Quark-gluon Plasma Formation Research Articles

Can doubly strange dibaryon resonances be discovered at RHIC?

The baryon-baryon continuum invariant mass spectrum generated from relativistic nucleus + nucleus collision data may reveal the existence of doubly-strange dibaryons not stable against strong decay if they lie within a few MeV of threshold. Furthermore, since the dominant component of these states is a superposition of two color-octet clusters which can be produced intermediately in a color-deconfined quark-gluon plasma (QGP), an enhanced production of dibaryon resonances could be a signal of QGP formation. A total of eight, doubly-strange dibaryon states are considered for experimental search using the STAR detector (Solenoidal Tracker at RHIC) at the new Relativistic Heavy Ion Collider (RHIC). These states may decay to Lambda-Lambda and/or proton-Cascade-minus, depending on the resonance energy. STAR's large acceptance, precision tracking and vertex reconstruction capabilities, and large data volume capacity, make it an ideal instrument to use for such a search. Detector performance and analysis sensitivity are studied as a function of resonance production rate and width for one particular dibaryon which can directly strong decay to proton-Cascade-minus but not Lambda-Lambda. Results indicate that such resonances may be discovered using STAR if the resonance production rates are comparable to coalescence model predictions for dibaryon bound states.

The directed flow maximum near cs = 0

We investigate the excitation function of quark-gluon plasma formation and of directed in-plane flow of nucleons in the energy range of the BNL-AGS and for the E Lab kin = 40AGeV Pb + Pb collisions performed recently at the CERN-SPS. We employ the three-fluid model with dynamical unification of kinetically equilibrated fluid elements. Within our model with first-order phase transition at high density, droplets of QGP coexisting with hadronic matter are produced already at BNL-AGS energies, E Lab kin ≃ 10AGeV. A substantial decrease of the isentropic velocity of sound, however, requires higher energies, E Lab kin ≃ 0AGeV. We show the effect on the flow of nucleons in the reaction plane. According to our model calculations, kinematic requirements and EoS effects work hand-in-hand at E Lab kin = 40AGeV to allow the observation of the dropping velocity of sound via an increase of the directed flow around midrapidity as compared to top BNL-AGS energy.

Microscopic study of energy and centrality dependence of transverse collective flow in heavy-ion collisions

The centrality dependence of directed and elliptic flow in light and heavy systems of colliding nuclei is studied within two microscopic transport models at energies from $1A$ GeV to $160A$ GeV. The pion-directed flow has negative slope in the midrapidity range irrespective of bombarding energy and mass number of the colliding ions. In contrast, the directed flow of nucleons vanishes and even develops antiflow in the midrapidity range in (semi)peripheral collisions at energies around $11.6A$ GeV and higher. The origin of the disappearance of flow is linked to nuclear shadowing. Since the effect is stronger for a light system, it can be distinguished from a similar phenomenon caused by quark-gluon plasma formation. In the latter case the disappearance of flow due to the softening of the equation of state should be most pronounced in collisions of heavy ions. The centrality dependence of the elliptic flow shows that the maximum in the $〈{v}_{2}(b)〉$ distribution is shifted to very peripheral events with rising incident energy, in accordance with experimental data. This is an indication of the transition from baryonic to mesonic degrees of freedom in hot hadronic matter.

Large hadron collider physics program: Compact muon solenoid experiment

The LHC physics program at CERN addresses some of the fundamental issues in particle physics and CMS experiment would concentrate on them. The CMS detector is designed for the search of Standard Model Higgs boson in the whole possible mass range. Also it will be sensitive to Higgs bosons in the minimal supersymmetric model and well adapted to searches for SUSY particles, new massive vector bosons, CP-violation in B-system, search for subtructure of quarks and leptons, etc. In the LHC heavy ion collisions the energy density would be well above the threshold for the possible formation of quark-gluon plasma.

Quark gluon plasma

Recent trends in the research of quark gluon plasma (QGP) are surveyed and the current experimental and theoretical status regarding the properties and signals of QGP is reported. We hope that the experiments commencing at relativistic heavy-ion collider (RHIC) in 2000 will provide a glimpse of the QGP formation.

Effect of Viscosity on One Dimensional Hydrodynamic Flow and Direct Photons from 200 AGeV S+Au Collisions at CERN SPS

We have analyzed the direct photon data obtained by the WA80 collaboration in 200 A GeV S+Au collision at CERN SPS, in a one dimensional hydrodynamical model. Two scenarios were considered: (i) formation of quark-gluon plasma and (ii) formation of hot hadronic gas. For both the scenarios, ideal as well as extremely viscous fluid was considered. It was found that direct photon yield from QGP is not affected much whether the fluid is treated as ideal or extremely viscous. The yield however differ substantially if hadron gas is produced. None of the scenarios gives satisfactory description of the data.

Energy Loss in Perturbative QCD

▪ Abstract We review the propagation of energetic partons in hot or cold QCD matter, as known from recent work. We summarize advances in the understanding of both collisional and radiative energy loss. Our emphasis is on radiative energy loss, which has very interesting properties that may help to detect the quark-gluon plasma produced in heavy-ion collisions. We describe two different theoretical approaches, which lead to the same radiated gluon energy spectrum. The case of a longitudinally expanding QCD plasma is investigated. The energy lost by a jet with given opening angle is calculated with the aim of making predictions for the suppression (quenching) of hard jet production. Phenomenological implications for the difference between hot and cold matter are discussed. Numerical estimates of the loss suggest that it may be significantly greater in hot matter than in cold. This makes the magnitude of the radiative energy loss a remarkable signal for quark-gluon plasma formation.

Quark dispersion relation and dilepton production in the quark-gluon plasma

Under very general assumptions we show that the quark dispersion relation in the quark-gluon plasma is given by two collective branches, of which one has a minimum at a nonvanishing momentum. This general feature of the quark dispersion relation leads to structures (van Hove singularities, gaps) in the low mass dilepton production rate, which might provide a unique signature for the quark-gluon plasma formation in relativistic heavy ion collisions.

A SIGNATURE FOR THE FORMATION OF THE QUARK-GLUON PLASMA IN RELATIVISTIC NUCLEUS-NUCLEUS COLLISIONS

From thermodynamic relations, the initial values of the quark-gloun plasma from relativistic nucleus-nucleus collisions are obtained. From these initial values we have studied the dilepton production on the basis of the (3+1) dimensional relativistic hydrodynamic model established by the present authors, and found that with increasing incident energy a characteristic plateau indicating the formation of quark-gluon plasma appears in the total yield, which may be tested in future experiments in CERN and Brookhaven.

Zero-Density Phase Transition to Quark-Gluon Plasma and the Relativistic Finite Baryon Volume Effect in Hadronic Matter**The project supported by National Natural Science Foundation of China under Grant No. 199775004 and the Chinese Science Foundation of Nuclear Industry under Grant No. H7196AY006

We examine the role of relativistic finite baryon volume effect in the phase transition between hadronic matter and quark-gluon plasma at vanishing baryon density. It is found that the appearance of abnormal state at high temperature may enhance the baryon-antibaryon excitation, which has large influence on the quark-gluon plasma formation. The numerical results show that the occurence of this phase transition depends on how to extrapolate the nucleon-scalar meson coupling constant to the baryonic resonance-scalar meson coupling.

Hadron production in nuclear collisions from the NA49 experiment at 158GeV/ c · A

With its large acceptance and particle identification coverage the NA49 experiment (Fig. 1) can study hadron production in a wide range of high energy reactions [1]. Originally aimed at examining central Pb+Pb collisions for signatures of quark-gluon plasma formation, the scope of the experiment has been enhanced with a systematic study of impact parameter and projectile size dependence, as well as the inclusion of the more elementary p+p and p+A interactions. The question is: are predicted signals of the quark-gluon plasma observed and are there discontinuities which would support the concept of hadronic phase transition?

Hadron production in nuclear collisions from the NA49 experiment at 158 a A

With its large acceptance and particle identification coverage the NA49 experiment (Fig. 1) can study hadron production in a wide range of high energy reactions [1]. Originally aimed at examining central Pb+Pb collisions for signatures of quark-gluon plasma formation, the scope of the experiment has been enhanced with a systematic study of impact parameter and projectile size dependence, as well as the inclusion of the more elementary p+p and p+A interactions. The question is: are predicted signals of the quark-gluon plasma observed and are there discontinuities which would support the concept of hadronic phase transition?

Hard $\pi^0$ and $\eta$ production in S + Au and Pb + Pb nuclear collisions at SPS energies and the possible signature of quark–gluon plasma

Hard $\pi^0$ and $\eta$ production is investigated in Landau and Bjorken hydrodynamical models, taking a great number of hadronic resonances (16 and 42) in the hadronic phase into account. Central and peripheral S + Au and Pb + Pb collisions are considered. We consider two different scenarios: one with quark–gluon plasma (QGP) formation and one with only hadronic gas without QGP. The Cronin effect and hard direct meson emission are taken into account. It is shown that these two scenarios give similar $p_{\perp}$ pion spectra, which approximately agree with experimental data for S + Au collisions obtained by the WA80 Collaboration. Therefore we conclude that it is difficult to extract proof of QGP formation from hadronic spectra. However, we also calculate the $\eta/\pi^0$ ratio. We show that this value agrees with experimental data only for the scenario with plasma and a phase transition for hadrons. The scenario of hadronic gas without plasma disagrees with experimental data.

Percolation approach to phase transitions in high energy nuclear collisions

We study continuum percolation in nuclear collisions for the realistic case in which the nuclear matter distribution is not uniform over the collision volume, whose finite size is considered, and show that the percolation threshold is increased compared to the standard, uniform situation. In terms of quark-gluon plasma formation this means that the phase transition threshold is pushed to higher energies.

Analysis of particle multiplicities in Pb+Pb collisions at158AGeV/cwithin hadron gas models

The preliminary data on hadron multiplicities measured in central Pb+Pb collisions at $158A$ $\mathrm{GeV}/c$ are analyzed. The ideal hadron gas model fails to give a reasonable explanation to the Pb+Pb data sets. We study the possible effects of pion enhancement due to different hard-core repulsion for pions and other hadrons and strangeness suppression because of incomplete chemical equilibrium. Each of these two modifications improves the results. The combined effect of these two mechanisms leads to an extremely good agreement with the data. An interpretation of the obtained results in terms of the possible quark-gluon plasma formation at the early stage of the collision is also discussed.

On Multiplicity Difference Correlators in Second-Order Quark-Gluon Plasma Phase Transition

The multiplicity difference correlators between two well-separated bins are studied in second-order quark-gluon plasma (QGP) phase transition within the Ginzburg-Landau model. For the case with very low mean multiplicities, an exponent γ is found, which has very weak dependence on the parameters of the model and can be used as a signal for the formation of QGP in ultra-relativistic heavy-ion collisions.

Finite size effects in quark-gluon plasma formation

Using lattice simulations of quenched QCD we estimate the finite size effects present when a gluon plasma equilibrates in a slab geometry, i.e., finite width but large transverse dimensions. Significant differences are observed in the free energy density for the slab when compared with bulk behavior. A small shift in the critical temperature is also seen. The free energy required to liberate heavy quarks relative to bulk is measured using Polyakov loops; the additional free energy required is on the order of 30 – 40 MeV at 2 – 3 T c .

First principles estimate of finite size effects in quark-gluon plasma formation

Using lattice simulations of quenched QCD we estimate the finite size effects present when a gluon plasma equilibrates in a slab geometry, i.e., finite width but large transverse dimensions. Significant differences are observed in the free energy density for the slab when compared with bulk behavior. A small shift in the critical temperature is also seen. The free energy required to liberate heavy quarks relative to bulk is measured using Polyakov loops; the additional free energy required is on the order of 30-40 MeV at 2-3 T_c.

Strangeness production in high density baryon matter

Strangeness production in heavy ion collisions, when compared with proton-proton collisions, is potentially a sensitive probe for collective energy deposition and therefore for reaction mechanisms in general. It may therefore provide insight into possible quark-gluon plasma formation in dense nuclear matter. To establish an understanding of the observed yields, a systematic study of high density baryon matter at different beam energies is essential. This might also reveal possible discontinuities in the energy dependence of the reaction mechanism. We present preliminary results for kaon production in Au + Au collisions at beam kinetic energies of 6, 8, and 10.7 GeV/u obtained by the E917 experiment at the AGS (BNL). These measurements complement those carried out by the E866 Collaboration at 2, 4, and 10.7 GeV/u with a significantly enlarged data sample. In both experiments a large range of rapidities was covered by taking data at different angular settings of the magnetic spectrometer.

Scaling behaviour of the multiplicity distribution under first-order quark-hadron phase transition

The multiplicity distribution in small bins is studied within the Ginzburg-Landau description for first-order quark-hadron phase transition. Direct comparison of the distribution with a Poisson one (with the same average) is made. Dynamical factor for the distribution and ratio are studied, and novel scaling behaviours between are found which can be used to detect the formation of quark-gluon plasma.