Quark-gluon Matter Research Articles

Thermal Charmed Quark Contribution to Dileptons in ChemicallyEquilibrating Quark-Gluon Matter

We find that in a chemically equilibrating baryon-rich quark-gluon matter,due to the slow cooling rate, high initial temperature, large gluondensity as well as large fusion cross section ofgg→cc̄ in the intermediate mass region, the gluonfusion gg→cc̄ provides a dominant contribution todileptons with intermediate masses, resulting in the significantenhancement of intermediate mass dileptons.

Generalized parton distributions and strong forces inside nucleons and nuclei

We argue that generalized parton distributions (GPDs), accessible in hard exclusive processes, carry information about the spatial distribution of forces experienced by quarks and gluons inside hadrons. This way the measurements of hard exclusive processes open a possibility for direct “measurements” of strong forces in different parts of nucleons and nuclei. Also such studies open avenue for addressing questions of the properties of the quark (gluon) matter inside hadrons and nuclei. We give a simple example of relations between GPDs and properties of “nuclear matter” in finite nuclei.

Phase transitions in neutron stars and gravitational wave emission

We review the detectability of gravitational waves generated by oscillations excited during a phase transition from hadronic matter to deconfined quark-gluon matter in the core of a neutron star. Neutron star properties were computed using a Boguta-Bodmer based model and the MIT bag model. The maximum energy available to excite mechanical oscillations into the star is estimated by the energy difference between the configurations with and without a quark-gluon matter core. On basis of the planned sensitivity of present laser interferometers (VIRGO or LIGO I) and those of the next generation (LIGO II), the maximum volume to be probed by these experiments is determined. These results are used as an indication of the potential detectability of neutron stars as sources of gravitational waves. Our results indicate that the maximum distance probed by the detectors of the first generation is well beyond M31, whereas the second generation detectors will probably see phase transition events at distances two times longer, but certainly not yet attaining the Virgo cluster.

Production and rescattering of heavy quarks in ultrarelativistic nucleus-nucleus collisions

Contributions of various mechanisms to the heavy-quark-production cross section in ultrarelativistic nucleus-nucleus collisions, including the direct pair production \(b\bar b\) and \(c\bar c\) as a result of hard primary collisions and pair production in showers induced by hard-produced partons in the initial and final states, are investigated. The sensitivity of the muon pair spectra with large invariant masses (from semileptonic \(B\bar B\) decays) and spectra of the secondary J/ψ (from individual B decays) to the multiple scattering and the energy losses of b quarks in dense quark-gluon matter is studied. The formation of such matter is expected in heavy-ion collisions at the LHC.

A method for analysing the jet azimuthal anisotropy in ultrarelativistic heavy ion collisions

The azimuthal anisotropy of jet spectra due to energy loss of jet partons in azimuthally non-symmetric volume of dense quark–gluon matter is considered for semi-central nuclear interactions at collider energies. We develop the techniques for event-by-event analysing the jet azimuthal anisotropy using particle and energy elliptic flow, and suggest a method for calculation of coefficient of jet azimuthal anisotropy without reconstruction of nuclear reaction plane.

Azimuthal asymmetry of jet production as a signal of parton energy losses in semicentral heavy-ion collisions

The question is investigated of whether an azimuthal asymmetry in the hadron-jet spectra can arise because of rescattering and energy losses of partons produced via hard processes in a dense quark-gluon matter formed in the region of the initial nuclear overlap in collisions characterized by a nonzero value of the impact parameter. Methods are discussed for determining the reaction-plane angle in ultrarelativistic heavy-ion collisions with the aid of the flux of semihard particles.

Intermediate Mass Dilepton Production in a ChemicallyEquilibrating Quark-Gluon Matter

Dilepton production during the chemical equilibration of quark-gluonmatter with a finite baryon density has been studied. We find that dueto the slowing down of the cooling rate and the high initial temperature of thequark-gluon matter produced at RHIC energies, the quark phasecontribution to dileptons with intermediate masses is significantlyheightened and is much larger than that calculated by the evolutionof the thermodynamic equilibrium system. The latter has shown anenhancement of intermediate mass dileptons from the quark phase.Therefore, such an enhancement of dileptons should be asignature for quark-gluon matter formation.

Hard QCD processes and impact-parameter dependence of parton energy losses in ultrarelativistic nuclear collisions

Special features of particle and jet production in hard QCD processes induced by ultrarelativistic heavy-ion collisions are investigated. The energy loss of partons produced in hard collisions that is due to their multiple scattering in dense quark-gluon matter is analyzed as a function of the impact parameter of a nucleus-nucleus collision. The possibility of experimentally observing effects that are expected in this connection and which include different impact-parameter dependences of radiative and collision hard-jet energy losses, a modification to the shape of the impact-parameter distribution of dijets, and the suppression of the yield of muon pairs having large invariant masses is discussed.

The Little Bang: searching for quark–gluon matter in relativistic heavy-ion collisions

I review the status of the search for quark-gluon plasma in relativistic heavy-ion collisions. The available data provide strong evidence for the "three pillars of the Little Bang model": strong radial expansion of the collision fireball with Hubble-like scaling, thermal hadron emission and primordial hadrosynthesis. It is argued that the initial state of the reaction zone exhibits features which cannot be understood with conventional hadronic dynamics, but are consistent with the formation of deconfined quark-gluon matter.

Enhancement of intermediate mass dileptons from expanding hot quark–gluon matter

A study of the production of intermediate mass dileptons from the quark phase and hadronic interactions as well as non-thermal contributions (Drell–Yan pairs, charmed hadronic decays) in the initial stage has been carried out based on a relativistic hydrodynamic model describing the evolution of a quark–gluon matter fire-cylinder. We find that due to the influence of the phase boundary on the evolution of the quark–gluon matter system and the high initial temperature of the quark–gluon matter produced at RHIC energies, the contribution of the quark phase to dileptons is obviously enhanced and is more important than those from hadronic interactions and even is comparable with those from non-thermal contributions. It is shown that the enhancement of intermediate mass dileptons is a possible signature for the quark–gluon matter formation in heavy ion collisions at RHIC energies.

Sudden hadronization in relativistic nuclear collisions

We formulate and study a mechanical instability criterion for sudden hadronization of dense matter fireballs formed in 158A GeV Pb-Pb collisions. Considering properties of quark-gluon matter and hadron gas we obtain the phase boundary between these two phases and demonstrate that the required deep quark-gluon-plasma supercooling prior to sudden hadronization has occurred.

Enhanced antiproton production in Pb(160 A GeV)+Pb reactions: evidence for quark gluon matter?

The centrality dependence of the antiproton per participant ratio is studied in Pb(160 A GeV)+Pb reactions. Antiproton production in collisions of heavy nuclei at the CERN/SPS seems considerably enhanced as compared to conventional hadronic physics, given by the antiproton production rates in pp and antiproton annihilation in pp reactions. This enhancement is consistent with the observation of strong in-medium effects in other hadronic observables and may be an indication of partial restoration of chiral symmetry.

Chemical equilibration of quarks and gluons at RHIC and LHC energies

We study chemical equilibration of quarks and gluons in central nuclear collisions at RHIC and LHC energies. The initial quark and gluon densities are taken from earlier studies as well as from recent perturbative QCD estimates and are then evolved via rate equations coupled to longitudinally boost-invariant fluid dynamics. We find that, for RHIC initial conditions, the lifetime of quark–gluon matter is too short in order for the quark and gluon number densities to chemically equilibrate prior to hadronization. In contrast, at LHC energies chemical equilibration is complete before the system hadronizes. Entropy production due to chemical equilibration can be as large as 30%.

On a possibility of adjoint colored states condensation at finite temperatures in lattice gauge model

Cooled down and diluted quark-gluon matter is considered. A possibility of condensation of multiquark clusters with zero N-alities is discussed.

Strangeness enhancement in heavy ion collisions – evidence for quark-gluon matter?

The centrality dependence of (multi-)strange hadron abundances is studied for Pb(158 A GeV)Pb reactions and compared to p(158 GeV)Pb collisions. The microscopic transport model UrQMD is used for this analysis. The predicted Λ/ π −, Ξ −/ π − and Ω −/π − ratios are enhanced due to rescattering in central Pb-Pb collisions as compared to peripheral Pb-Pb or p-Pb collisions. However, the enhancement is substantially smaller than observed experimentally. The enhancement depends strongly on the kinematical cuts. The maximum enhancement is predicted around midrapidity. For Λ's, strangeness suppression is predicted at projectile/target rapidity. For Ω's, the predicted enhancement can be as large as one order of magnitude. Comparisons of Pb-Pb data to proton induced asymmetric (p- A) collisions are hampered due to the predicted strong asymmetry in the various rapidity distributions of the different (strange) particle species. In p-Pb collisions, strangeness is locally (in rapidity) not conserved. The present comparison to the data of the WA97 and NA49 Collaborations clearly supports the suggestion that conventional (free) hadronic scenarios are unable to describe the observed high (anti-)hyperon yields in central collisions. A reduction of the constituent quark masses to the current quark masses m s ∼230 MeV, m q ∼10 MeV, as motivated by chiral symmetry restoration, yields hyperon production close to the experimentally observed high values. An ad hoc overall increase of the color electric field strength (effective string tension of κ=3 GeV/fm) yields similar results. It has been suggested that these findings might be interpreted as a signal of a phase of nearly massless particles.

SINGULAR SHELLS OF QUARK-GLUON MATTER

The spherically symmetric thin shells of the macroscopically stable quark-gluon matter are considered within the frameworks of the bag model and theory of discontinuities in general relativity. The equation of state for the two-dimensional matter is suggested, and its features are discussed. The exact equations of motion of such shells are obtained. Distinguishing the two cases, circumstellar and microscopical shells, we calculate the parameters of equilibrium configurations, including the conditions of decay (deconfinement).

Dilepton Production in an Expanding Hot Baryon-Rich Quark-Gluon Matter

From the full stopping scenario, a relativistic hydrodynamic model describing the evolution of the baryon-rich quark-gluon matter fire-cylinder has been established. Based on it, the rapidity distribution of dileptons with intermediate invariant masses has been studied. It is found that the quark phase gives the dominant contribution to the dilepton spectrum due to the influence of the phase boundary on the evolution of the system, especially the dilepton yield is strongly suppressed with the increasing rapidity.

Dilepton Distribution as a Signature for the Baryon-Rich Quark-Gluon Matter

From the full stopping scenario, the dilepton production in the baryon-rich quark-gluon fireball based on a relativistic hydrodynamic model is studied, and it is found that with increasing incident energy a characteristic plateau indicating the formation of the baryon-rich quark-gluon matter appears in the total yield.

Diphoton rates from thermalized matter resulting in ultra-relativistic heavy-ion collisions

Diphoton radiation off strongly interacting matter resulting from ultrarelativistic heavy-ion collisions is estimated for SPS and RHIC conditions. At SPS energies the thermal diphoton signal competes strongly with the Drell — Yan like background. For RHIC energies we find that radiation from initially undersaturated (but gluon-rich) deconfined matter overwhelms the hadron gas at invariant mass M > 1 GeV; the Drell — Yan like diphotons dominate at M > 2.5 GeV. Due to this the so-called M⊥ scaling is approximately obtained in a narrow window and can serve as additional tool for discriminating radiation from quark-gluon matter.

Dilepton production in an expanding baryon-rich quark-gluon fireball

From the full stopping scenario, the dilepton production in a baryon-rich quark-gluon fireball has been studied based on a relativistic hydrodynamic model. Due to the influence of the phase boundary on the evolution of the system, the quark phase gives the dominant contribution to the dilepton spectrum. In particular, with increasing initial baryon density, the total dilepton yield first rapidly rises when the initial hadronic system goes to the initial quark system, then goes down owing to the suppression of the dilepton production, thus making a characteristic peak signaling the formation of the baryon-rich quark-gluon matter which appears in the total yield. These characteristics can be tested in future experiments at CERN and Brookhaven.