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.     

Low mass lepton pair production and pion dynamics in ultrarelativistic heavy ion collisions

The production of lepton pairs with low invariant mass as probes for the pion dynamics in relativistic nuclear collisions is discussed in some detail. We emphasize the implications of a possible large positive pion chemical potential for the production rate of lepton pairs. We find a strong enhancement in the low invariant mass region, \(2m_\pi \leqslant M_{\mu ^ + \mu ^ - } \leqslant 600\,MeV\) , for lepton pairs which are produced via the annihilation of pions in the hot and dense collision zone. Various sources of background which could mask this enhancement are discussed in some detail. We present and compare calculations for dilepton mass spectra based on different nuclear collision scenarios. It is found that the di-muon mass spectrum can serve as a useful probe to distinguish between these different scenarios.     

Dilepton creation based on an analytic hydrodynamic solution

High-energy collisions of various nuclei, so called “Little Bangs” are observed in various experiments of heavy ion colliders. The time evolution of the strongly interacting quark-gluon plasma created in heavy ion collisions can be described by hydrodynamical models. After expansion and cooling, the hadrons are created in a freeze-out. Their distribution describes the final state of this medium. To investigate the time evolution one needs to analyze penetrating probes, such as direct photon or dilepton observables, as these particles are created throughout the evolution of the medium. In this paper we analyze an 1+3 dimensional analytic solution of relativistic hydrodynamics, and we calculate dilepton transverse momentum and invariant mass distributions. We investigate the dependence of dilepton production on time evolution parameters, such as emission duration and equation of state. Using parameters from earlier fits of this model to photon and hadron spectra, we compare our calculations to measurements as well. The most important feature of this work is that dilepton observables are calculated from an exact, analytic, 1+3D solution of relativistic hydrodynamics that is also compatible with hadronic and direct photon observables.     

A non-plasma approach to the soft dilepton production inhA andAA collisions

We propose a method of evaluating the soft dilepton production rate in hadron-nucleus and nucleus-nucleus collisions at high energies based on summing the contributions from quark-antiquark systems (chains) created in individual hadron-nucleon interactions. The individual contributions are determined by using the square dependence of the soft dilepton production rate on hadron multiplicity predicted for hadron-hadron collisions by the soft-quarkannihilation model and observed recently inpp collisions at the CERN ISR. The possibility of additional contributions originating from the annihilation of quarks and antiquarks that belong to different chains is taken into account in a phenomenological way, which nevertheless enables to correlate different phenomena in soft dilepton production. More detailed predictions are given for proton-nucleus collisions at 200 GeV/c.     

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.     

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.     

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.     

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.     

Feasibility study of heavy ion physics program at NICA

There are strong experimental and theoretical evidences that in collisions of heavy ions at relativistic energies nuclear matter undergoes a phase transition to the deconfined state—Quark Gluon Plasma. The caused energy region of such transition was not found at high energy at SPS and RHIC and search for this energy is shifted to lower energies, which will be covered by the future NICA (Dubna), FAIR (Darmstadt) facilities and BES II at RHIC. Fixed target and collider experiments at the NICA facility will work at the energy range from a few AGeV up to \(\sqrt {\;{S_{NN}}} \; = \;11\;GeV\) GeV and will study the most interesting area on the nuclear matter phase diagram. The most remarkable results were observed in the study of collective phenomena occurring in the early stage of nuclear collisions. Investigation of the collective flow will provide information on Equation of State (EoS) for nuclear matter. Study of the Event-by-Event fluctuations and correlations can give us signals of critical behavior of the system. Femtoscopy analysis provides the space-time history of the collisions. Also, it was found that baryon stopping power revealing itself as a “wiggle” in excitation function of curvature of the (net)proton rapidity spectrum relates to the order of the phase transition. The available observations of an enhancement of dilepton rates at low invariant masses may serve as a signal of the chiral symmetry restoration in hot and dense matter. Due to this fact, measurements of the dilepton spectra are considered to be an important part of the NICA physics program. The study of strange particles and hypernuclei production gives additional information on the EoS and “strange” axis of the QCD phase diagram. In this paper a feasibility of the considered investigations is shown by the detailed Monte Carlo simulations applied to the planned experiments (BM@N, MPD) at NICA.     

In-Medium η′ Mass Reduction

In this talk, we present the reduction of the mass of η′-meson at finite temperature which leads to the enhancement of η′ contributions to the dilepton spectra from relativistic heavy ion collisions. QCD low energy theorem, together with the Witten–Veneziano formula, provides the relation of the η′ mass with gluon condensates.     

Dilepton production in proton-nucleus and nucleus-nucleus collisions at SPS energies

Dilepton production in proton- and nucleus-induced reactions is studied in the relativistic transport model using initial conditions determined by the string dynamics from RQMD. It is found that both the CERES and HELIOS-3 data for dilepton spectra in proton-nucleus reactions can be well described by the ‘conventional’ mechanism of Dalitz decay and direct vector meson decay. However, to provide a quantitative explanation of the observed dilepton spectra in central S+Au and S+W collisions requires contributions other than these direct decays. Introducing a decrease of vector meson masses in hot and dense medium, we find that these heavy-ion data can also be satisfactorily explained. This agrees with our earlier conclusions based on a fire-cylinder model. We also give predictions for Pb+Au collisions at 160 GeV/nucleon using current CERES mass resolution and acceptance.     

A microscopic calculation of secondary Drell-Yan production in heavy ion collisions

A study of secondary Drell-Yan production in nuclear collisions is presented for SPS energies. In addition to the lepton pairs produced in the initial collisions of the projectile and target nucleons, we consider the potentially high dilepton yield from hard valence antiquarks in produced mesons and antibaryons. We calculate the secondary Drell-Yan contributions taking the collision spectrum of hadrons from the microscopic model URQMD. The contributions from meson-baryon interactions, small in hadron-nucleus interactions, are found to be substantial in nucleus-nucleus collisions at low dilepton masses. Preresonance collisions of partons may further increase the yields. Received: 9 July 1997 / Revised version: 17 December 1997 / Published online: 10 March 1998     

Hadrons in nuclei

Changes of hadronic properties in dense nuclear matter as predicted by theory have usually been investigated by means of relativistic heavy-ion reactions. In this talk I show that observable consequences of such changes can also be seen in more elementary reactions on nuclei. Particular emphasis is put on a discussion of photonuclear reactions; examples are the dilepton production at ≈1 GeV and the hadron production in nuclei at 10–20 GeV photon energies. The observable effects are expected to be as large as in relativistic heavy-ion collisions and can be more directly related to the underlying hadronic changes.     

Momentum dependence of the pion cloud for ϱ-mesons in nuclear matter

We extend hadronic models for ϱ-meson propagation in cold nuclear matter via coupling to in-medium pions to include finite three-momentum. Special care is taken to preserve gauge invariance. Consequences for photoabsorption on the proton and on nuclei as well as for the dilepton production in relativistic heavy-ion collisions are discussed.     

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。     

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：     

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.     

Dilepton production from ρ-mesons in a quark-gluon plasma

Assuming that ρ-mesons exist in a quark-gluon plasma at temperatures close to the QCD phase transition, we calculate the dilepton production rate from q-ˉq annihilation via a ρ-meson state using Vector Meson Dominance. The result is compared to the rates from direct q-ˉq annihilation and from π⁺-π⁻ annihilation. Furthermore we discuss the suppression of low mass dileptons if the quarks assume an effective mass in the quark-gluon plasma. Received: 8 September 1999 / Revised version: 22 October 1999