Neutrino propagation in a medium with a magnetic field

Data on the mean multiplicity of strange hadrons produced in minimum bias proton-proton and central nucleus-nucleus collisions at momenta between 2.8 and 400 GeV/c per nucleon have been compiled. The multiplicities for nucleon-nucleon interactions were constructed. The ratios of strange particle multiplicity to participant nucleon as well as to pion multiplicity are larger for central nucleus-nucleus collisions than for nucleon-nucleon interactions at all studied energies. The data at AGS energies suggest that the latter ratio saturates with increasing masses of the colliding nuclei. The strangeness to pion multiplicity ratio observed in nucleon-nucleon interactions increases with collision energy in the whole energy range studied. A qualitatively different behaviour is observed for central nucleus-nucleus collisions: the ratio rapidly increases when going from Dubna to AGS energies and changes little between AGS and SPS energies. This change in the behaviour can be related to the increase in the entropy production observed in central nucleus-nucleus collisions at the same energy range. The results are interpreted within a statistical approach. They are consistent with the hypothesis that the Quark Gluon Plasma is created at SPS energies, the critical collision energy being between AGS and SPS energies.     

Centrality dependence of high-p(T) hadron suppression in Au+Au collisions at sqrt[s(NN)]=130 GeV

Inclusive transverse momentum distributions of charged hadrons within 0.2相似文献   

Nuclear stopping as a probe for in-medium nucleon-nucleon cross sections in intermediate energy heavy ion collisions

Using an isospin-dependent quantum molecular dynamics, nuclear stopping in intermediate heavy ion collisions has been studied. The calculation has been done for colliding systems with different neutron-proton ratios in beam energy ranging from 15 MeV/ u to 150 MeV/ u. It is found that, in the energy region from above Fermi energy to 150 MeV/ u, nuclear stopping is very sensitive to the isospin dependence of in-medium nucleon-nucleon cross section, but insensitive to symmetry potential. From this investigation, we propose that nuclear stopping can be used as a new probe to extract the information on the isospin dependence of in-medium nucleon-nucleon cross section in intermediate energy heavy ion collisions.     

碰撞参数的混杂效应及其对观测量的影响

基于改进型量子分子动力学（ImQMD）模型,讨论了中低能区重离子碰撞实验上利用多重数定义的中心碰撞和理论上利用固定碰撞参数定义的中心碰撞的差别及其影响。在束流能量为50 MeV/u时,发现两种方法得到的碎块电荷多重数分布、碎块的平均动能的结果的最大偏差约有50%。两种方法对于同位旋敏感观测量--并合不变的中子-质子产额比的影响在5%~15%之间。研究表明,利用中低能重离子碰撞精确获取核物质状态方程或者介质中核子-核子散射截面时,必须考虑碰撞参数混杂的效应。     

Classical microscopic calculation of fast heavy-ion collisions

We consider heavy-ion collisions with beam energies of a few hundred MeV/nucleon, because such collisions seem favorable for producing compressed nuclear matter. As an alternative to hydrodynamic ways of calculating such collisions, we are investigating a microscopic, rapid (and therefore economical) simulation method. There are two simplifications basic to this method: (i) using classical particle kinematics, and (ii) taking nucleonnucleon interactions into account via cross sections rather than explicit forces. Some other simplifications, concerning nuclear binding etc., are presently crude but will be improved. Even at normal density, nuclear matter is not so dilute; therefore our calculations show some sensitivity to details of the mechanism assume for nucleon-nucleon scattering. For head-on collision of two mass-235 nuclei, our present calculations yield maximum densities between 2 and 3 times that of normal nuclear matter.     

Pion multiplicity in nuclear collisions

Data on the mean multiplicity of ^- produced in minimum bias proton-proton, proton-neutron and proton-nucleus interactions as well as central nucleus-nucleus collisions at momenta of 1.4–400 GeV/c per nucleon have been compiled and studied. The results for neutron-neutron and nucleon-nucleon interactions were then constructed. The dependence of the mean pion multiplicity in proton-nucleus interactions and central collisions of identical nuclei are studied as a function of the collision energy and the nucleus mass number. The number of produced pions per participant nucleon in central collisions of identical nuclei is found to be independent of the number of participants at a fixed incident momentum per nucleon. The mean multiplicity of negatively charged hadrons per participant nucleon for central nucleus-nucleus collisions is lower by about 0.12 than the corresponding multiplicity for nucleon-nucleon interactions atp _LAB 15 A·GeV/c, whereas the result at 200 A·GeV/c is above the corresponding nucleon-nucleon multiplicity. This may indicate change of the collision dynamics at high energy.     

A simplified model for calculating RAA and RCP of cold nuclear matter effects

An incoherent binary nucleon-nucleon collision model of AA collisions is presented for simulating particle production in cold nuclear matter. With a simple phenomenological parameter, the mean nucleon energy loss fraction, this model yields pseudorapidity density distributions that are comparable to those of experiment, as well as those of HIJING. Particle production data for a given binary collision is extracted from the PYTHIA event generator. The nuclear geometry is described by the Glauber model. The preliminary R _AA and R _CP results are also presented and discussed, with a proposal that R _AA be redefined.     

利用超相对论量子分子动力学模型研究交变梯度同步加速器能区Au+Au碰撞中的核阻止效应

运用修正的超相对论量子分子动力学模型研究了交变梯度同步加速器(AGS)能区Au+Au碰撞中的核阻止效应. 该模型考虑了形成和"预形成"粒子的平均场势、核子-核子弹性散射反应截面的介质修正和碎块形成的判断条件. 研究发现: 在AGS能区, 核阻止效应受到形成和“预形成”粒子的平均场势和核子-核子弹性散射反应截面介质修正的影响; 在中心快度区自由质子的产额偏大, 考虑新的碎块形成判断条件后, 此模型的理论计算结果与自由质子的实验结果符合得很好. 关键词： 超相对论量子分子动力学模型 交变梯度同步加速器能区 Au+Au碰撞 核阻止效应     

QCD predictions for charm and bottom quark production at RHIC

We make up-to-date QCD predictions for open charm and bottom production at RHIC in nucleon-nucleon collisions at square root of S=200 GeV. We also calculate the electron spectrum resulting from heavy flavor decays to allow direct comparison to the data. A rigorous benchmark, including the theoretical uncertainties, is established against which nuclear collision data can be compared to obtain evidence for nuclear effects.     

INDRA 能区重离子输运过程的系统研究及对称能探测

基于改进的极端相对论量子分子动力学(UrQMD) 输运模型,通过细致考察初始化、状态方程及两体碰撞等系统研究了INDRA 能区重离子碰撞后出射的自由核子及轻粒子的集体流和核阻止本领等观测量。研究发现,一个采用带动量依赖的软的状态方程及动量修正的密度依赖的核子核子弹性散射截面的动力学输运过程能够很好描述INDRA 全能域内实验获得的中心快度区的氢同位素的直接流。利用该套输运系统还研究了直接流平衡能对对称势能密度依赖强度因子的敏感性。发现,出射的自由中子平衡能敏感依赖于对称势能的密度依赖,而自由质子却不会。同时还发现,利用两Sn 同位素系列反应出射的自由中子直接流的平衡能与初始中子/质子比的关系可以很好地探测对称能的密度依赖。The terms of initialization, equation of state (EoS), and two-body collision in the updated ultrarelativistic quantum molecular dynamics (UrQMD) model are examined in details so as to systematically study the collective flows and the nuclear stopping of free nucleons and light clusters from heavy-ion collisions at INDRA energies. It is seen that at INDRA energies the dynamic transport with a soft EoS with momentum dependence and with the momentum-modified density-dependent nucleon-nucleon elastic cross sections describes the directed flow exhibited by hydrogen isotopes (Z= 1) emitted at midrapidity fairly well. The sensitivity of the balance energy (Ebal) of the directed flow to the strength parameter of the density dependence of symmetry potential energy is further studied with the same parameter set. It is found that the Ebal of neutrons from HICs is particularly sensitive to the density dependence of the symmetry potential energy, while that of protons is not. And, the initial neutron/proton ratio dependence of the balance energy of neutrons from Sn isotopes can be taken as a useful probe to constrain the stiffness of the nuclear symmetry energy.     

Cold nuclear matter effects on J/ψ yields as a function of rapidity and nuclear geometry in d+A collisions at sqrt[s(NN)]=200 GeV

We present measurements of J/ψ yields in d+Au collisions at sqrt[s(NN)]=200 GeV recorded by the PHENIX experiment and compare them with yields in p+p collisions at the same energy per nucleon-nucleon collision. The measurements cover a large kinematic range in J/ψ rapidity (-2.2相似文献   

重离子碰撞中原子核阻止的同位旋效应

利用含有3种对称势形式的同位旋相关的量子分子动力学,研究了中能重离子碰撞中原子核阻止的同位旋效应和随入射道条件的系统演化过程.计算结果表明,原子核阻止灵敏地依赖束流能量、碰撞参数、碰撞系统的质量和核子–核子碰撞截面的同位旋相关性,而3种对称势和碰撞系统的中质比对它的影响不很明显,但在大约费米能量以下能区,原子核阻止同时依赖于介质中核子–核子碰撞截面和对称势.故认为在费米能量以上能区直至150MeV/u,原子核阻止是提取介质中核子–核子碰撞截面的一个新的物理观测量.     

密度分布对原子核阻止同位旋效应的影响

利用同位旋相关的量子分子动力学（IQMD）模型,分析了中能重离子碰撞过程中相对论平均场和SkyrmeHartrere－Fork等核结构模型给出的核密度分布对原子核阻止的影响。研究表明,从费米能到的较大能量范围内,无论小质量体系还是大质量体系,原子核阻止对同位旋相关的核子－核子碰撞截面都非常灵敏,而不同模型给出的核密度分布对原子核阻止影响不大,说明原子核阻止作为提取同位旋相关核子－核子碰撞截面的灵敏探针是与原子核结构模型无关的。 Influence of densitydistributionwhichisgivenbyRMFandSkyrme－Hartrere－Fork etalnuclearstructuremodelonnuclearstoppinginheavy－ioncollisionsatintermediateenergiesarestudiedbyusingisospin－dependentquantummoleculardynamics(IQMD)model.ResearchshowsthatrangeformtheFermienergytothelargerenergyof 100 Mev/u, for bothsmallandlargemasssystem,nuclearstoppingisfoundtobestronglydependentonthenucleon－nucleoncrosssections,andweaklyondifferentdensitydistribution.ThisshowsthatnuclearstoppingcanbeusedasaprobetoextracttheinformationontheisospindependenceN－Ncrosssectionsandhasnorelationshipwithnuclearstructuremodels.     

Nuclear Matter Density,Collision Number and Entropy in Dynamics Process of Intermediate High Energy Heavy Ion Collision

The nuclear matter density,collision number and entropy in the dynamic process of heavy ion collision ⁴⁰Ca+⁴⁰Ca at E_L=400MeV/A for very central collision were calculated.The results show that the nuclear spatial density distributions depend mainly on the mean field but rarely on the nucleonnucleon collisions.The density distribution in the momentum space,collision number and entropy are influenced by the mean field and nucleon-nucleon collision,especially the medium effects.     

Relativistic heavy-ion collisions: An approach based on non-equilibrium thermodynamics

A new theory of particle production in high energy collisions is proposed which is based on non-equilibrium thermodynamics. The non-equilibrium model is a major extension of the equilibrium thermodynamic model of relativistic heavy-ion collisions developed earlier. While the equilibrium thermodynamic theory is appropriate for the formation of light nuclei and for pions, the non-equilibrium theory applies to the creation of particles heavier than the pion, which include such particles as the strange mesons, strange baryons and the anti-nucleons. Using an approach based on the degree of the reaction of kinetic theory, the time evolution of the composition of hadronic systems in incomplete equilibrium is investigated. Densities of produced particles are related to space-time quantities and to the production cross sections of the underlying dynamic processes. An application of the non-equilibrium approach to the production of strange matter is given. The importance of secondary processes, following pion production, in the formation of strange matter is shown. In fact, the secondary production process for kaons is as important as the direct production process arising from initial nucleon-nucleon (NN) collision of a first collision picture. Thus, kaons can be produced in a late stage of the collision of two nuclei and they do not necessarily reflect the early stages of the collision as first thought. Using the experimental number of kaons, the time of reaction is also estimated. No evidence for a long-lived state of the nuclear system is found. Expressions for particle production ratios are developed. The results of an equilibrium theory and a non-equilibrium theory are found to be similar for such ratios. The chemical equilibrium constant is shown to be present in the non-equilibrium theory; the Boltzmann factor in the production threshold energy appears in the equilibrium theory. The K^?/K⁺ ratio is estimated. Surprisingly, reasonable agreement with experiment is found in the K^?/K⁺ ratio using the equilibrium theory, even though the production processes for K⁺'s and K^?'s treated individually, are not ones for which the equilibrium theory applies. It is shown that a fundamental difference between the equilibrium and non-equilibrium theory is lost when particle ratios for non-equilibrium particles are taken. Expressions for the production of complex composite structures made of strange particles are developed. The non-equilibrium model with some modifications may be useful for high energy NN and pion-nucleon collisions.     

200A GeV硫打原子核反应中奇异压低的约化和奇异反重子产生

用事件产生器LUCIAE分析了200AGeV硫打原子核反应中p和Λ产额､横动量分布以及Λ/p比值的NA35数据,结果表明:NA35数据反映了极端相对论性核一核碰撞中青异压低与同能量的核子-核子碰撞相比有约化的现象;但是核-核碰撞中Λ/p比值趋向1,并不必然意味着夸克的味对称性.     

The dynamical ingredients governing the multifragmentation process and collision dynamics in heavy ion collisions

By simulating numerically the reaction dynamics of heavy ion collisions within the modified quantum molecular dynamics (MQMD), we have studied the influences of the nucleon-nucleon (n-n) collision cross section with and without medium effect, momentum dependent interactions (MDI), equation of state (EOS) and the aggregating method of fragments on the multifragmentation process of heavy ion collisions with different beam energies. It is found that multifragmentation patterns of the final fragment distributions, the collective flows of fragments and single particles, collision number and nuclear matter density depend strongly on then-n cross section and momentum dependent interactions and the nuclear equation of state, especially these dependences are associated with beam energies. But the fragment multiplicity distribution patterns depend very weakly on the equation of state.     

Quark-gluon plasma formation in the central rapidity region and finite size effects of colliding nuclei

The energy density of the central products in the ultra-relativistic heavy ion collisions are calculated. In our estimation, the special attention is paid to the space-time extension of the emission points of the secondary hadrons which are originate in the successive nucleon-nucleon interactions within the finite size of colliding nuclei. The average collision number per produced particle, in the final state interaction, is also calculated and is used as a criterion whether the system is thermalized or not. It turns out that the attainable energy density in the central heavy ion collision is sensitive to the spacetime extension of the emission points. However, if the incident energy and the mass numbers of colliding nuclei are high and large, we can get high enough energy density for the phase transition from a hadronic state to a quark-gluon plasma state.     

Observation of a threshold effect in the anomalous J/ψ suppression

We report on a search for a phase transition from ordinary nuclear matter to a state of deconfined quarks and gluons as predicted by lattice QCD calculations. A new measurement of charmonium production in Pb-Pb interactions at 158 GeV/c per nucleon agrees with our previous results and confirms the anomalous J/ψ suppression we had already observed on a significantly smaller data sample. New event selection and analysis techniques show that, for peripheral collisions, the J/ψ cross-section per nucleon-nucleon collision agrees with the precise suppression pattern inferred from a wide range of measurements extending from p-p up to S-U collisions. As the collisions become more central, the Pb-Pb cross-section exhibits a clear departure from this normal behaviour. The onset of the anomalous J/ψ suppression reported here is the first clear observation of a threshold effect in heavy ion collisions and can be considered as a strong indication of the production of a deconfined quark-gluon phase in central Pb-Pb collisions.