Re-hardening of hadron transverse mass spectra in relativistic heavy-ion collisions

We analyze the spectra of pions and protons in heavy-ion collisions at relativistic energies from 2 A GeV to 65+65 A GeV by using a jet-implemented hadron-string cascade model. In this energy region, hadron transverse mass spectra first show softening until SPS energies, and re-hardening may emerge at RHIC energies. Since hadronic matter is expected to show only softening at higher energy densities, this re-hardening of spectra can be interpreted as a good signature of the quark-gluon plasma formation     

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.     

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.     

Formation time of hadrons and density of matter produced in relativistic heavy-ion collisions

Density of matter produced in relativistic heavy-ion collisions depends substantially on the spacetime evolution of the collision and on the formation time of hadrons produced. Interactions of hadrons younger than their formation time are attenuated with respect to their normal values (transparency of hadronic matter for newly formed hadrons). The system of secondary hadrons produced in a heavy-ion collision thus expands as a gas of almost non interacting particles before hadrons reach their formation time. Densities of interacting hadronic matter produced in oxygen-lead and sulphur-lead collisions at 200 GeV/nucleon are estimated as a function of the formation time of hadrons. Uncertainties in our knowledge of the critical temepratureT _c and of the formation time of hadrons τ₀ permit at present three scenarios: an optimistic one (QGP has already been produced in collisions of oxygen and sulphur with heavy ions and will be copiously produced in Lead collisions), a pessimistic one (QGP cannot be produced at 200 GeV/nucleon) and an intermediate one (QGP has not been produced in oxygen and sulphur interactions with heavy ions and will be at best produced only marginally in Pb-collisions). We find the last opinion as most probable.     

Do we really understand nuclear reactions within thick targets using GeV-hadrons?

Three experimental evidences are presented to show that, using classical concepts of physics, we are unable to understand the interaction of 72 GeV ⁴⁰Ar (and 44 GeV ¹²C) and their relativistic secondary fragments in thick Cu-targets. The first observation is connected with fragmentation products of mass A in the range of 10A40, in particular to the production of more-than-calculated yields of ²⁴Na. The reason for this unexpected behavior is due to some enhanced interaction properties for relativistic secondary fragments. Moreover, due to the observation of constant classical total interaction cross sections for primary and secondary hadrons, this enforces a reduced production of spallation products being close to the target, as is in fact experimentally observed (the second evidence). The third experimental observation is a more-than-linear increase in the production of secondary neutrons for reactions induced by 44 GeV ¹²C in thick Pb- and Cu-targets as compared to 12 GeV ¹²C. This can be understood within the context of the first two experimental evidences. These effects are experimentally confirmed above about 40 GeV for hadronic primaries, however, they may even extend down to 1 GeV primary hadron energy.

Results are discussed within the context of a new theoretical approach.     

TWO TEMPERATURES EMISSION OF RELATIVISTIC α-PARTICLES IN HIGH-ENERGY HEAVY-ION COLLISIONS

We have performed an experimental study of the α-fragments emitted from collisions between emulsion nuclei and heavy-ion projectiles at beam energies beyond 1A GeV. It is shown that the transverse momentum distributions of relativistic α-particles give evidence for two effective temperatures emission in high-energy heavy-ion collisions. The data might take on a new signature for the phase transition from hadron matter to quark matter.     

The three-dimensional (2 + 1)-fluid model for relativistic nuclear collisions

A new multi-fluid model is constructed for describing high-energy heavy-ion collisions. It is assumed that two baryonic fluids formed by the projectile and target nucleons produce a third hadronic fluid via inelastic nucleon-nucleon collisions. The production and expansion dynamics of the hadronic fluid are investigated in detail. Two equations of state for this fluid are considered: one corresponding to an ideal gas of pions and resonances and another one corresponding to an interacting hadron gas described by the relativistic mean-field model. The effects of freeze-out and non-zero pion chemical potential are investigated. The rapidity and transverse momentum spectra of secondary pions are compared with the experimental data forS + S collisions at 200 GeV/nucleon.The authors thank J. Schaffner for his most valuable assistance in the application of the mean-field model. The authors are also grateful to H. Sorge and A. Jahns for fruitful discussions. This work was supported by the Gesellschaft für Schwerionenforschung (GSI) and the Bundesministerium für Forschung und Technologie (BMFT).     

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.     

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.     

相对论重离子碰撞中背景的选取对2π干涉学分析的影响

用小相对动量区域2π干涉学分析方法,研究了相对论重离子碰撞中背景的选取对2π干涉学分析所得π源参数的影响,得到了在不同背景下2π干涉学分析所得π源参数之间的解析关系,与相对论重离子中心碰撞1.8A GeV Ar+Pb的实验结果进行了对比.     

Evidence for nonhadronic degrees of freedom in the transverse mass spectra of kaons from relativistic nucleus-nucleus collisions?

We investigate transverse hadron spectra from relativistic nucleus-nucleus collisions within two independent transport approaches that are based on quark, diquark, string, and hadronic degrees of freedom. Both transport models show their reliability for elementary pp as well as light-ion (C+C, Si+Si) reactions. However, for central Au+Au (Pb+Pb) collisions above approximately 5A GeV the measured K+/- transverse mass spectra have a larger inverse slope parameter than expected from the calculation. Thus, the pressure generated by hadronic interactions in the transport models above approximately 5A GeV is lower than observed in the experimental data. This finding shows that the additional pressure--as expected from lattice QCD calculations at finite quark chemical potential and temperature--is generated by strong partonic interactions in the early phase of central Au+Au (Pb+Pb) collisions.     

Clocking hadronization at RHIC

The RHIC accelerator collided Au on Au beams at $\sqrt {s_{NN} } = 130$ GeV in the summer of 2000. To study these collisions, the RHIC experiments have been using a number of observables. One new observable in the field of relativistic heavy ions is the balance function. The balance function can be used to measure the correlation of charged particle pairs in rapidity. The rapidity separation of a particle pair that is created at the same point indicates the time of hadronization for that pair. Preliminary balance function analyses of STAR data are reported. The calculation of a balance function for hadron gas simulations (HIJING) is also discussed in this paper. These preliminary results indicate that the balance function is a useful observable in heavy-ion collisions.     

宇宙线超高能相互作用的QCD部分子模型产生器

在pQCD的部分子模型和独立碎裂方案的部分子强子化模型基础上,建立了用于超高能宇宙线与空气核相互作用的产生器.通过拟合PP散射和e⁺e^－对撞实验数据来决定产生器中的参数.再现了从s=546GeV到1800GeVpp散射中喷注的产生和从s=14GeV到91GeV e⁺e^－对撞等实验的基本特征,并外推到≤22TeV的超高能区.     

Measurement of nuclear-modification factors for <Emphasis Type="Italic">π</Emphasis><Superscript>0</Superscript>, <Emphasis Type="Italic">η</Emphasis>, and <Emphasis Type="Italic">ϕ</Emphasis> mesons and protons in heavy-ion interactions in the PHENIX experiment

Light hadrons provide a convenient tool for studying the properties of hot and dense media formed in central collisions of relativistic heavy nuclei. The results obtained in the PHENIX experiment at the relativistic heavy-ion collider (RHIC, Brookhaven National Laboratory, USA) by measuring nuclearmodification factors for light hadrons in various colliding systems (pp, dAu, CuCu, and AuAu) at the c.m. energies of √s _NN = 62.4 and 200 GeV are presented.     

Effect of the Choice of the Background on Two-Pion Inter-Ferometry

The effect of the choice of the background on the pion source parameters in two-Pion interferometry analyses in relativistic heavy-ion collisions is studied with the two-Pion interferometry at small relative momentum. The analytic relations between the pion source parameters from two-Pion interferometry analyses with different backgrounds are obtained, and comparison is made with the experimental measurements of the cental relativistic heavy-ion collisions of 1.8 A GeV Ar+Pb.     

Neutron productions in the fragmentation of relativistic heavy nuclei and formation of a beam of high-energy neutrons

The production of quasimonoenergetic high-energy neutrons at zero angle (0°) in the spallation of relativistic heavy nuclei is discussed by considering the example of the interaction of lead nuclei with light target nuclei. It is shown that this process can be used to generate a beam of high-energy neutrons at existing heavy ion accelerators. At the same time, itmay lead to the appearance of a parasitic neutron beam because of the interaction of the heavy-ion beam used with beam line and experimental setup materials.     

Charmonium and bottomonium in heavy-ion collisions

We review the present status in the theoretical and phenomenological understanding of charmonium and bottomonium production in heavy-ion collisions. We start by recapitulating the basic notion of “anomalous quarkonium suppression” in heavy-ion collisions and its recent amendments involving regeneration reactions. We then survey in some detail concepts and ingredients needed for a comprehensive approach to utilize heavy quarkonia as a probe of hot and dense matter. The theoretical discussion encompasses recent lattice QCD computations of quarkonium properties in the Quark-Gluon Plasma, their interpretations using effective potential models, inelastic rate calculations and insights from analyses of electromagnetic plasmas. We illustrate the powerful techniques of thermodynamic Green functions (T-matrices) to provide a general framework for implementing microscopic properties of heavy quarkonia into a kinetic theory of suppression and regeneration reactions. The theoretical concepts are tested in applications to heavy-ion reactions at SPS, RHIC and LHC. We outline perspectives for future experiments on charmonium and bottomonium production in heavy-ion collisions over a large range of energies (FAIR, RHIC-II and LHC). These are expected to provide key insights into hadronic matter under extreme conditions using quarkonium observables.     

Hadrons in medium

In these lectures I first give the motivation for investigations of in-medium properties of hadrons. I discuss the relevant symmetries of QCD and how they might affect the observed hadron properties. I then discuss at length the observable consequences of in-medium changes of hadronic properties in reactions with elementary probes, and in particular photons, on nuclei. Here I put an emphasis on new experiments on changes of the σ- and ω-mesons in medium     

New kind of phase transition in hot nuclei

The thermodynamic properties of finite nuclei at excitation energies between 10 MeV and 2 GeV are quite distinct from those of ordinary many-body systems. Due to the small size and the high charge the accessible phase space is dominated by the interplay of surface and long-range Coulomb energies. In a microcanonical Monte Carlo simulation of the decay channels of ¹³¹Xe phase transitions are found which are characterized by the opening of pseudo-fission or cracking modes. These can be identified with anomalies found in recent heavy-ion and high-energetic proton-induced reactions. They, especially the first one, have nothing to do with the liquid—gas transition.