Shower Particle Multiplicity Distribution in High Energy Nuclear Reactions

The multiplicity distributions of charged shower particles in high energy reactions are studied in the framework of a statistical model based on the approximation of the phase space integral. The model is first applied to the proton-proton collisions and then further extended on geometrical basis to include proton-nucleus and nucleus-nucleus collisions. Although the model is parameter free, it gives quite satisfactory results in comparison with experimental data.     

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 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.     

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.     

Some general characteristics of Si-emulsion interactions at 4.5 GeV/c per nucleon

We present our observations on the salient features of the interactions produced by ²⁸Si nuclei of 4.5 GeV/c per nucleon in photographic emulsions. The results on the inelastic mean-free path and interaction cross section are presented and compared with the predictions of a geometrical model. Measurements on the multiplicity distribution, mean multiplicities, multiplicity correlations and multiplicity scaling of charged secondary particles are studied. A comparison with other available data at same/nearby incident momentum per nucleon from proton-nucleus and carbon-nucleus collisions is presented in order to examine the dependence on the projectile mass.     

Inclusive production of neutral strange particles in\bar pp interactions at 32 GeV/c

Multiplicities of charged particles produced in the forward and backward hemispheres are investigated for proton-nucleus and proton-nucleon collisions in the range from 20 to 400 GeV/c. Weak, but growing with incident beam momentum, forward-backward multiplicity correlations are observed which cannot be reduced to the inelastic diffraction. These correlations are more pronounced in proton-nucleus than proton-nucleon interactions. The experimental data are discussed in the frameworks of current cluster and multiple scattering models. It is shown that multiple scattering models describe well the forward-backward multiplicity correlations in nuclear production.     

400 AMeV 12C诱发乳胶核反应重电离粒子前后关联研究

对400 AMeV 12C诱发乳胶核反应重电离粒子的前后关联进行了研究。重电离粒子来源于靶核碎片,分为灰径迹粒子和黑径迹粒子两种。实验结果很好地验证了核-核碰撞几何模型。灰径迹粒子的发射在前、后半球是各向异性的,而黑径迹粒子的发射基本是一致的。向前、向后发射的灰径迹粒子、黑径迹粒子平均多重数对重电离粒子数n_h呈线性依赖,黑径迹粒子、灰径迹粒子的向前发射随n_h的增加而增加的幅度分别大于其向后发射随n_h的增加而增加的幅度,但核内级联效应受靶核大小的影响,向后半球内发射的灰径迹粒子平均多重数随n_h的增加有饱和现象,这些多重数关联特性基本上可以利用基于碰撞几何的旁观体-反应体模型及级联蒸发模型来解释。The forward-backward multiplicity correlation of heavily ionized particles produced in 12C-emulsion interactions at 400 AMeV is investigated. The heavy ionized particles, come from the target fragments, are divided into grey track particle and black track particle. The experimental results can be well explained by the geometry model of the nucleus-nucleus collisions. The emission of grey track particles in forward and backward hemisphere is not isotropic, but the emission of black track particle is almost isotropic. The averaged multiplicity of grey track particles and black track particles in forward and backward hemisphere linearly depend on the number of heavily ionized particle n_h, the correlation strength in forward hemisphere is greater than that in backward hemisphere, but the dependence of grey track particle in backward hemisphere on the number of heavily ionized particle n_h shows the saturation because of the intranuclar cascade effect is influenced by target size. The characteristics of multiplicity correlations can be well explained by the participant-spectator model based on the colliding geometrical picture and the cascade evaporation model of high energy nucleus-nucleus collisions.     

Enhancement of the K+/π+ Ratio in High-Energy Proton-Nucleus and Nucleus-Nucleus Collisions due to the Final State Interactions

The pion-meson secondary collisions with surrounding nucleons in the nucleus can influence the K⁺/π⁺ ratio. We calculate this effect in proton-nucleus and nucleus-nucleus collisions by the Glauber model and find that the final state interactions are important.     

Leading baryon spectrum in high-energy nuclear collisions

We compute the leading baryon spectrum in high-energy nuclear collisions in the framework of the Dual parton model. Nucleus-nucleus collisions are treated in full detail. The average rapidity shift of the leading baryon (nuclear stopping power) is studied as a function of the degree of centrality of the collision and the cross-section of the selected events is given. Important differences are found here between proton-nucleus and nucleus-nucleus interactions. The agreement with existing data on proton-nucleus and α?α collisions is quite good.     

Analysis of multiparticle production data on proton-nucleus collisions using a new variable

Multiparticle production data on proton-nucleus collisions have been analyzed taking the number of ‘created’ charged particles instead of the observed number of shower particles as the variable. The mean normalized multiplicity,R _A , has been found to be independent of energy in the energy range (7–8000) GeV and its mass number dependence has been obtained. The modified analysis introduces some more regularities in the experimental results onp-nucleus collisions like the invariance with respect to energy of the relationshipR _A = α + βN _h and the KNO-like scaling of the multiplicity distributions of the created charged particles. The functional form of the scaling function has been calculated.     

Strangeness enhancement and strangeness suppression in nuclear collisions at 200 GeV/N

Data on yields of strange and negative hadrons in full phase space produced in proton-nucleus and central S+S collisions at 200 GeV/nucleon are reviewed. The total number of produced strange and nonstrange valence quark-antiquark pairs is determined on the basis of the hadron multiplicity at freezout. It is shown that the strangeness production factor λ _s is increased by a factor of 2 in central S+S collisions, whereas no increase is observed in proton-nucleus collisions.     

Strange baryons production and projectile fragmentation in p−A collisions at the AGS

Experiment 910 (E910) has studied proton-nucleus collisions at 12.3 and 17.5 GeV incident energies using a variety of targets. Centrality selection is provided via “grey” track multiplicity which can be statistically related to ν, the number of scatterings of the proton in the target nucleus. Previously published semi-inclusive measurements of Ν production are discussed and preliminary measurements of “leading” Ν production in 17.5 GeV p-Au collisions are presented as a function of ν. The data indicate that the probability for the projectile to fragment into a strange baryon increases significantly with increasing ν for ν≤3; an extrapolation of the E910 data to nucleus-nucleus collisions can reproduce most of the enhancement in Ν production observed in Pb-Pb collisions at 40, 80 and 160 A. GeV. Preliminary measurements of Ξ^? production in 17.5 GeV p-Au collisions show a rapid increase in yield with increasing ν. The data are discussed in the context of the Van Hove fragmentation model. Preliminary proton and pion spectra are presented for 12.3 GeV p-Be collisions. The data show a rapid degradation of outgoing proton momenta with increasing N _grey while only modest changes are observed in the pion fragmentation spectra.     

Flow effects in nucleus-nucleus collisions at (4.5×A) GeV/c

The multiplicity distributions of the negative particles are analysed using the moments method, for obtaining some experimental information on the possibility to observe flow processes and correlation between the particles in nucleus-nucleus collisions. The role of the collision geometry is important. The calculations are compared with experimental results for nucleus-nucleus collisions at (4.5×A) GeV/c obtained at the Synchrophasotron from JINR Dubna.Presented (by the first author) at the International School-Workshop Relativistic Heavy-Ion Physics, Prague (Czech Republic), 19–23 September 1994.     

AGS和RHIC能区核-核碰撞中的领头粒子效应与双柱模型

在总结双柱模型计算结果的基础上, 用该模型对交变梯度同步加速器(AGS)和相对论性重离子对撞机(RHIC)能区核－核碰撞中的领头粒子效应进行了统一分析. 结果表明:在AGS和RHIC能区, 不同中心性核－核碰撞中带电粒子(赝)快度分布的不同, 主要由领头粒子的贡献影响; 如果扣除领头粒子的贡献, 不同中心性核－核碰撞中带电粒子的(赝)快度分布有相同的形状.     

Microscopic theory of photon production in proton-nucleus and nucleus-nucleus collisions

The production of energetic photons in medium-energy proton and heavy-ion induced reactions is studied on the basis of incoherent nucleon-nucleon collisions. For this purpose we first evaluate covariantly the photon production from proton-neutron collisions in a vector (ω) and scalar meson (σ) exchange model with coupling constants given by the M2Y G-matrix in the nonrelativistic limit. We furthermore follow the proton-neutron collisional history by means of a phase-space simulation based on the Boltzmann-Uehling-Uhlenbeck approach for proton-nucleus and nucleus-nucleus collisions adding up incoherently the yields from each individual collision. The satisfactory agreement we obtain in comparison with experimental data allows to conclude that energetic photons predominantly arise from proton-neutron bremsstrahlung during the early stage of the collision.     

Canonical hadron gas interpretation ofp-A E802 data

Hadron gas models have proved successful in predicting particle production in relativistic nucleus-nucleus collisions. The extension of these models to the smaller systems formed in proton-nucleus collisions requires that the finite size of the system be considered. We study two features introduced by the finite size: the need to conserve strangeness and baryon number exactly by performing calculations in the canonical ensemble, and the inclusion of a finite size geometrical correction term in the single particle density of states. We find significant differences between the grand canonical and canonical ensembles and a strong dependence on the baryon number of the system.     

Charged-particle multiplicity density at midrapidity in central Pb-Pb collisions at sqrt[S(NN)] = 2.76 TeV

The first measurement of the charged-particle multiplicity density at midrapidity in Pb-Pb collisions at a center-of-mass energy per nucleon pair √ S NN = 2.76 TeV is presented. For an event sample corresponding to the most central 5% of the hadronic cross section, the pseudorapidity density of primary charged particles at midrapidity is 1584 ± 4(stat) ± 76(syst), which corresponds to 8.3 ± 0.4(syst) per participating nucleon pair. This represents an increase of about a factor 1.9 relative to pp collisions at similar collision energies, and about a factor 2.2 to central Au-Au collisions at √ S NN = 2.76 TeV. This measurement provides the first experimental constraint for models of nucleus-nucleus collisions at LHC energies.     

Can the fluctuations in head-on ultra-relativistic nuclear collisions be large?

We apply the Low-nussinov model of hadron-hadron collisions to nucleus-nucleus collisions. We show that fluctuations in the multiplicity are larger than those for models which assume that the nucleons interact incoherently. The dependence of the total multiplicity onA is difficult to estimate. String formation and breaking is discussed using classical chromodynamic equations to describe the gluon fields. These equations display amusing collective phenomena.     

高能核-核碰撞中蒸发碎片的研究

在高能核－核碰撞中,基于简单的模型计算,描述了来自弹核和靶核旁观体的蒸发碎片的多重数分布与角分布． Based on the simple modelling calculation,the multiplicity and angular distribution of evaporated fragments from both projectile and target spectators in high energy nucleus-nucleus collisions are described.