Production of secondaries in relativistic nuclei collisions at high energies

Using the multiple-scattering theory and quark-gluon string model, the averaged and central collisions between high-energy nuclei are studied. Production of light flavour hadrons as well as charmed and beauty mesons and baryons is considered. We present predictions for mean multiplicities of secondaries, inclusive spectra and distributions over multiplicity for the case of Au?Au collisions with energy 100 GeV per nucleon in every beam.     

Multiplicities in highp t hadron-nucleus interactions

We present results obtained in π^? A (A=C, Cu, Pb) — collisions at 38 GeV/c. A single particle trigger selects events with one charged particle in the central region and large transverse momentum. The effect of this trigger on the multiplicities of all charged particles and of protons is shown.     

Formation of multinucleon systems and nuclei with mass numbers of 6 and 7 in <Superscript>16</Superscript>O<Emphasis Type="Italic">p</Emphasis> interactions at a momentum of 3.25 GeV/<Emphasis Type="Italic">c</Emphasis> per nucleon

The results of a comparative analysis of the multiplicities of singly and doubly charged accompanying particles in the channels involving the production of six- and seven-nucleon systems in ¹⁶Op collisions at a momentum of 3.25 GeV/c per nucleon are reported. Cross sections for the yield of multinucleon systems and mean multiplicities of accompanying particles are found. The experimental data are compared with the predictions of the cascade—fragmentation—evaporation model.     

Multiplicity of charged secondaries emitted in association with neutral strange particles in antiproton-nucleus collisions at 40 GeV/c

In collisions of 40-GeV/c antiprotons with D, Li, C, S, Cu, and Pb nuclei, mean multiplicities of various secondary particles are investigated as functions of the mass number A. The mass-number dependence of the mean multiplicities of positively charged particles suggests that the effect of intranuclear cascades is strong for the emission of Λ hyperons, but that it is relatively weak for the emission of either K ⁰ or $\bar \Lambda $ . Also measured are the yields of various neutral strange particles with respect to those of charged secondaries.     

Structures and angular distributions of slow and relativistic charged particles produced in 7Li-emulsion interactions at 3 GeV/c per projectile nucleon

Experimental results on the multiplicity distributions of various particles produced in the interactions of ⁷Li with emulsion nuclei at a momentum of 3 GeV/c per projectile nucleon are reported. A comparison with data on collisions induced by other nuclei at a nearly identical momentum per nucleon is presented in order to reveal the dependence on the projectile mass. The internal structure of ⁷Li is explored by studying the projectile fragment. The mean multiplicity of shower particles, 〈n _s〉, induced by ⁷Li is found to be less than that in the case of ⁶Li projectiles. The angular distributions of target fragments and relativistic charged secondaries are investigated. No shock-wave phenomena are observed. Forward-to-backward ratios are calculated for each case. The probability distributions for relativistic secondaries produced per unit rapidity are studied in detail, along with the rapidity densities and their dependence on the projectile and the target mass. A comparison of the angular spectra of shower particles produced in central and peripheral collisions supports the limiting-fragmentation hypothesis. The collisions in question seem to become more central with increasing shower-particle multiplicity.     

Highp T hadrons as leading particles in jets produced at the ISR

The density of charged particles near a single highp _T trigger particle in proton-proton collisions has been studied with the SFM detector at the CERN-ISR. It is shown that:—the secondaries cluster in a jet-like manner about the trigger particle; —the momentum distributions in this trigger jet can be unambiguously separated from the spectator background distributions;—the momentum distributions in this trigger jet can be unambiguously separated from the spectator background distributions;—the momentum component transverse to the jet direction is exponentially damped;—the width of the transverse distributions increases with the momentum component along the jet;—the fractional longitudinal momentum density depends strongly on the trigger transverse momentum and scales with \(x_T = {{2p_T } \mathord{\left/ {\vphantom {{2p_T } {\sqrt s }}} \right. \kern-0em} {\sqrt s }}\) . All these findings support the hard constituent scattering origin of highp _T hadrons.     

Multiplicity and pseudorapidity distributions of charged particles from32S induced heavy ion interactions at 200A GeV

Multiplicity and pseudorapidity distributions have been measured for³²S+Al, Cu, Ag and Au at 200A GeV. The widths of the pseudorapidity distributions increase from central to peripheral collisions. The main contribution is assumed to come from the increasing fraction of charged particles stemming from the fragmentation of the target for peripheral collisions as compared to central collisions. On the average 170–180 charged particles per unit of pseudorapidity are attained for the most central³²S+Au events at the maximum of the distribution. The target dependence of the yield of charged particles for central collisions is investigated. In the target rapidity region the yield is directly proportional to the target mass. The transverse energy per charged particle has been studied as a function of centrality and pseudorapidity. The experimental results are compared to the results from the Monte-Carlo model VENUS 3.11 which includes rescattering among secondaries. A comparison between different centrality triggers is made.     

中低能速度能区离子与H,He原子碰撞中的经典电子俘获截面

Bohr-Lindhard模型被用来描述中低能速度能区离子—原子碰撞中的经典电子俘获过程。根据离子与原子的作用时间与碰撞参数的关系,建立俘获几率对碰撞参数的依赖性,碰撞参数范围被限定在俘获半径之内。在该模型的框架内,人们试图通过电子的空间分布函数来研究所有碰撞参数的贡献,但存在较为复杂的数值计算。基于Bohr-Lindhard模型,本工作提出通过简单的指数衰减函数来描述电子俘获几率对碰撞参数的依赖性,计算了Aq+(q = 2~6)-H碰撞中的单电子俘获截面和Aq+(q = 3~6)-He碰撞中的双电子俘获截面,计算结果与已有实验数据符合很好,很好地描述了低能和中能区的电子俘获截面随能量和电荷态的变化,该工作还可计算其他不同电荷态离子与He和H靶的电子俘获截面。     

Jet production from nuclei at 400 GeV/c

Results are presented from a calorimeter study on the production of jets in large-E_T proton-nucleus collisions at 400 GeV/c. Jet-like large-p_T events from eight nuclear targets are seen with a special jet trigger. As previously observed, the cross section for such events increases slightly faster than the atomic number, much like in the production of large-p_T single hadrons. Data on energy flows and multiplicities suggest that a heavy nucleus does not significantly widen the angular distribution of the beam remmants. In the central region the particle flow in hard nuclear collisions is similar to that in soft nuclear collisions.     

Analysis of shower particle multiplicity in hadron-nucleus collisions in the few hundred GeV region

An analysis of relativistic charged secondaries produced in hadron-nucleus collisions in the few hundred GeV region is presented from the point of view of the structure of hadrons and to examine the possibility of discriminating different theoretical models. An attempt has been made to study the mean normalised multiplicity ratio in the central region and its dependence on the mean number of intranuclear collisionsv. The rate of increase in the production of particles exhibits good agreement with our proposed approach of the constituent supercluster model.     

Entrance-Channel Dependence of Isospin Effects on Double n/p Ratio in HIC

Within the hadronic transport model IBUU04, we investigate the effect of density-dependent symmetry energy on double neutron/proton (n/p) ratio of free nucleons in heavy ion collisions by taking four isotopic Sn+Sn reaction systems. Especially the entrance-channel asymmetry and impact-parameter dependence of the effect of symmetry energy are discussed. It is found that in both central and semi-central collisions the sensitivity of the double n/p ratio to the density-dependent symmetry energy is more pronounced in neutron-richer systems. Our results also indicate clearly that the effect of symmetry energy is stronger in central collisions than that in semi-central collisions.     

On the semiclassical impulse approximation for electron capture in asymmetric ion-atom collisions

A derivation of the impulse approximation for the capture of a targetK-shell electron by a light projectile in ion-atom collisions is given in the framework of the semiclassical approximation. The impact-parameter dependence of the capture probability is calculated numerically without further approximations, and shows good agreement with recent experimental results for protons colliding with Ne and Ar. The validity of several peaking approximations and the relation to ionisation theories is briefly discussed.     

A study of deep inelastic hadron-hadron collisions with a large acceptance calorimeter trigger

Large transverse energy cross sections of 300 GeV/c pions and protons on hydrogen have been measured with a segmented calorimeter covering the central rapidity region ?0.88 < y < 0.67 and 2π in azimuth. The selected events show large multiplicities and no jet-like event structure. Processes more complicated than the scattering of two constituents appear to dominate these inelastic collisions.     

Hadron Multiplicities in Pb＋Pb Collisions at the Large Hadron Collider and Pomeron Loop Effects

We study the pseudo-rapidity distribution of hadron multiplicities of high energy Pb＋Pb collisions by using color glass condensate dynamics at LHC/ALICE in the fixed coupling case. It is found that after including the pomeron loop effects the charged hadron multiplicities at central rapidity are about 1500 for central Pb＋Pb collisions, which are significantly smaller than the saturation based calculations, ～ 1700 ÷ 2500 and compatible with that based on a study of multiplicities in the fragmentation region.     

Central events in the interactions of 28Si and 32S with heavy emulsion targets

Data on the multiplicity of secondaries in central events of ²⁸Si (14.6 AGeV) and ³²S (3.7 AGeV) interactions with AgBr emulsion nuclei have been compiled and studied. The dependence of the multiplicities of the outgoing charged stripping particles on the number of interacting nucleons and therefore on the impact parameter, as indicated by the target size, and consequently, on the degree of centrality is investigated. The resultant multiplicity distribution of the produced pions for each studied case is fitted by both Negative Binomial (NB) and Poisson distributions. The NB distribution is valid for most of the considered cases. The transparency of the target for a projectile was found to become more pronounced as the incident energy increased. Received: 20 June 2000 / Accepted: 17 January 2001     

Experimental study of structure functions and sum rules in charge-changing interactions of neutrinos and antineutrinos on nucleons

High-energy exclusive and inclusive cross sections are discussed in terms of their dependence on the transverse-position variables (impact parameters) of incident and outgoing particles. The objective is to clarify the points of conflict and agreement of various models with each other and with intuitive ideas based on macroscopic collisions. First the impact parameter representation of states and amplitudes is reviewed. New impact parameter conservation laws and sum rules are derived from Lorentz invariance. The generalized optical theorem of Mueller is extended to give the impact parameter distribution of produced particles. Mueller-Regge behaviour is shown to imply three-dimensional limiting fragmentation in (impact parameter, rapidity)- space and a specific linear structure of the final-state particle density in the central region of this space. The predictions of weak- and strong-coupling multiperipheral models, the φ³ ladder eikonal model and the dual resonance model are presented, with emphasis on the dependence of average multiplicity on impact parameter and the dependence of mean square impact parameter on multiplicity. Special techniques for strong-coupling multiperipheral models are used to study the breakdown of the random-walk impact-parameter structure of the weak-coupling case, and to show explicitly that the average multiplicity is a decreasing function of impact parameter in the strongly coupled ABFST model.     

Strange baryon production in heavy ion collisions

The rapidity distribution of Λ and ?Λ produced in nucleus-nucleus collisions at CERN energies is studied in the framework of an independent string model — with quark-antiquark as well as diquark-antidiquark pairs in the nucleon sea. It is shown that, besides the Λ-?Λ pair production resulting from the fragmentation of sea diquarks, final state interactions of co-moving secondaries π + N → K + Λ and ?N → K → ?Λ are needed in order to reproduce the data. Predictions for Pb-Pb collisions are presented.     

A study of pion production in 4.5 GeV/c)/nucleon 4He interactions with nuclear targets

Inelastic interactions of ⁴He nuclei at the incident momentum of 4.5 GeV/c)/nucleon with pure Li, C, Al, Cu and Pb targets were studied in the SKM-200 streamer chamber. Cross sections and their dependence on the target mass number for fragmentation processes were obtained. The multiplicities, transverse momenta and rapidities of negative pions as well as their correlations were analyzed separately for “central” and “peripheral” collisions. A comparison of the results with the predictions of some theoretical models is presented.     

Hydrodynamic flow at RHIC

We review the apparently hydrodynamic behaviour of low transverse momentum particles (pT ≤ 1.5 GeV/c) produced in central and semicentral (b ≤ 7 fm) heavy ion collisions at RHIC. We investigate the impact parameter dependence of various observables, elaborating on radial and elliptic flow and particle multiplicities. We also discuss possible ambiguities in the initialization of the hydrodynamic system and present observables that should allow for their resolution.     

Possible relation between scaled hadronic multiplicity distributions in pp collisions and +e−→γ→ hadrons

An impact-parameter representation of the empirical scaled charged-prong multiplicity distributions in high-energy pp collisions leads to the speculation that a corresponding multiplicity distribution in ⁺e^?→γ→ hadrons should approximately scale and be more sharply peaked about a larger average multiplicity than the pp case.