nucleus collisions. An arbitrary and very short summary of the advantages and motivations for the use of heavier nuclei in the quark-gluon plasma search will be presented.

Suppression of Dilepton in Expanding Hot Baryon-Rich Quark-Gluon Matter

With the increase of the initial baryon density, the dilepton production in invariant masses between 2mπ and 1 GeV is shown to be suppressed and an abnormal peak of the dilepton spectrum appears near the invariant mass 0.75 GeV if a baryon-rich quark-gluon matter was formed in the ultrarelativistic nucleus-nucleus collision. These predictions are important for several large-scale experiments at CERN and Brookhaven.

Effect of Bose-Einstein correlations on intermittency in ultrarelativistic nucleus-nucleus collisions

The role of quantum correlations, the rescattering of the produced particles and mixing of events with various impact parameters on intermittency in nucleus-nucleus collisions at 200 A GeV is studied in the framework of the RQMD model. We generated an RQMD data sample with Bose-Einstein correlations of identical pion pairs included. One can get quantitative agreement with the measured intermittent behaviour of the pion pseudorapidity spectra if these quantum correlations are taken into account.

“Effective-a” effect in ultra-relativistic nucleus-nucleus collisions and consequences on search for QGP

We compute an effective-A dependence of cross sections in ultra-relativistic nucleusnucleus collisions, which varies with the impact parameter and might simulate some QGP signatures. This effect can explain, without QGP, some measured ratios of particle production.

Effective source sizes of low rapidity soft particle emission

Correlations of positive pions and protons measured with the Plastic Ball detector in ultrarelativistic nucleus-nucleus collisions are studied. Source parameters are extracted for various projectile-target combinations. While the proton source can be explained by geometry, the pion source shows more subtle effects, which may be related to a very large source component. A comparison with the RQMD model for 200 A GeV 32S + Au reactions reveals discrepancies for the pion source.

Transverse flow effects on high-energy photons emitted by expanding quark-gluon plasma.

High-energy photons emitted in ultrarelativistic nucleus-nucleus collisions at energies reached at the CERN SPS and LHC and the BNL RHIC are discussed as a source of information on the collision dynamics and as a signature of the deconfining phase transition. The energy density and velocity fields of the expanding matter have been computed assuming cylindrical symmetry along the transverse direction and boost invariance along the longitudinal direction. The emission of photons from an expanding quark-gluon plasma undergoing a first-order phase transition has been evaluated. When the transverse flow of the system is accounted for, it is found that the photons having their origin in the hadronic matter are comparable to those originating in the quark-gluon plasma for [ital p][sub [ital T]] greater than 2 GeV, at energies reached at the SPS and RHIC. At energies reached at the LHC, photons having a [ital p][sub [ital T]] larger than about 4 GeV are shown to have their origin predominantly in the quark-gluon plasma. The location of the [ital p][sub [ital T]] window is shown to be very sensitive to the value of the freeze-out temperature chosen. The feasibility of evaluating the direct (QCD) photons has been studied. Photons fragmented off final more » state quarks in parton-parton scattering are shown to provide the largest background beyond [ital p][sub [ital T]][approx]3 GeV at energies reached at the LHC. Accounting for hadronic degrees of freedom beyond the pion is found to be important. « less

An [formula omitted] dependence of cross sections in ultra-relativistic heavy-ion collisions and its interference with quark-gluon plasma signatures

It is shown that in ultra-relativistic nucleus-nucleus collisions, as a consequence of the geometry of the collision, there is a dependence of the total and differential cross sections on effective atomic mass numbers of the target and beam nuclei. The general expressions of the effective atomic mass numbers are deduced and they are computed for 0+Cu, O+U, S+U, Si+Au and U+U collisions. This particular A-dependence of the cross sections can have a strong effect on the production ratio of two particles of different species. As a consequence it might simulate a signature of quark-gluon plasma which is expected to be given by the yields ratio of two different particles. It is shown that this effect can explain quantitatively the K/π ratios measured in Si+Au and S+W collisions as well as the decrease of the J/ψ production relative to Drell-Yan lepton pairs measured in O+Cu, O+U and S+U collisions. The need of precise measurements of the powers α and α( x F, P T) in inclusive nuleon-nuleus interactions for the understanding of particle production in nucleus-nucleus collisions is stressed. Expectations for future experiments are presented.

The hadronization of a quark-gluon plasma

The hadronization process of an expanding quark-gluon plasma is studied within two different non-equilibrium approaches. In the flavour kinetic model, we consider a bulk transition mechanism including resonance decay and particle evaporation, while in the second model we explore the decay through the evaporation of hadrons from the surface of a plasma droplet. The yield of differents hadrons are calculated. In particular the production of strange hadrons and of vector mesons are discussed in the light of recent data from ultra-relativistic nucleus-nucleus collisions.

J/ψ yield with suppression and its correlation with secondary-pion emission in quark-hadron matter

The production of J/ψ's resulting from ultrarelativistic nucleus-nucleus collisions is described by the semi-local duality model. The J/ψ production rate is studied in Bjorken's (1 + 1)-dimensional scaling expansion system. Its correlation with the secondary-hadron multiplicity is analyzed in detail. Furthermore, the J/ψ suppression caused by the deconfinement and absorption mechanisms is examined and their effects on this correlation are analyzed once again. In the plots of this correlation only a plateau remains in the case of the J/ψ suppression and this character could be used as a signal for diagnosing the mixed phase of QCD matter.

Colour rope formation and strange baryon production in ultrarelativistic heavy ion collisions

Enhanced strangenees production in recent experiments with S projectiles on different targets at a beam energy of 200 A GeV has been reported by several experimental groups (NA35, WA85, NA34). Here we consider that elementary triplet colour charges which are excited in the initial stage of a nucleus-nucleus collision may combine forming highly “ropes”. The colour ropes decay by quark-antiquark pair creation creation screening the initial field. We implemented the generated and decay of ropes into a new version of the Monte Carlo program RQMD which is based on string excitation in ultrarelativistic nucleus-nucleus collisions. We find that all measured strange particle yields in S+S can be quantitatively understood employing the rope mechanism in addition to hadronic rescattering.

Dilepton radiation from non-equilibrized parton matter produced in ultra-relativistic heavy-ion collisions

Dilepton radiation from parton (quark-gluon) matter, produced in ultrarelativistic nucleus-nucleus collisions, is calculated within a kinetic theory approach in order to study non-equilibrium effects. We investigate (i) the approach towards equilibrium in early collisions stages and (ii) the departure from a supposed equilibrium stage due to the subsequent longitudinal expansion. Independent of the details of the parton equilibration process the continuum dilepton rates from quark fusion in deconfined matter are found to exceed the Drell-Yan background in the J ψ region at RHIC and LHC energies. The appearance of higher mass dileptons depends on the degree of thermalization.

Production of strangeness and vector mesons from a decaying quark-gluon plasma

Production of strange hadrons and vector mesons from a decaying quark-gluon plasma is studied within two non-equilibrium scenarios. In the flavour kinetic model, we consider a bulk transition mechanism including resonance decay and particle evaporation, while in the second model we explore the decay through the evaporation of hadrons from the surface of a plasma droplet. The results are discussed in the light of recent data from ultra-relativistic nucleus-nucleus collisions.

Anisotropy as a signature of transverse collective flow.

We show that anisotropies in transverse-momentum distributions provide an unambiguous signature of transverse collective flow in ultrarelativistic nucleus-nucleus collisions. We define a measure of the anisotropy from experimental observables. The anisotropy coming from collective effects is estimated quantitatively using a hydrodynamical model, and compared to the anisotropy originating from finite multiplicity fluctuations. We conclude that collective behavior could be seen in Pb-Pb collisions if a few hundred particle momenta were measured in a central event.

Inclusive π0 production in ultrarelativistic nucleus-nucleus collisions

For several ion species we compare thermal to nonthermal contributions for inclusive π0 production after having used a hydrodynamical model to estimate the energy density and temperature of the quark-gluon plasma produced by each. For the high (low)-pT region quantum chromodynamics (the Fermi model) is used to estimate the nonthermal rates. The production of lowpT, lowxF π0’s, which clearly stand out from the background, is energetically favored by the quark-gluon plasma.