Energy dependence of transverse quark flow in heavy ion collisions

Energy dependence of quark transverse flow carries information about dynamical properties (equation of state, initial conditions) of deconfined matter produced in heavy ion collisions. We assume quark-antiquark matter formation in Pb+Pb collisions at CERN SPS and Au+Au collisions at RHIC energies and determine quark transverse flow at the critical temperature of the quark-hadron phase transition. Coalescence of massive quarks is calculated in the MICOR hadronization model and hadronic final state effects are considered using the GROMIT cascade program. Comparing theoretical results to data, transverse flow values are determined and energy dependence is discussed.     

System size in relativistic heavy ion collisions

System size is more than a geometrical quantity in relativistic heavy ion collisions; it is closely related to evolution process, i.e. a different system size corresponds to a different evolution process, and whether QGP is produced depends on the system size. We propose that the system size should be under the same level when comparing the measurements from different colliding nuclei. The equivalence of the peripheral collisions of Au-Au and the central collisions of smaller nuclei is studied using the Monte Carlo method. Comparing the transverse overlapping area of the colliding nuclei, the number of participant nucleons and the number of nucleon-nucleon binary collisions in various colliding nuclei, we give an estimate of the correspondence in system size. This is helpful in the experimental comparison of the measurements from different colliding nuclei.     

Rapidity distributions and energy densities from hydrodynamical studies at 5–200 GeV/n

3+1 dimensional relativistic calculations of the space-time evolution of heavy ion collisions at bombarding energies from 5 to 200 GeV/n are presented. Collisions with heavier projectiles seem to be more rewarding to form extended regimes of highly excited nuclear matter containing enough baryons for a sufficient time span to enable a transition of the hadron matter into a quark gluon plasma. A strong impact parameter dependence has to be taken into account when comparing the final baryon rapidity distributions with experimental results. Experimental results of the reactions¹⁶O(60, 200 GeV/n)→Pb are compared with hydrodynamical calculations.     

Constraint Molecular Dynamics and results on isotopic distributions produced in heavy ion collision — The Sn + Ni systems

The Constraint Molecular Dynamics approach is illustrated together with calculation on different heavy ion collisions. In particular the charge-mass distribution produced in the collision ¹²⁴Sn + ⁶⁴Ni and ¹¹²Sn + ⁵⁸Ni at 35 MeV/nucleon is discussed. By comparing the Y = (N ? Z)/A distributions for fragments produced in central collisions it results that this observable can be sensitive to the dynamic effects induced on the two systems by the different charge/mass ratio.     

On chemical equilibrium in nuclear collisions

The data on average hadron multiplicities in central A+A collisions measured at CERN SPS are analysed with the ideal hadron gas model. It is shown that the full chemical equilibrium version of the model fails to describe the experimental results. The agreement of the data with the off–equilibrium version allowing for partial strangeness saturation is significantly better. The chemical freeze–out temperature of about 180 MeV seems to be independent of the system size (from S+S to Pb+Pb) and in agreement with that extracted in , and collisions. The strangeness suppression is discussed at both hadron and valence quark level. It is found that the hadronic strangeness saturation factor increases from about 0.45 for interactions to about 0.7 for central A+A collisions with no significant change from S+S to Pb+Pb collisions indicating that the strangeness enhancement in heavy ion collisions cannot be fully attributed to the increased system size. The quark strangeness suppression factor is found to be about 0.2 for elementary collisions and about 0.4 for heavy ion collisions independently of collision energy and type of colliding system. Received: 31 October 1997 / Published online: 26 February 1998     

在RHIC和LHC中光子-核子碰撞产生的大横动量双轻子

研究了RHIC和LHC能区的Au-Au和Pb-Pb周边重离子碰撞中,来自光子-核子相互作用产生的大横动量双轻子。利用微扰QCD 参数化和Weizs?cker-Williams近似计算了双轻子的产率。经过与领头阶和碎裂过程的双轻子产生数值计算结果相比较,光-核碰撞过程产生的大横动量双轻子在RHIC能区是可忽略的。但是在LHC能区,光-核碰撞在大横动量区域是周边重离子碰撞的一个重要的双轻子源。     

Particle spectra from the ALCOR model

We introduce the Transchemistry Model as a dynamical extension of the Algebraic Coalescence Rehadronization (ALCOR) model in describing the hadronization of quark matter which is expected to be produced in relativistic heavy ion collisions. Results are presented for CERN SPS NA49 Pb+Pb experiment calculating hadron multiplicities and momentum spectra. The freeze-out properties of different hadrons are characterized by similar temperature, density and flow profiles in cylindrically symmetric geometry.     

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.     

System size in relativistic heavy ion collisions

System size is more than a geometrical quantity in relativistic heavy ion collisions; it is closely related to evolution process,i.e.a different system size corresponds to a different evolution process,and whether QGP is produced depends on the system size.We propose that the system size should be under the same level when comparing the measurements from different colliding nuclei.The equivalence of the peripheral collisions of Au-Au and the central collisions of smaller nuclei is studied using the Monte Carlo method.Comparing the transverse overlapping area of the colliding nuclei,the number of participant nucleons and the number of nucleon-nucleon binary collisions in various colliding nuclei,we give an estimate of the correspondence in system size.This is helpful in the experimental comparison of the measurements from different colliding nuclei.     

Probing the quark-gluon phase transition using energy and system-size dependence of long-range multiplicity correlations in heavy ion collisions from the STAR experiment

Long-range forward-backward multiplicity correlations have been measured in heavy ion collisions at RHIC. Results for short and long-range multiplicity correlations (forward-backward) are presented for several systems (Au+Au, Cu+Cu, and pp) and energies (e.g. ?{s_NN }\sqrt {s_{NN} } = 200 and 62.4 GeV). A strong, long-range correlation is seen for central heavy ion collisions at ?{s_NN }\sqrt {s_{NN} } = 200 GeV that vanishes in semi-peripheral events and pp collisions. There is no apparent scaling with the number of participants (N _part) involved in the collision. These correlations provide information about the longitudinal behavior of the system formed in heavy ion collisions. To access the transverse behavior, the clusters produced in the same heavy ion collisions have been characterized by a study of the energy and system size dependence of the percolation density parameter (ρ). The relationship between the long-range correlation and percolation has been explored to characterize the hadron-quark/gluon phase transition and rapid thermalization of the system.     

相对论重离子碰撞的趋平衡问题

从相对论ＢＵＵ理论出发,研究了１ＧｅＶ／ｕ能区重离子碰撞全局和局域平衡性质．研究结果表明,在该能区平均场对反应动力学过程仍有相当的作用;对有限核系统的反应动力学过程的时间演化的研究表明,有限核系统未能达到全局平衡,在中心区基本达到局域平衡．因而在该能区的重离子碰撞中引入完全热平衡概念时,需谨慎考虑．The equilibration in relativistic heavy ion collisions for systems 16 O+ 16 O, 40 Ca+ 40 La and 139 La+ 139 La is studied with RBUU theory. We have found that the mean field still plays a role in addition to the collision term in the equilibration process in relativistic heavy ion collisions at energy around 1 GeV/u. For finite systems, the systems do not reach complete equilibrium. But at the center zone, the local equilibrium is almost reached.     

相对论非对称重离子碰撞中小相对动量区域2π干涉学

用小相对动量区域2π干涉学分析方法研究了相对论非对称重离子碰撞中π介子的吸收存在时对2π干涉学分析所得π源参数的影响,得到了不考虑和考虑π的吸收情况下所得到的源的参数的解析关系.与相对论非对称重离子碰撞1.8A GeVAr+Pb的实验结果进行了对比.     

From RHIC to LHC: a relativistic diffusion approach

We investigate the energy dependence of stopping and hadron production in high‐energy heavy‐ion collisions based on a three‐sources Relativistic Diffusion Model. The transport coefficients are extrapolated from Au + Au and Cu + Cu at RHIC energies ( = 19.6–200 GeV) to Pb + Pb at LHC energies = 5.52 TeV. Rapidity distributions for net protons, and pseudorapidity spectra for produced charged particles in central collisions are compared to data at RHIC energies, and discussed for several extrapolations to LHC energies.     

Strange particle production in relativistic collisions of heavy ions

This review examines data on strange particle production in Pb + Pb collisions obtained in heavy ion experiments at CERN SPS. The yields of K, Λ, Ξ and Ω are considered, as well as their rapidity and transverse mass distributions, depending on the centrality of Pb + Pb collisions at SPS energies. Experimental results are compared with predictions of the statistical nuclear fireball and microscopic parton -string models. We discuss in detail the experimentally observed effect of strangeness enhancement in nucleus-nucleus collisions as compared to nucleon-nuclear interactions and its interpretation within the framework of various theoretical models. Also, we analyze the behavior of hyperon yields and nuclear modification factors in passing from SPS to RHIC energies.     

Current status of quark-gluon plasma signals

Compelling evidence for the creation of a new form of matter has been claimed to be found in Pb+Pb collisions at SPS. We discuss the uniqueness of often proposed experimental signatures for quark matter formation in relativistic heavy ion collisions. It is demonstrated that so far none of the proposed signals like J/? meson production/suppression, strangeness enhancement, dileptons, and directed flow unambiguously show that a phase of deconfined matter has been formed in SPS Pb+Pb collisions. We emphasize the need for systematic future measurements to search for simultaneous irregularities in the excitation functions of several observables in order to come close to pinning the properties of hot, dense QCD matter from data.     

Omega- and Omega+ production in central Pb + Pb collisions at 40 and 158A GeV

Results are presented on Omega production in central Pb+Pb collisions at 40 and 158A GeV beam energy. For the first time in heavy ion reactions, rapidity distributions and total yields were measured for the sum Omega(-) + Omega(+) at 40A GeV and for Omega(-) and Omega(+) separately at 158A GeV. The yields are strongly underpredicted by the string-hadronic UrQMD model but agree better with predictions from hadron gas models.     

超相对论重离子碰撞中小相对动量区域2π干涉学

用小相对动量区域π干涉学方法给出了超相对论重离子碰撞中两种模型下π源参数的关系,并与200A GeV O+Au碰撞的实验结果进行了对比.     

Three dimensional hydrodynamics of ultrarelativistic heavy ion collisions

We have utilized a 2+1 dimensional numerical code based on Flux Corrected Transport method to find a solution for 3+1 dimensional cylindrically symmetric hydrodynamic flow of hadronic matter which is assumed to be formed in extremely high energy heavy ion collisions. The hydrodynamics is supplemented with a decoupling calculation in order to produce measurable particle distributions. This numerical procedure is applied here to Landau type initial conditions which have been fixed using a simple geometrical picture for a central O+Pb collision at 200 GeV/nucleon. The bag equation of state for nonbaryonic matter is used to simulate the deconfinement phase transition to quark gluon plasma. The resulting pseudorapidity distributions of transverse energy for the final massless pions are calculated.     

Φ Meson Production in Heavy Ion Collisions at RHIC

We present Φ meson production in Cu+Cu and Au+Au collisions measured by the STAR experiment at RHIC.The hadronic decay mode Φ→K~+K~- is used in the analysis.The yields for Φ meson in Cu+Cu and Au+Au collisions at a given beam energy are scaled by the number of participant.The N_(part) normalized Φ meson yields in heavy ion collisions over those from p+p collisions are larger than 1 and increase with collision energy.These results suggest that the source of enhancement of strange hadrons is related to the formation of a dense medium in high energy heavy ion collisions and can not be only due to canonical suppression of their production in smaller systems.We also present STAR results on the Φ meson elliptic flow υ_2 from 2~(1/SNN)=200 GeV Cu+Cu at RHIC.The elliptic flow in Cu+Cu system that has the similar relative magnitude and qualitative features as that in Au+Au system.The observations imply the hot and dense matter with partonic collectivity has been formed in heavy ion collisions at RHIC.However,eccentrality normalized υ_2,υ_2/(n_qε_(part)) is lower for Cu+Cu than for Au+Au collisions at 200 GeV.So this might indicate thermalization has not been reached in 200 GeV Cu+Cu collisions.     

Event-by-event fluctuations of the Kaon-to-Pion ratio in central Pb+Pb collisions at 158 GeV per nucleon

We present the first measurement of fluctuations from event to event in the production of strange particles in collisions of heavy nuclei. The ratio of charged kaons to charged pions is determined for individual central Pb+Pb collisions. After accounting for the fluctuations due to detector resolution and finite number statistics we derive an upper limit on genuine nonstatistical fluctuations, which could be related to a first- or second-order QCD phase transition. Such fluctuations are shown to be very small.