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.     

富重子夸克–胶子物质双轻子产生的快度依赖

考虑Drell–Yan本底,基于相对论性流体力学模型,研究了来自¹⁹⁷Au十¹⁹⁷Au中心碰撞的富重子夸克–胶子物质的双轻子产生的快度依赖.发现随着快度的增加,双轻子产额呈现出强烈的抑制.这个标志富重子夸克–胶子物质形成的特征能够在CERN和Brookhaven将来的实验中得到检验.     

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.     

Photon production from quark gluon plasma at finite baryon density

The photon yield from a baryon-rich quark gluon plasma (QGP) at SPS energy has been estimated. In the QGP phase, rate of photon production is evaluated up to two-loop level. In the hadron phase, dominant contribution from π,ρ, ω mesons has been considered. The evolution of the plasma has been studied with appropriate equation of state in both QGP and hadron phase for a baryon-rich system. At SPS energy, the total photon yield is found to increase marginally in the presence of baryon density.     

夸克-胶子的等离子体在核-核碰撞中形成的信号

从热力学关系,得到来自相对论核碰撞形成的夸克－胶子系统的初始值,基于这些初始值,在相对论流体力学模型下研究了双轻子的产生,发现随着入射能量的增加,一个标志夸克－胶子等离子体形成的特征平台出现的总产额中,这些特上可在ＣＥＲＮ（西欧中心）和Ｂｒｏｏｋｈａｖｅｎ未来的实验中得到检验。     

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.     

膨胀的热夸克胶子物质的中等质量双轻子的增强

基于一个描述夸克胶子火柱演化的相对论流体力学模型,研究了夸克相、强子相互作用以及非热过程(DrellYan对、粲强子衰变)的中等质量双轻子的产生.发现由于相边对夸克胶子物质演化的影响和RHIC能量核碰撞产生的夸克胶子物质具有高的初始温度,夸克相对双轻子的贡献显著增强,比那些来自强子相互作用的贡献重要,甚至能与来自非热的贡献比较.表明中等质量双轻子的增强是一个在核碰撞中产生了夸克胶子物质的可能信号. 关键词： 夸克-胶子物质 双轻子增强 相对论流体力学模型     

膨胀的热的富重子夸克–胶子物质的双轻子产生

从核完全阻止的假定出发,基于一个相对论性流体力学模型,研究了富重子夸克-胶子物质的双轻子产生,发现随着入射能量的增加,在双轻子总产额中出现一个标志富重子夸克-胶子物质形成的特征平台,这样显著的特征可在CEEN和Brookhaven即将开展的实验中得到检验.     

夸克-胶子等离子体在相对论性核-核碰撞中形成的一种信号

从热力学关系,计算来自相对论性核-核碰撞的富重子夸克-胶子等离子体的初值.接着基于(3+1)维相对论性流体力学模型研究了系统的双轻子产生.发现随着碰撞核入射能量的增加,一个标志夸克-胶子等离子体形成的特征平台出现在双轻子的总产额中.这样的特征可在CERN和Brookhaven未来的实验中得到检验. 关键词：     

Photon Production in a Chemically Equilibrating Quark-Gluon Plasma at Finite Baryon Density： Complete Leading Order Results

We investigate hard photon production of the near-collinear bremsstrahlung and a new process called the inelastic pair annihilation, fully including the LPM effect, in a chemically equilibrating quark-gluon plasma at finite baryon density, and find that the effect of the system evolution on the photon production and large contribution of the bremsstrahlung make the total photon yield of the two processes as a strongly increasing function of the initial quark chemical potential.     

Low mass dilepton production in relativistic heavy ion collisions

Mechanisms of formation for low mass dilepton excess observed in the relativistic heavy ion collisions are considered. The experimental data are reviewed. In addition to discussing the standard mechanisms of dilepton production specific to the collisions of relativistic nuclei (the pion annihilation in the hadron-gas stage and the quark-antiquark annihilation in the quark-gluon phase), the mechanism of dilepton production in the mixed phase of nuclear matter is proposed, and its contribution to the low mass dilepton spectrum is estimated. In addition, the first-order corrections in the strong-interaction coupling constant to the dilepton production in the parton medium and the nonperturbative approaches are considered.     

Quark-gluon plasma diagnostics in a successive equilibrium scenario

The relativistic Fokker-Planck equation has been used to study the evolution of the quark distribution in the quark-gluon phase expected to be formed in ultra-relativistic heavy-ion collisions. The effect of thermal masses for quarks and gluons is incorporated to take account of the in-medium properties. We find that the kinetic equilibrium is achieved before the system reaches the critical temperature of quark-hadron phase transition. We find that chemical equilibrium is not achieved during this time. We have evaluated the electromagnetic probes of quark-gluon plasma from the non-equilibrated quark-gluon phase and compared them with those in completely equilibrated scenario. The hard QCD production rates for the electromagnetic ejectiles as well as the heavy quark production rates are also calculated.     

Dilepton Spectrum from An Expanding Hot Nuclear Matter

In this paper the dilepton spectrum from an expanding hot nuclear matter is studied.Comparing with the spectrum from hot quark matter,we find that they are quite different.Especially the former with respect to the initial temperature or particle density is monotonic,the latter is non-monotonic.This may serve as theoretical examination for the production of quark phase transition.     

Measuring quark-gluon-plasma thermalization time with dileptons

We calculate the medium dilepton yield from a quark-gluon plasma which has a time-dependent local momentum-space anisotropy. A phenomenological model for the hard momentum scale, p(hard)(tau), and plasma anisotropy parameter, xi(tau), is constructed which interpolates between longitudinal free streaming at early times (tautau(iso)). We show that high-energy dilepton production is sensitive to the plasma isotropization time, tau(iso), and can therefore be used to experimentally determine the time of onset for hydrodynamic expansion of a quark-gluon plasma and the magnitude of expected early-time momentum-space anisotropies.     

化学非平衡夸克-胶子物质中等质量双轻子的产生

研究了正在进行化学平衡的富重子夸克-胶子物质的双轻子产生，发现由于产生在RHIC能量 的化学非平衡的富重子夸克-胶子物质冷却慢和高的初始温度，导致中等质量双轻子产生重 大增强.因此，中等质量双轻子的增强可以是夸克-胶子物质形成的信号.同时，这个增强能 补偿由于初始夸克化学势增加引起的双轻子抑制，因而双轻子产额的抑制不再是夸克-胶子 物质产生的信号. 关键词： 化学非平衡夸克-胶子物质 热粲夸克 双轻子     

Rapidity dependence of thermal dileptons resulting from hadronizing quark-gluon matter with finite baryon charge

The influence of a non-vanishing baryon charge on the rapidity distribution of dileptons produced in ultrarelativistic heavy-ion collisions is studied. We employ a frozen motion model with scaling invariant expansion of the hadronizing quark-gluon plasma as well as a realistic rapidity distribution of secondary particles (i.e., pions and baryons) expected for RHIC energies. We demonstrate a sizeable suppression of the thermal dilepton yield at large rapidities due to the finite baryon density. This affects the most favorable rapidity window for using dileptons for the diagnostic of the quark-gluon plasma and the early temperature distribution of the hot reaction zone. To discriminate the thermal dileptons from Drell-Yan background we propose to utilize the dilepton yield scaled suitably by the pion multiplicity as function of rapidity.     

Photon radiation and dilepton production induced by rescattering in strong interacting medium

Using the opacity expansion technique we investigate the photon radiation and dilepton production induced by multiple rescattering as an energetic parton jet passing through the strong interacting medium. The real photon radiation and dilepton invariant-mass spectra of the bremsstrahlung contribution from an energetic quark jet are presented. The leading contribution of total energy loss by photon emission in the medium of a high energetic quark jet is found to be proportional to the jet energy and has a linear dependence on the thickness of the nuclear target. The rescattering contribution to the dilepton production is important only when the pair has a small invariant mass and the jet has relatively low energy. The contribution fraction of the dilepton induced by rescattering in medium is found to be nearly a constant when the ratio of the jet energy to Debye screening mass E/μ is large.     

胶子非弹性散射过程对夸克胶子等离子体中双轻子产生的影响

双轻子是研究夸克物质的形成和性质的重要探针.本文基于化学平衡化的黏滞性夸克胶子等离子体演化模型,计算了相对论重离子碰撞能量下金-金对心碰撞形成的夸克胶子等离子体中的双轻子产额.在黏滞性计算中加入了胶子非弹性散射过程对黏滞系数的贡献.相较仅考虑夸克和胶子弹性散射的情况,双轻子的产额有较明显的降低.这表明在黏滞系数中加入胶子非弹性散射的贡献使得系统的演化过程加快,演化时间变短.     

A new state of hadronic matter at high density

We propose in this article that if the chemical potential exceeds a critical value in dense hadronic medium, a first-order phase transition to a new state of matter with Lorentz symmetry spontaneously broken (in addition to the explicit breaking) takes place. As a consequence, light vector mesons get excited as “almost” Goldstone bosons. Since the light vector mesons dominantly couple to photons, the presence of these new vector mesons could lead to an enhancement in the dilepton production from dense medium at an invariant mass lower than the free-space vectormeson mass. We provide a low-energy quark model which demonstrates that the above scenario is a generic case for quark theories with a strong interaction in the vector channel. We discuss possible relevance of this phase to the phenomenon of the enhanced dilepton production at low invariant masses in relativistic heavy-ion collisions.     

THE CHARACTERISTIC DILEPTON MASS DISTRIBUTION FROM HOT QUARK MATTER IN ULTRARELATIVISTIC NUCLEUS-NUCLEUS COLLISIONS

The characteristic dilepton mass distributions from the decay of high temperature-density quark-gluon matter in ultrarelativistic nucleusnucleus collisions were calculated via the relativistic hydrodynamics.Both longitudinal expansion accompanying transverse expansion of the matter and the effect of phase transition leading to the re-distribution of the temperature were considered.The characteristic phenomena which have been suggested as a test for the existence of quark-gluon plasma were given and provided as a theoretical basis of experiment.