v<Subscript>2</Subscript> and R<Subscript>AA</Subscript> of non-photonic single electrons: which restriction do the current measurements pose on the production of charm,beauty?

In relativistic heavy ion collisions, heavy flavor is expected to result predominately from initial hard parton–parton scatterings. Hence, in the absence of later stage effects, the production of heavy flavor in A+A collisions can be viewed as a superposition of N+N collisions. Measurements of v₂ or R_AA of heavy flavor (or their decay electrons) in A+A collisions violate the simple superposition picture and therefore present themselves as probes of the medium formed in such collisions. On the basis of the measured v₂ and R_AA of non-photonic single electrons in A+A collisions at RHIC, we will investigate the interplay between these observables as well as the restrictions they pose for charm and bottom production.     

Femtoscopy in p+p vs. heavy ion collisions in STAR

The geometric substructure of the particle-emitting source has been characterized via two-particle interferometry by the STAR collaboration for several energies and colliding systems at RHIC. In heavy ion collisions the m_T dependence of femtoscopic radii has been observed for all particle types by several experiments at different collision energies. This dependence has been thought to arise from space-momentum correlations generated by collective flow. On the other hand, there are several reports of a similar m_T dependence by experiments measuring elementary particle collisions. Here, quite different physical mechanisms – including resonances, strings, and uncertainty arguments – have been proposed to explain the dependence. Determining the differences or similarities in the space-time physics driving the signal in heavy ion versus p+p collisions requires a direct comparison of m_T dependence of the radii. Such a comparison has, until now, been sorely lacking. STAR data allow, for the first time, such a direct comparison between A+A, d+A, and p+p collisions, at the same energy, measured in the same detector, and using the same analysis techniques. Surprisingly, our preliminary results indicate an m_T-independent scaling of the femtoscopic radii with overall system size. Possible physics implications of these observations will be discussed, and the importance of long-range non-femtoscopic correlations for low multiplicity collisions will be emphasized. PACS 25.75.Gz; 25.75.-q     

Heavy quarks at RHIC from parton transport theory

There are several indications that an opaque partonic medium is created in energetic Au+Au collisions at the relativistic heavy ion collider (RHIC). At the extreme densities of ∼10–100 times normal nuclear density reached, even heavy-flavor hadrons are affected significantly. Heavy-quark observables are presented from the parton transport model MPC, focusing on the nuclear suppression pattern, azimuthal anisotropy (“elliptic flow”), and azimuthal correlations. Comparison with Au+Au data at top RHIC energy indicates significant heavy-quark rescattering, corresponding roughly five times higher opacities than estimates based on leading-order perturbative QCD. We propose measurements of charm–anticharm, e.g., D-meson azimuthal correlations as a sensitive, independent probe to corroborate these findings. PACS 25.75.-q; 25.75.Ld; 25.75.Gz     

Interaction of heavy ions at the Large Hadron Collider (LHC)

The present-day understanding of the problem of the deconfinement of nuclear matter as applied to experiments that would study heavy ion collisions at the Large Hadron Collider (LHC) is discussed briefly. The elliptic energy flow, jet quenching, and the production of J/Ψ and Y quarkonia are appropriate observables here.     

The strange border of the QCD phases

We address the flavour composition along the border between the hadronic and the quark–gluon plasma phases of QCD. The ratio of strange to up and down antiquarks () produced in particle and nuclear collisions is found to increase in collisions with an initially reached energy density () up to GeV/fm. Above this value it decreases approximately linearly and reaches its asymptotic value at zero baryon chemical potential (). We demonstrate that in nuclear collisions approaches its asymptotic value at –9 GeV/fm, corresponding to –8 TeV per pair, which will be reached at the LHC. After correcting for the difference in the chemical potentials of various colliding systems, universally saturates across the QCD phase boundary, following the temperature. Recent experimental puzzles as the increase in the ratio in collisions at 40 GeV per nucleon, its different behaviour at midrapidity, the decrease of the double ratio of in nucleus–nucleus over collisions with increasing , and the increase of in over collisions at the same , are naturally explained. We study the approach of thermodynamic observables at to the transition point and extract an estimate of the critical temperature. Received: 17 April 2001 / Published online: 24 August 2001     

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     

The thermodynamic model for nuclear multifragmentation

A great many observables seen in intermediate energy heavy ion collisions can be explained on the basis of statistical equilibrium. Calculations based on statistical equilibrium can be implemented in microcanonical ensemble (energy and number of particles in the system are kept fixed), canonical ensemble (temperature and number of particles are kept fixed) or grand canonical ensemble (fixed temperature and a variable number of particles but with an assigned average). This paper deals with calculations with canonical ensembles. A recursive relation developed recently allows calculations with arbitrary precision for many nuclear problems. Calculations are done to study the nature of phase transition in intermediate energy heavy ion collision, to study the caloric curves for nuclei and to explore the possibility of negative specific heat because of the finiteness of nuclear systems. The model can also be used for detailed calculations of other observables not connected with phase transitions, such as populations of selected isotopes in a heavy ion collision.The model also serves a pedagogical purpose. For the problems at hand, both the canonical and grand canonical solutions are obtainable with arbitrary accuracy hence we can compare the values of observables obtained from the canonical calculations with those from the grand canonical. Sometimes, very interesting discrepancies are found.To illustrate the predictive power of the model, calculated observables are compared with data from the central collisions of Sn isotopes.     

Pion suppression in nuclear collisions

The pion multiplicity per participating nucleon in central nucleus–nucleus collisions at the energies 2–15 AGeV is significantly smaller than in nucleon–nucleon interactions at the same collision energy. This effect of pion suppression is argued to appear due to the evolution of the system produced at the early stage of heavy–ion collisions towards a local thermodynamic equilibrium and further isentropic expansion. Received: 21 July 1997 / Revised version: 12 November 1997 / Published online: 26 February 1998     

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.     

Charmed exotics in heavy ion collisions

Based on the color–spin interaction in diquarks, we argue that charmed multiquark hadrons are likely to exist. Because of the appreciable number of charm quarks produced in central nucleus–nucleus collisions at ultrarelativistic energies, the production of charmed multiquark hadrons is expected to be enhanced in these collisions. Using both the quark coalescence model and the statistical hadronization model, we estimate the yield of charmed tetraquark mesons, T_cc, and pentaquark baryons, Θ_cs, in heavy ion collisions at RHIC and LHC. We further discuss the decay modes of these charmed exotic hadrons in order to facilitate their detections in experiments. PACS 25.75.Dw; 14.20.Lq; 14.40.Lb     

Jet tomography in the forward direction at RHIC

Hadron production at high p_T displays a strong suppression pattern in a wide rapidity region in heavy ion collisions at RHIC energies. This finding indicates the presence of strong final state effects for both transversally and longitudinally traveling partons, namely induced energy loss. We have developed a perturbative QCD based model to describe hadron production in pp collisions, which can be combined with the Glauber–Gribov model to describe hadron production in heavy ion collisions. Investigating AuAu and CuCu collisions at energy at mid-rapidity, we find the opacity of the strongly interacting hot matter to be proportional to the participant nucleon number. Considering forward rapidities, the suppression pattern indicates the formation of a longitudinally contracted dense deconfined zone in central heavy ion collisions. We determine the parameters for the initial geometry from the existing data. PACS 12.38.Mh, 24.85.+p, 25.75.-q     

同位旋依赖的碎块判断方法对重离子碰撞观测量的影响(英文)

该工作提出了一个同位旋依赖碎块判断方法用以描述重离子碰撞过程中的碎块形成。利用该方法可以改善输运理论对于核子以及轻带电粒子产额的描写,其在计算中降低了发射核子产额、增加碎块(特别是丰中子碎块) 的产额。对于丰中子轻带电粒子的增强主要出现在中心快度区。研究表明,该方法对于同位旋敏感的观测量,如,n/p,t/3He,Rmidyield 和isoscaling parameters 等,以及系统的平衡度的量度都会产生影响。We introduce isospin dependence in the cluster recognition algorithms used in the Quantum Molecular Dynamics model to describe fragment formation in heavy ion collisions. This change reduces the yields of emitted nucleons and enhances the yields of fragments, especially for heavier fragments. The enhancement of neutron-rich lighter fragments mainly occurs at mid-rapidity. Consequently, isospin dependent observables, such as isotope distributions, yield ratios of n/p,t/3He, Rmid yield and isoscaling parameters are affected. We also investigate how equilibration in heavy ion collisions is affected by this change.     

Working group report: Quark gluon plasma

The 10th Workshop on High Energy Physics Phenomenology (WHEPP-10) was held at the Institute of Mathematical Sciences, Chennai during January 2–13, 2008. One of our working grops (WG) is QCD and QGP. The discussions of QGP WG include matter at high density, lattice QCD, charmonium states in QGP, viscous hydrodynamics and jet quenching, colour factor in heavy ion collisions and RHIC results on photons, dileptons and heavy quark. There were two plenary talks and several working group talks with intense discussions regarding the future activities that are going to be persued.      

Strangeness enhancement

Highly effective conversion of kinetic energy into abundant particle multiplicity is the remarkable feature discovered in high energy heavy ion collisions. This short and pedagogic review addresses topical issues related to the understanding of this phenomenon, originating in the creation of the deconfined quark–gluon plasma phase. I consider in depth the apparently simple, yet sometimes misunderstood, intricate issues: a) statistical hadro-chemistry, chemical parameters, b) strange flavor chemical equilibration in quark–gluon plasma, and c) particle yields and sudden hadronization, in the historic perspective of work and competition with my friend József Zimányi.     

对称能密度依赖的输运理论模型研究中的一些问题的讨论

本文非常简要地介绍了目前通过重离子碰撞中的对称能敏感观测量获取对称能密度依赖的研究现状,讨论了在输运理论模型计算中可能引起计算结果的不确定性的几个问题。特别与目前广泛采用的输运理论模型计算相关的3个方面的问题,即对称势动量依赖项和中子、质子有效质量劈裂;不同电荷态△产生的阈能和相关截面的介质修正;输运理论模型中的多体关联和涨落等的处理等问题。     

Lepton-pair production from deep inelastic scatteringin peripheral relativistic heavy ion collisions

We calculate the inelastic electron- and muon-pair production in peripheral relativistic heavy ion collisions in the region of large Q² of one of the photons. This offers a possibility to study the quark distribution functions in ions in "ultraperipheral heavy ion collisions". The calculations are compared with those making use of the equivalent photon and the equivalent lepton approximation. We compare the results for Pb-Pb and Pb-p collisions at RHIC (γ ≈ 100) and LHC (γ ≈ 3000) energies. Furthermore we include nuclear modifications to the parton distribution functions in our calculations to study their effect on the cross sections.     

大入射能量范围内重离子输运过程的动力学性质研究

基于一个改进的微观动力学输运模型——极端相对论量子分子动力学（UrQMD）, 较为系统地研究了从SIS能区到AGS和SPS能区, 再到RHIC能区, 入射能量跨5个数量级的重离子核反应及多个人们感兴趣的实验观测量, 如粒子产额、 集体流、 核阻止以及两粒子HBT关联等。研究表明, 不论单粒子观测量, 还是两粒子关联观测量, 都能自洽地由同一个输运模型加以描述。Based on one updated microscopic transport model Ultra relativistic Quantum Molecular Dynamics (UrQMD) with modifications of both potentials and two body scattering cross sections, quite a few interesting observables, such as yields, collective flows, the nuclear stopping, and the HBT two particle interferometry, are systematically investigated for heavy ion collisions within a large beam energy regime of five orders of magnitude (from SIS, AGS, SPS up to RHIC). It is shown that a consistent explanation can be received from both single particle and two particle related observables.