Quark-gluon Plasma Formation Research Articles

PHYSICS OF STRANGE MATTER FOR RELATIVISTIC HEAVY-ION COLLISIONS

Relativistic heavy-ion collisions offer the possibility to produce exotic metastable or even absolutely stable states of nuclear matter containing (roughly) equal number of strangeness compared to the baryon number: Strangelets, small pieces of strange quark matter, were proposed as a signal of quark-gluon plasma formation. As their hadronic counterpart, also small pieces of strange hadronic matter may also show up with rather similar properties. The reasoning of both their stability and existence, the possible separation of strangeness necessary for their formation, and the chances for their detection, are reviewed.

Diffusion model for relativistic heavy-ion collisions

A transport model is formulated in order to represent the nonequilibrium-statistical nature of heavy-ion collisions at relativistic and ultra-relativistic energies. The time evolution of macroscopic variables such as transverse energyE⊺ and longitudinal energyE‖, or rapidityY, is described by means of a diffusion equation. Analytical solutions for mean values and fluctuations ofE⊺ andE‖ are derived and evaluated at impact-parameter dependent values of the interaction times. Agreement with the available data on transverse energy distributions in ultra-relativistic reactions fromp+Au to Au+Au is obtained, and calculated rapidity distributions are found to agree with the data. A prediction for Pb+Pb at SPS-energies is compared with recent experimental results, and a possible signature of quark-gluon plasma formation is discussed.

REVIEW OF RAPIDITY DENSITY DISTRIBUTIONS IN HEAVY-ION INDUCED INTERACTIONS AT RELATIVISTIC ENERGIES

Violent interactions among colliding nuclei ultimately provide the suitable conditions for creating new and hitherto unexplored phenomena like color rope and quark-gluon plasma formation. The systemmatics achieved so far in pseudorapidity and density distributions of charged particles emitted from relativistic nucleus-nucleus collisions is reviewed and comparisons to model predictions are made. Data from both emulsion and counter experiments are utilized and the dependence of the angular spectra on projectile energy, centrality measured by means of forward charge or enegy flow, and on the projectile/target masses are reported. For a few central events, energy densities of the hot overlap zone are estimated to be beyond onset of deconfinement phase transition.

Strangeness enhancement in p+A and S+A interactions at energies near 200A GeV.

The systematics of strangeness enhancement is calculated using the hijing and venus models and compared to recent data on pp, pA, and AA collisions at CERN/SPS energies (200A GeV). The hijing model is used to perform a linear extrapolation from pp to AA. venus is used to estimate the effects of final state cascading and possible nonconventional production mechanisms. This comparison shows that the large enhancement of strangeness observed in S+Au collisions, interpreted previously as possible evidence for quark-gluon plasma formation, has its origins in nonequilibrium dynamics of few nucleon systems. A factor of 2 enhancement of ${\mathrm{\ensuremath{\Lambda}}}^{0}$ at midrapidity is indicated by recent pS data, where on the average one projectile nucleon interacts with only two target nucleons. There appears to be another factor of 2 enhancement in the light ion reaction SS relative to pS, when on the average only two projectile nucleons interact with two target ones.

Nucleation of quark-gluon plasma from hadronic matter

The energy densities achieved during central collisions of large nuclei at Brookhaven's AGS may be high enough to allow the formation of quark-gluon plasma. Calculations based on relativistic nucleation theory suggest that rare events, perhaps one in every 10 2 or 10 3, undergo the phase transition. Experimental ramifications may include an enhancement in the ratio of pions to baryons, a suppression of deuteron production and a larger source size as seen by hadron interferometry.

Formation and evolution of quark-gluon plasma at RHIC and LHC

Initial conditions for quark-gluon plasma formation at τ = 0.1 fm are considered in lowest order perturbative QCD. Chemical composition, thermalization of the formed semihard quark-gluon system and especially implications of the new HERA parton distributions with the enhancement at small x are studied. The plasma at τ = 0.1 fm is shown to be strongly gluon dominated both at RHIC and LHC, and a possibility for rapid thermalization of gluons at LHC is pointed out. Uncertainties in the calculations, particularly shadowing corrections to the parton distributions, are discussed. Free streaming and ideal hydro limits for the evolution of the gluonic plasma with the calculated minijet initial conditions are demonstrated, and a lower limit for final multiplicities obtained for the LHC nuclear collisions.

Measurement of the [formula omitted] production ratio in central S-W interactions at 200 A GeV/c

Strange baryon and in particular multi-strange baryon production is suggested to be a useful probe in the search for quark gluon plasma formation in heavy ion collisions. We have measured the (Ω − + Ω +) (Ξ − + Ξ +) production ratio to be 0.8±0.4 at central rapidity and ϱ T > 1.6 GeV/c.

Collective modes in a strangelet.

Possible existence of various collective modes, such as the compressional, the polarizational, and the surface vibrational modes in a strangelet, are discussed in the framework of hydrodynamic model. The eigenenergies of these collective modes are estimated using an equation of state for the interacting quark matter based on the quark bag model and are compared with the corresponding modes in the nuclei. It is found that, within the uncertainties associated with the bag model parameters, the collective eigenenergies in a strangelet are 1.5\char21{}4 times higher than those in the nuclei of same baryon number. The collective mode behavior may distinguish a strangelet from a large nucleus and give signature of the formation of quark-gluon plasma in ultrarelativistic heavy-ion collision experiments.

Nucleation of quark-gluon plasma from hadronic matter.

The energy densities achieved during central collisions of large nuclei at Brookhaven's AGS may be high enough to allow the formation of quark-gluon plasma. Calculations based on relativistic nucleation theory suggest that rare events, perhaps one in every ${10}^{2}$ or ${10}^{3}$, undergo the phase transition. Experimental ramifications may include an enhancement in the ratio of pions to baryons, a reduction in the ratio of deuterons to protons, and a larger source size as seen by hadron interferometry.

Momentum- and energy-dependence ofJ/Ψ suppression in relativistic heavy ion collisions

With a view to understand the available data on the momentum- and energy-dependences of theJ/Ψ suppression, we compute the suppression rate within the framework of a hydrodynamical evolution model for the quark-gluon plasma (QGP) formation. We make use of an ansatz for the temperature profile function which accounts not only for the space and time evolutions of QGP but also for the possibility of a mixed phase in the boundary region of the deconfined phase. While the predictions are made for the longitudinal momentum- and energy-dependences of the suppression rate, a satisfactory agreement with the available data is found for its dependences on the transverse momentum and the transverse energy.

On the systematic behavior of the chaoticity parameter in HBT analyses

We study the systematic behavior of the chaoticity parameter extracted in very high energy collision experiment. We find that this systematic behavior is understood with the assumption that the bosons are produced through the decay of coherent sources. This approach not only gives a hint to the quantum structure of these processes but also provides interesting predictions about a possible signature of Quark-Gluon Plasma formation.

Dual Higgs Mechanism for Quarks in Hadrons

We study nonperturbative features of QCD using the dual Ginzburg-Landau (DGL) theory, where the color confinement is realized through the dual Higgs mechanism brought by QCD-monopole condensation. The linear confinement potential appears in the QCD-monopole condensed vacuum. We study the infrared screening effect to the confinement potential by the light-quark pair creation, and derive a compact formula for the screened quark potential. We study the dynamical chiral-symmetry breaking (D$\chi $SB) in the DGL theory by solving the Schwinger-Dyson equation. QCD-monopole condensation plays an essential role to D$\chi $SB. The QCD phase transition at finite temperature is studied using the effective potential formalism in the DGL theory. We find the reduction of QCD-monopole condensation and the string tension at high temperatures. The surface tension is calculated using the effective potential at the critical temperature. The DGL theory predicts a large mass reduction of glueballs near the critical temperature. We apply the DGL theory to the quark-gluon-plasma (QGP) physics in the ultrarelativistic heavy-ion collisions. We propose a new scenario of the QGP formation via the annihilation of color-electric flux tubes based on the attractive force between them.

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.

Hadron production by ep collisions at HERA

TheP⊥ distribution of hadrons from deep-inelastic ep collisions of the H1 experiment determines for the hadrons in the final state, a temperature 277±14 MeV≫220 MeV of the critical temperature predicted by QCD, suggesting the formation of quark-gluon plasma (QGP). From their forward to backward ratio, it is found an excess of hadrons arising from the deconfinement of quarks inside the proton.

Rescattering effect of pion on the k/ π ratio in relativistic nucleus-nucleus collisions

The enhancement of k +/ π + ratio in A+B collisions comparing to p+A and/or p+p collisions at similar incident energy is predicted as a possible signal of the quark-gluon plasma formation in ultra-relativistic nucleus- nucleus collisions. Several hadronic models have been developed to see whether the enhancement can also be explained in hadronic scheme. Most of the hadronic thermal models 1−4 give positive answer. However,the answers of two hadronic transport models 5−6 were really opposite.

INTERMITTENCY PATTERNS IN PROTON-NUCLEUS INTERACTIONS AT HIGH ENERGY

A study of intermittency in hadron-nucleus and the comparison with nucleus-nucleus interactions is presented. The power law behavior of the factorial moments and the variation of intermittency index with the center-of-mass energy are shown. Results favor the formation of quark-gluon plasma in preference to a cascade mechanism.

Monofractal density fluctuations and scaling laws for count probabilities and combinants

The relation of combinants to various statistics characterizing the fluctuation pattern of multihadron final states is discussed. Scaling laws are derived for count probabilities and combinants in the presence of homogeneous and clustered monofractal density fluctuations. It is argued that both types of scaling rules are well suited to signal quark-gluon plasma formation in a second-order QCD phase transition.

Observation of Ω − and [formula omitted] in sulphur-tungsten interactions at 200 GeV/ c per nucleon

Multistrange baryon and antibaryon production is suggested to be a useful probe in the search for Quark-Gluon Plasma formation. We report the detection of an Ω − + Ω − signal in central S+ W interactions at 200 A GeV/c and measure the ratio Ω − Ω − = 0.57±0.41 at central rapidity and p T>1.6 GeV/ c.

Status of Quark-Gluon Plasma

I review the current status of the experimental searches for a Quark-Gluon Plasma in heavy ion collisions. The brief introduction covers important results of finite temperature lattice QCD and the intuitive Bjorken picture of Quark-Gluon Plasma (QGP) formation. Global features of the data, such as transverse energy distributions and the various signatures of QGP such asJ/ϕ-suppression, ψ-enhancement as well as thermal dileptons and photons are then discussed.

The shape of dilepton spectra in heavy ion collisions as a signature of quark-gluon plasma

The formation of quark-gluon plasma in heavy ion collisions should manifest itself by the increase of the ratio of medium to large mass dileptons, the former being produced predominantly in the QGP and the mixed phase whereas the latter are produced mostly by the Drell-Yan process. The increase of this ratio should be visible either when comparing collisions of lighter with heavier ions, e.g. O+O with Pb+Pb or when comparing low and large transverse energy events in heavy ion e.g. Pb+Pb collisions.