Can van Hove singularities be observed in relativistic heavy-ion collisions?

Based on general arguments, the in-medium quark propagator in a quark-gluon plasma leads to a quark dispersion relation consisting of two branches, of which one exhibits a minimum at some finite momentum. This results in a vanishing group velocity for collective quark modes. Important quantities such as the production rate of low mass lepton pairs and mesonic correlators depend inversely on this group velocity. Therefore these quantities, which follow from self energy diagrams containing a quark loop, are strongly affected by van Hove singularities (peaks and gaps). If these sharp structures could be observed in relativistic heavy-ion collisions it would reveal the physical picture of the quark-gluon plasma (QGP) as a gas of quasiparticles.     

Dynamic color screening in a classical quark plasma

Screening of a moving infinite color sheet source is examined in a quark plasma at finite temperature. The classical chromohydrodynamic equations for quarks are integrated, to obtain profiles for quark current density, which in turn are used to solve the SU(2) Yang-Mills equations numerically. This provides a classical but non-perturbative treatment for the screening of a moving source in quark plasma. The results show two interesting features. We observe that if the test source is at rest the screening does not depend on the color dynamics and the behavior is very similar to that in Coulomb plasma. When the test source is moving with non-relativistic velocity the non-abelian features manifest themselves by weakening the screening and also by exhibiting an oscillatory profile with distance.     

Charm production in a quark-gluon plasma

A calculation is made for charm quark production in a longitudinally expanding quark-gluon plasma. A comparison is made with hadronic charm production in ultra-relativistic nuclear collisions assuming an extrapolation from p-p collisions and no plasma formation. The charm yield from a QGP begins to dominate purely hadronic production for plasma temperatures above 315–440 MeV, depending on the bombarding energy of the colliding nuclei and the value assumed for the charm quark mass. Implications for plasma signals, most notably dilepton emission and J/ψ suppression, are discussed.     

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.     

The changing scenario of the atomic nucleus—from nucleons and mesons to quarks and gluons

The general characteristics of the transition from hadronic matter of nucleons, three quark bags, mesons of quark antiquark pairs to quark gluon plasma is discussed. The phenomenological approach essentially guided by the MIT bag model and general thermodynamic criteria of first-order phase transition is elaborated. The more realistic calculations using the QCD lattice renormalization quark are touched upon. Possible signals of quark-gluon plasma are discussed. The central issue of deciphering plasma signals from the signals of hot hadronic matter is discussed in detail. The signals of the quark-gluon plasma, a subject of considerable interest in contemporary literature are focussed only on (i) dileptons (ii) photon photon pairs and (iii)J/Ψ suppression (with special emphasis on CERN experiments). The lingering shadow of “EMC” effect is also mentioned. Relics of the very early universe microseconds after the big bang in today’s universe (∼ 15 billion years later) are discussed. Finally, the outlook of this very exciting field is presented, a purely personal viewpoint, generalized eventually to poetic signals of the creation of the universe.     

Jet flavor conversions in the quark-gluon plasma

A quark or gluon jet traversing through a quark-gluon plasma can be converted to a gluon or quark jet through scattering with the thermal quarks and gluons in the quark-gluon plasma. Their conversion rates due to two-body elastic and inelastic scattering have recently been evaluated in the lowest order in QCD. Including both energy loss and conversions of quark and gluon jets in the expanding quark-gluon plasma produced in relativistic heavy ion collisions, a net conversion of quark jets to gluon jets has been found. This reduces the difference between the nuclear modification factors for quark and gluon jets in heavy ion collisions and thus enhances the ratios of high transverse momentum protons and antiprotons to pions that are produced from the fragmentation of these jets. To account for the observed similar ratios in central Au + Au and p + p collisions at same energy requires, however, a much larger net quark to gluon jet conversion rate than that given by the lowest-order QCD, indicating the importance of higher-order processes and the strongly coupling properties of the quark-gluon plasma in describing the propagation of jets in the quark-gluon plasma. B.-W. Zhang: On leave from: Institute of Particle Physics, Huazhong Normal University, Wuhan 430079, China Correspondence: C.-M. Ko, Cyclotron Institute and Physics Department, Texas A&M University, College Station, TX 77843-3366, USA     

Quark number susceptibility in hard thermal loop approximation

We calculate the quark number susceptibility in the deconfined phase of QCD using the hard thermal loop (HTL) approximation for the quark propagator and quark–meson vertices. This improved perturbation theory takes into account important medium effects such as thermal quark masses and Landau damping in the quark–gluon plasma. We explicitly show that the Landau damping part in the quark propagator for space-like quark momenta does not contribute to the quark number susceptibility due to the quark number conservation. We find that the quark number susceptibility only due to the collective quark modes deviates from the free one around the critical temperature but approaches free results in the infinite temperature limit. The results are in conformity with recent lattice calculations. Received: 7 December 2001 / Published online: 15 March 2002     

化学非平衡夸克–胶子等离子体中的双轻子产生

研究了正在进行化学平衡的具有有限重子密度的夸克–胶子等离子体系统的演化和双轻子产生.结果发现由于夸克相的寿命随初始夸克化学势的增加而增加,以及其他一些因素,如较高的初始温度、较大的胶子密度和较大的胶子聚变和夸克湮没反应截面,导致热粲夸克对双轻子产生提供了占统治的贡献.这个效应造成中等质量双轻子的重大增强.     

Tetraquarks Production in Quark-Gluon Plasma with Diquarks

We present a way to calculate tetraquarks ratios for quark gluon plasma with diquarks, The ratios of tetraquarks over baryons produced from quark matter are high than hadronic gas model limits. It is a better way to search for four-quark states in relativistic heavy ion collisions. It may become a criterion to judge whether quark-gluon plasma has formed to search for four-quark states in relativistic heavy ion collisions.     

Photons from Quark and Hadron Phases in Au + Au Collisions

Based on a relativistic hydrodynamic model describing the evolution of the chemically equilibrating quark-gluon plasma system with finite baryon density in a 3＋1-dimensional spacetime, we compute photons from the quark phase, hadronic phase and initial non-thermal contributions. It is found that due to the effects of the initial quark chemical potential, chemical equilibration and rapid expansion of the system, the photon yield of the quark-gluon plasma is strongly suppressed, and photons from hadronic matter and initial non-thermal contributions almost reproduce experimental data.     

Constraints for critical temperature of the phase transition into the quark-gluon plasma

Low temperature of the phase transition into the quark-gluon plasma correspond to low values of the bag model constant and to absolutely stable strange quark matter. Some of the observed pulsars are identified quite reliably as neutron stars. If strange matter is stable, the central density of these pulsars is to be smaller that critical density of the phase transition into the nonstrange quark matter. The nonstrange quark matter being formed turns to more stable strange matter on a weak interaction timescale converting neutron stars into strange stars. The requirement of stability of old and newly born neutron stars is used to constrain the bag model constant and the critical temperature of the phase transition into the quark-gluon plasma at zero chemical potential.     

On the energy loss of high energy quarks in a finite-size quark-gluon plasma

The induced gluon emission from a fast quark passing through a finite-size QCD plasma is studied within the light-cone path integral approach. It is shown that the leading log approximation used in previous studies fails when the gluon formation length becomes on the order of the length of the medium traversed by the quark. Calculation of the energy loss beyond the leading log approximation gives the energy loss which grows logarithmically with quark energy, contrary to the energy-independent prediction of the leading log approximation.     

Quark condensate in a dilute hadronic gas containing an excess of baryons

We calculate the quark condensate within a simple model for the hadronic phase by evaluating the derivative of the pressure with respect to the quark mass. The corresponding phase diagram for the transition from the hadrons to a quark-gluon plasma is discussed and we also describe the composition of the hadron gas for several temperatures and baryon densities.     

Kertész line and embedded monopoles in QCD

We propose a new class of defects in QCD which can be viewed as embedded monopoles made of quark and gluon fields. These objects are explicitly gauge invariant and they closely resemble the Nambu monopoles in the standard electroweak model. We argue that the "embedded QCD monopoles" are proliferating in the quark-gluon plasma phase while in the low-temperature hadronic phase the spatial proliferation of these objects is suppressed. At realistic quark masses and zero chemical potential the hadronic and quark-gluon phases are generally believed to be connected by a smooth crossover across which all thermodynamic quantities are nonsingular. We argue that these QCD phases are separated by a well-defined boundary-known as the Kertész line in condensed matter systems-associated with the onset of the proliferation of the embedded QCD monopoles in the quark-gluon plasma phase.     

夸克物质研究现状

本文简要评述了夸克物质(夸克-胶子等离子体)在理论和实验两个方面的研究现状。讨论了退禁闭相变、夸克物质形成及信号、夸克物质时空演化等问题,并列举了当前国际上正在进行和即将进行的有关实验。     

Electromagnetic signals of quark gluon plasma

Successive equilibration of quark degrees of freedom and its effects on electromagnetic signals of quark gluon plasma are discussed. The effects of the variation of vector meson masses and decay widths on photon production from hot strongly interacting matter formed after Pb + Pb and S + Au collisions at CERN SPS energies are considered. It has been shown that the present photon spectra measured by WA80 and WA98 Collaborations can not distinguish between the formation of quark matter and hadronic matter in the initial state.     

Open charm and beauty at ultrarelativistic heavy ion colliders

Important goals of BNL RHIC and CERN LHC experiments with ion beams include the creation and study of new forms of matter, such as the quark gluon plasma. Heavy quark production and attenuation provide unique tomographic probes of that matter. We predict the suppression pattern of open charm and beauty in Au+Au collisions at RHIC and LHC energies based on the DGLV formalism of radiative energy loss. A cancellation between effects due to the sqrt[s] energy dependence of the high p(T) slope and heavy quark energy loss is predicted to lead to surprising similarity of heavy quark suppression at RHIC and LHC.     

Strangeness in paradise: The strange quark as a probe of QCD

I review the arguments for considering the strange quark as a probe of changes in the QCD vacuum, and of the formation of a quark-gluon plasma, in particular. I survey existing data showing enhanced production of strange hadrons in relativistic heavy ion collisions and analyse their implications in the context of thermal and microscopic reaction models. My lecture closes with some remarks on strange quark matter and hypernuclei.     

Relationship between quark-antiquark potential and quark-antiquark free energy in hadronic matter

In high-temperature quark-gluon plasma and its subsequent hadronic matter created in a high-energy nucleus-nucleus collision, the quark-antiquark potential depends on the temperature. The temperature-dependent potential is expected to be derived from the free energy obtained in lattice gauge theory calculations. This requires one to study the relationship between the quark-antiquark potential and the quark-antiquark free energy. When the system's temperature is above the critical temperature, the potential of a heavy quark and a heavy antiquark almost equals the free energy, but the potential of a light quark and a light antiquark, of a heavy quark and a light antiquark and of a light quark and a heavy antiquark is substantially larger than the free energy. When the system's temperature is below the critical temperature, the quark-antiquark free energy can be taken as the quark-antiquark potential. This allows one to apply the quark-antiquark free energy to study hadron properties and hadron-hadron reactions in hadronic matter.     

Photon production from baryon rich quark-gluon plasma

We study high energy photon production from a quark-gluon plasma at finite baryon density. We find that the photon production spectrum from the quark-gluon plasma maintained at constant temperature is only mildly dependent on the quark chemical potential.