Direct hadron production in ultrarelativistic heavy-ion collisions

Hadrons emitted by the pre-surface layer of a quark-gluon plasma (QGP) before the phase transition into a hadronic gas are considered as possible sources of direct information about QGP. It is shown that if QGP is created in ultrarelativistic heavy-ion collisions, then these hadrons strongly contribute at soft p_t at SpS energy and dominate up to an order of magnitude at LHC energy.     

Influence of shear viscosity of quark-gluon plasma on elliptic flow in ultrarelativistic heavy-ion collisions

We investigate the influence of a temperature-dependent shear viscosity over entropy density ratio η/s on the transverse momentum spectra and elliptic flow of hadrons in ultrarelativistic heavy-ion collisions. We find that the elliptic flow in √S(NN)=200 GeV Au+Au collisions at RHIC is dominated by the viscosity in the hadronic phase and in the phase transition region, but largely insensitive to the viscosity of the quark-gluon plasma (QGP). At the highest LHC energy, the elliptic flow becomes sensitive to the QGP viscosity and insensitive to the hadronic viscosity.     

Flavor-dependent photoproduction in heavy-ion collisions

Strong electromagnetic fields produced in the non-central heavy-ion collisions can induce vector meson photoproduction. In this paper, we study the photoproduction J/ψ and φ mesons in the relativistic heavy-ion collision from ultra-peripheral nuclear collisions to peripheral hadronic heavy ion collisions. And then include both initial hadronic production and thermal production in quark-gluon plasma (QGP). We find, for the charm anti-charm bound state J/ψ, the photoproduced J/ψs are mainly in the very low momentum region and clearly exceed the hadronic production. However, considering the thermal production of strange quark anti-quark pairs in QGP produced in relativistic heavy-ion collisions, the photoproduced φ is usually smaller than the thermal production and only evident at very peripheral collisions as even their photoproduction is much larger than J/ψ.     

Transition to hot quark matter in relativistic heavy-ion collision

The nuclear matter-quark matter phase transition density is calculated as a function of temperature. The result suggests a transition to quark matter in heavy-ion collision at laboratory kinetic energies of a few GeV per nucleon. The transition may be inferred by observing a limiting temperature for the hadrons produced by the collision.     

Baryon number projection for an interacting hadron gas

Calculations of hadronic matter usually enforce conservation of the average baryon number density using the grand canonical ensemble. We have performed calculations for an interacting system in the canonical ensemble with fixed baryon numberN _b , as appropriate for a finite fireball of the type produced in ultra relativistic heavy ion collisions. These results are compared with those obtained from calculations in the grand canonical ensemble. For an interacting nucleon gas the two ensembles yield free energies which differ by approximately 5%.     

The 3rd flow component as a QGP signal

Earlier fluid dynamical calculations with QGP show a softening of the directed flow while with hadronic matter this effect is absent. On the other hand, we indicated that a third flow component shows up in the reaction plane as an enhanced emission, which is orthogonal to the directed flow. This is not shadowed by the deflected projectile and target, and shows up at measurable rapidities, y _CM=1?2. To study the formation of this effect initial stages of relativistic heavy ion collisions are studied. An effective string rope model is presented for heavy ion collisions at RHIC energies. Our model takes into account baryon recoil for both target and projectile, arising from the acceleration of partons in an effective field. The typical field strength (string tension) for RHIC energies is about 5–12 GeV/fm, what allows us to talk about “string ropes”. The results show that QGP forms a tilted disk, such that the direction of the largest pressure gradient stays in the reaction plane, but deviates from both the beam and the usual transverse flow directions. The produced initial state can be used as an initial condition for further hydrodynamical calculations. Such initial conditions lead to the creation of third flow component. Recent v ₁ measurements are promising that this effect can be used as a diagnostic tool of the QGP.     

Physics perspectives of the ALICE experiment at the large hadron collider

The large hadron collider (LHC) under construction at CERN will deliver ion beams up to centre of mass energies of the order of 5.5 TeV per nucleon, in case of lead. If compared to the available facilities for the study of nucleus-nucleus collisions (SpS and RHIC), this represents a huge step forward in terms of both volume and energy density that can be attained in nuclear interactions. ALICE (a large ion collider experiment) is the only detector specifically designed for the physics of nuclear collisions at LHC, even though it can also study high cross-section processes occurring in proton-proton collisions. The main goal of the experiment is to observe and study the phase transition from hadronic matter to deconfined partonic matter (quark gluon plasma —QGP). ALICE is conceived as a general-purpose detector and will address most of the phenomena related to the QGP formation at LHC energies: for this purpose, a large fraction of the hadrons, leptons and photons produced in each interaction will be measured and identified.     

String dynamics in hadronic matter

Hadron production in soft hadronic collisions is successfully described by a longitudinal excitation and subsequent decay of color flux tubes. We consider the dynamics of interacting unstable strings as a generalization designed forhA andAA interactions at ultrarelativistic energies. The constituent quarks at the ends of the decaying strings and the produced hadrons can interact with the surrounding matter. The effect of secondary interactions in molecular dynamics calculations forAA collisions at CERN energies (200A GeV) can be seen in an enhancement of transverse energy, particle production and the mean transverse momenta. The results agree very well with the experimental measurements at ultrarelativistic beam energies inpp, hA and the recentAA collisions.     

Enhancement of high PT hadrons due to collapsing Z(3) walls in heavy-ion collisions

We discuss enhancement of multiplicities of hadrons at high transverse momentum due to multiple reflections of quarks from collapsing Z(3) interfaces in the QGP produced in relativistic heavy-ion collisions. By modeling the dependence of effective mass of the quarks on the Polyakov loop order parameter, we evaluate the reflection coefficient of quarks from collapsing Z(3) interfaces. We use the effective potential proposed by Pisarski for the Polyakov loop to determine the profile of the Z(3) interfaces and calculate the reflection probability for quarks. We discuss the formation of a network of these Z(3) walls in relativistic heavy-ion collisions, in the QGP phase. We do a numerical simulation to calculate the modifications in the thermal transverse momentum spectra of the quarks/anti-quarks that results from a collapsing wall. We then use the recombination model to calculate the transverse momentum spectrum of final hadrons. Our results show enhancement of high P _T hadrons, with the enhancement being stronger for heavier quarks. Further, we find that due to larger reflection coefficient for heavier quarks, the density of strange and charm quarks/anti-quarks increases inside the collapsing walls. This implies enhancement in the multiplicities of multi-strange and multi-charmed hadrons.     

Exploration for Degradation of Λ Polarization Produced in Relativistic Nucleus Nucleus Collisions

The degradation of Λ transverse polarization produced in S+Pb central collisions at energy 200 GeV per nucleon has been studied in detail. The S+Pb central collision events have been generated using Monte-Carlo generator——LUCIAE at 200 GeV per nucleon. The various factors degrading Λ transverse polarization have been analysed quantitatively. The ratios of Λ′s produced from rescattering, secondary production and decays of Σ, Ξ hyperons to the total measurable Λ′s in experiment have been investigeted and the degradation effect of these Λ on the total polarization has been determined. The simulation and calculation show that above three factors decrease the Λ transverse polarization strongly, however, can not eliminate the polarization completely when the Λ′s are assumed to be produced from hadronic gas in the final state. To explan the experimental data of vanished Λ polarization, it probably needs to consider new mechanisms of Λ production, including a weak effect of QGP formation.     

Search for Exotic Strange Dibaryon in Relativistic Heavy Ion Collisions

The exotic strange dibaryon particle (ΩΩ)0 with S = -6 can be produced in relativistic heavy ion collisions. The yields of this kind of exotic strange dibaryon particles can increase signitlcantly soon as the formation of QGP does exhibit after the collision. If there is no phase transition after the collision, the upper bound of the production of this diomega can be estimated from the free hadronic gas model for nuclear matter. The relative yield ratio of diomega to deuteron is less than 0.000205, this means that if there is no QGP creation it is difficult to observe the production of diomega in relativistic heavy ion collisions.     

Strangeness enhancement and strangeness suppression in nuclear collisions at 200 GeV/N

Data on yields of strange and negative hadrons in full phase space produced in proton-nucleus and central S+S collisions at 200 GeV/nucleon are reviewed. The total number of produced strange and nonstrange valence quark-antiquark pairs is determined on the basis of the hadron multiplicity at freezout. It is shown that the strangeness production factor λ _s is increased by a factor of 2 in central S+S collisions, whereas no increase is observed in proton-nucleus collisions.     

Pion multiplicity in nuclear collisions

Data on the mean multiplicity of ^- produced in minimum bias proton-proton, proton-neutron and proton-nucleus interactions as well as central nucleus-nucleus collisions at momenta of 1.4–400 GeV/c per nucleon have been compiled and studied. The results for neutron-neutron and nucleon-nucleon interactions were then constructed. The dependence of the mean pion multiplicity in proton-nucleus interactions and central collisions of identical nuclei are studied as a function of the collision energy and the nucleus mass number. The number of produced pions per participant nucleon in central collisions of identical nuclei is found to be independent of the number of participants at a fixed incident momentum per nucleon. The mean multiplicity of negatively charged hadrons per participant nucleon for central nucleus-nucleus collisions is lower by about 0.12 than the corresponding multiplicity for nucleon-nucleon interactions atp _LAB 15 A·GeV/c, whereas the result at 200 A·GeV/c is above the corresponding nucleon-nucleon multiplicity. This may indicate change of the collision dynamics at high energy.     

Dile pton production in p+ p,Cu+Cu and Au+Au collisions at $\sqrt{s} = 200$ GeV

We study dilepton production in proton–proton, Cu+Cu as well as in Au+Au collisions at the center-of-mass energy \(\sqrt{s} =200\) GeV per participating nucleon pair within an extended statistical hadronization model. In extension to earlier studies we incorporate transport calculations for an estimate of uncorrelated e ⁺ e ^? -pairs from semileptonic D meson decays. While the invariant mass spectrum of dielectrons is well understood in the p+p collisions, severe discrepancies among different model scenarios based on hadronic degrees of freedom and recent data from the PHENIX Collaboration are found in heavy-ion collisions in the low-mass region from 0.15 to 0.6 GeV as well as in the intermediate mass regime from 1.1 to 3 GeV when employing the standard dilepton sources. We investigate, furthermore, the background from correlated dileptons that are not emitted as a pair from a parent hadron but emerge from semileptonic decays of two correlated daughter hadrons. Our calculations suggest a sizeable contribution of such sources in central heavy-ion collisions in the low-mass region. However, even the upper limits of our calculations are found to be far below the dilepton mass spectra of the PHENIX Collaboration.     

Kaon interferometry: a sensitive probe of the QCD equation of state?

We calculate the kaon-interferometry radius parameters for high-energy heavy-ion collisions, assuming a first-order phase transition from a thermalized quark-gluon plasma (QGP) to a gas of hadrons. At high transverse momenta K(T) approximately 1 GeV/c direct emission from the phase boundary becomes important; the emission duration signal, i.e., the R(out)/R(side) ratio, and its sensitivity to T(c) (and thus to the latent heat) are enlarged. The QGP+hadronic rescattering transport model calculations do not yield unusually large radii (R(i) < or = 9 fm). Finite-momentum-resolution effects have a strong impact on the extracted interferometry parameters ( R(i) and lambda), as well as on the ratio R(out)/R(side).     

Quarkochemistry in relativistic heavy-ion collisions

The time necessary to achieve the equilibrium ratio of strange to non-strange quarks in heavy-ion reactions is estimated in the framework of perturbative QCD. It is found, in the present approximation, to be much larger than the total collision time of even a central U + U collision at E_lab=2.1 GeV/nucleon bombarding energy.     

Anisotropic flow from AGS to LHC energies

Within hydrodynamics we study the effects of the initial spatial anisotropy in non-central heavy-ion collisions on the momentum distributions of the emitted hadrons. We show that the elliptic flow measured at midrapidity in 158 A GeV/c Pb+Pb collisions can be quantitatively reproduced by hydrodynamic expansion, indicating early thermalization in the collision. We predict the excitation functions of the 2nd and 4th harmonic flow coefficients from AGS to LHC energies and discuss their sensitivity to the quark-hadron phase transition.     

Search for Tetraquarks in Relativistic Heavy-Ion Collisions

Tetraquarks can be produced in relativistic heavy-ion collision. The yield of this kind of tetraquarks can increase significantly soon as the formation of QGP after the collision. If there is no phase transition after collision, the upper bound of the production of this four-quark states can be estimated from the free hadronlc gas model for nuclear matter. The relative yield ratio of tetraquark cs^-s^-s to Ω is less than 0.0164.     

相对论重离子碰撞中的软探针和硬探针

简要回顾了高能核碰撞中夸克胶子等离子体的软探针和硬探针的一些最新进展,主要内容集中在相对论重离子对撞机和大型强子对撞机实验中各向异性集体流和喷注淬火的理论和唯象研究,对小系统中集体流的来源也做了简要的讨论。对于软探针,讨论了初态三维涨落和碰撞几何各向异性、相对论流体力学演化、末态各向异性集体流以及集体流的涨落、关联和纵向去关联等。通过与实验数据作系统的比较,可以探测重离子碰撞中夸克胶子等离子体的动力演化和各种输运性质。对于硬探针,集中讨论了部分子能量损失和喷注淬火对部分子味道的依赖性、重味夸克在夸克胶子等离子体中的强子化、整体喷注在核介质中的演化以及核介质对喷注的响应等。细致分析相关的观测量,可以帮助我们更全面地了解相对论核碰撞中喷注与核介质的相互作用以及重味粒子的生成。对于小系统,讨论初态和末态效应在解释小系统中轻强子和重味强子的集体流方面的贡献,这有助于我们理解大碰撞系统中集体流的起源成因。