Charge distribution in scattering off nuclei: a test of the production mechanism

In the dual parton model, the absolute value of the charge in the forward hemisphere in collisions of negative particles with nucleons increases with energy. This effect is strongly amplified in collisions with nuclei when the number of struck nucleons increases. It can provide a crucial test of the particle production mechanism.     

On a present status of the gathering model for cumulative meson production

Physical grounds of the “gathering” model are discussed. It is shown that the “gathering” of nucleons in a collective interaction is a direct consequence of the behaviour of colour forces at large distances. An analytical variant of the model is presented which allows a large simplification of the calculation procedure. The results obtained are compared with the data onπ- andK-meson cumulative production. The model applicability for relativistic nucleus-nucleus collisions is discussed.     

Dispersion of the multiplicity distribution in hadron-nucleus collisions

It is argued that the measurements of the dispersion of the multiplicity distribution in hadron-nucleus collisions favour the processes of particle production in a sequence of collisions of the incident hadron with target nucleons. On the other hand, generation of particles on groups of nucleons treated as single units leads to large discrepancies.     

Pion and strangeness puzzles

The dependence of pion and strangeness production on the number of participant nucleons and collision energy is discussed for central A+A collisions. A possible interpretation of the experimental results assuming transition to QGP is sketched within a simple statistical approach.     

Model for describing the production of Centauro events and strangelets in heavy-ion collisions

A phenomenological model for describing the production of Centauro events in relativistic heavy-ion collisions is discussed. The model provides quantitative predictions for kinematical variables, for the baryon number, and for the masses of a Centauro fireball and of its decay products. A Centauro fireball decays predominantly into nucleons, strange hyperons, and possibly strangelets. Centauro events in Pb + Pb collisions at the LHC energy are simulated for the CASTOR detector. The signatures of these events are discussed in detail.     

Deuterons from 300 GeV proton collisions calculated from the Lund model

We present an alternative approach to describe deuteron production in high energy particle collisions. The phenomenological Lund-model has been used to give the number and momentum distribution of secondary nucleons produced in 300 GeV proton-proton collisions. Deuterons are assumed to be produced in a final state interaction between the nucleons inNN d reactions inside a volume of 1 F³. The results are compared to experimental data.Data became available for antideuteron production in e⁺e^– annihilation. * A Lund Monte Carlo calculation for the production of secondary nucleons in e⁺e^– collisions exist. The results from a final state interaction were too small by a factor 10³. A calculation for a coalescent model which uses the overlap of the proton-neutron momentum distribution with a distribution from a Hulthen wave function is in good agreement with the data. The coalescent model works well in this case. It is because the relative momentum of the coalescent nucleons produced for 10 GeV cm energy in e⁺e^– collisions is very much smaller than the relative momentum of the nucleons produced in 300 GeV fixed target energy ofpp collisions.     

Study of the energy flow in sulphur-and oxygen-nucleus collisions at 60 and 200 GeV/nucleon

Data on multiplicities of charged particles produced in proton-nucleus and nucleus-nucleus collisions at 200 GeV per nucleon are presented. It is shown that the mean multiplicity of negative particles is proportional to the mean number of nucleons participating in the collision both for nucleus-nucleus and proton-nucleus collisions. The apparent consistency of pion multiplicity data with the assumption of an incoherent superposition of nucleon-nucleon collisions is critically discussed.     

MULTIPLICITY DISTRIBUTIONS OF PRODUCED PARTICLES IN α-α COLLISIONS AT HIGH ENERGIES

We analyse the multiplicity distributions of negative particles produced in α-α collisions at S_NN=31.2GeV by using a Glauber-type multiple collision model in which the projectile nucleons degrade in energy as they make collisions with target nucleons.The energy loss suffered by the projectile nucleons in the passage through the target nucleus is explicitly considered in the calculation.The agreement with experiments is good.     

Evolution of pion phase-space density and Bose-enhancement effects in high-energy heavy-ion collisions

The evolution of the pion phase-space density in the ultrarelativistic heavy-ion collision is investigated on the basis of an analytical solution of the kinetic equation. The rate of pion production is treated as a superposition of binary collisions of the projectile and target nucleons. It is calculated in accordance with the experimentalNN→πX cross section, neglecting the deceleration of nuclei during their interpenetration. Very large occupation numbers of the lowp _T -pions in the central rapidity region are predicted for typical heavy-ion collisions at Brookhaven and CERN energies. The role of the Boseenhancement of pion production is discussed. It is shown that this effect may be essentially suppressed due to the finite formation time of pions.     

Pickup in the surface — The origin of deuterons in nuclear precompound reactions

The formation of deuterons in nucleon- and pion-induced precompound reactions is discussed as a multistep direct reaction: several collisions of nucleons inside the nucleus are followed by a pickup in the surface as last step.     

Correlations between nucleons in high-energy nuclear collisions

The correlation between nucleons ejected in high-energy nuclear collisions is studied in a classical Glauber model treating multiple collisions between nucleons and delta resonances. The microscopic binary kinematics produces pronounced structure in the rapidity-rapidity and azimuthal-angle projections.     

Inclusive (γ, N), (γ, NN) and (γ, Nπ) reactions in nuclei at intermediate energies

Differential cross sections of nucleons excited in photonuclear reactions in medium and heavy nuclei are studied by considering all relevant reaction mechanisms leading to the excitation of protons or neutrons. We take advantage of previous microscopic studies for the absorption and scattering of photons and photoproduced pions, and implement a simulation code in order to take into account the propagation of the nucleons as well as their collisions with other nucleons in the nuclear medium, which generate secondary excited nucleons. Comparison with experimental data is done. Cross sections for nucleon emission in coincidence with one pion are also calculated, and some coincidence observables are discussed.     

Using nuclei to probe hadronization in QCD

The behavior of quasi-exclusive and inclusive ϱ and J/ψ photoproduction, electroproduction and hadroproduction in nuclei are discussed for small and large p_⊥. In particular we argue that J/ψ production in ion-ion collisions is likely to be suppressed relative to the background lepton pair production, independent of whether or not a QCD plasma is formed. We point out that previous extractions of the J/ψ inelastic cross section do not actually measure the cross section for the interaction of physical J/ψ's with nucleons.     

The expansion of the fireball formed by a high-energy heavy-ion collision

Employing the Wigner density formalism we present a model for the expansion of a fireball created in a relativistic heavy-ion collision. In a simplified model we first study how many collisions a nucleon suffers during the expansion of the fireball. Knowing this we can determine the Wigner densities of the nucleons in the entrance channel of the elementary deuteron production. It turns out that, even if the nucleons in the entrance channel have a thermal distribution, the particles in the exit channel do not. Comparison with experiments shows that the energy distributions of pions, protons and deuterons can be described by the same temperature. The number of deuterons allows the determination of the average density at the beginning of the expansion. Finally, we compare these results with those of other models, especially of a chemical rate equation.     

Large momentum pion production in proton nucleus collisions and the idea of “fluctuons” in nuclei

It is shown that in proton-nucleus collisions, the production of pions with large momenta can be explained by the assumption of the existence of nuclear density fluctuations (“fluctuons”) at short distances of the nucleon core radius order, with the mass of several nucleons.     

Nuclear stopping power

Inclusive cross sections for protons emitted in 100 GeV proton-nucleus collisions are used to estimate the stopping power of nuclear matter for fast nucleons. The typical recoil momentum obtained for a nucleon struck by the center of a lead nucleus is 4–10 GeV/c, an order of magnitude greater than in p-p collisions, and an order of magnitude smaller than in a naive cascade model. Possible implications for high energy heavy ion collisions are discussed.     

Fast deuteron production in proton-nucleus interactions

Deuteron formation in proton-nucleus reactions due to the coalescence mechanism with final-state interaction between nucleons is discussed in order to establish the kinematical region of the validity of this approach. The experimental data on fast deuteron production in proton-nucleus collisions at bombarding energies above 3 GeV are analyzed. It is shown that most experimental results were obtained for deuteron momenta below the critical limit of the coalescence model. It is discussed to which extent further experimental studies could help to identify contributions from the coalescence mechanism and to get better understanding of the fast deuteron-production process.This article was processed by the author using the LATEX style filepljour2 from Springer-Verlag.     

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.     

Microscopic calculation of pre-equilibrium emission

Previous TDHF calculations have shown that a pre-equilibrium emission of nucleons should be seen for asymmetric collisions of nuclei. Recent calculations indicate that the added effect of two-body collisions is to enhance this pre-equilibrium emission, it being seen also for symmetric collisions. More detailed calculations of this phenomenon are presenied here. The two-body collisions are treated by the time-relaxation method. This method is reviewed, and a revised formula for the relaxation time is introduced.Calculations are made in a slab geometry. For real nuclei, as much as 6% of all nucleons are estimated to be emitted at a beam energy of E_c.m. = 20 MeV/A. The energy distribution of the emitted nucleons relates to a temperature of 13 MeV at this energy. At 5 MeV/A, the emission is reduced to about 1 %.     

System-size dependence of strangeness production in nucleus-nucleus collisions at squareroot[sNN]=17.3 GeV

Emission of pi+/-, K+/-, phi, and Lambda was measured in near-central C+C and Si+Si collisions at 158 AGeV beam energy. Together with earlier data for p+p, S+S, and Pb+Pb, the system-size dependence of relative strangeness production in nucleus-nucleus collisions is obtained. Its fast rise and the saturation observed at about 60 participating nucleons can be understood as the onset of the formation of coherent systems of increasing size.