Pion production in central high energy nuclear collisions

A model for pion production is studied in the context of a cascade calculation. The pions are produced through Δ-resonances which are allowed to decay. The emitted pions are assumed to interact with the other nucleons by forming new Δ -resonances. The time evolution of the pion and Δ-population is studied; it is found that Δs are always more numerous than pions during the sequence of baryon-baryon collisions. The spectrum of the pions is in considerably better agreement with experiment than the one obtained with frozen Δ-isobars. The presence of Δ-resonances appears to be important for the cooling of the pion system. The pion multiplicity is found to deviate from a Poisson distribution. The pion yield is overestimated by at least 25%; this result is discussed within the framework of conventional dynamics.     

Anomalous behavior of pion production in high energy particle collisions

The shape of the invariant differential cross section for charged hadron production as a function of transverse momentum measured in various collider experiments is analyzed. Contrary to the behavior of produced charged kaons, protons, and antiprotons, the pion spectra require an anomalously high contribution of an exponential term to describe the shape.     

Dependence of particle production in high energy heavy ion collisions on the nuclear equation of state

Recent experimental results for ₈₁ ¹⁹⁶ Tl₁₁₅ along with that of ₈₁ ¹⁹⁸ Tl₁₁₅ are analysed. It is shown that staggering observed in the excitation energy of the even and odd spin states in these odd-odd mass transitional nuclei is due to the residual interaction between the two odd particles.     

Charged particle multiplicities and inelastic cross sections in high energy nuclear collisions

Inelastic cross sections at 60 and 200 GeV/nucleon are determined in a streamer chamber for ¹⁶O on several nuclear targets. Charged particle multiplicity distributions for inelastic and central collisions are studied and compared with theoretical predictions. The inelastic cross section exhibit a geometrical dependence on nuclear radii. The multiplicity data are governed by the collision geometry. They are consistent with a picture of superposition of independent nucleon-nucleus interactions.     

Thermalization in high energy nuclear collisions

complete events of the symmetric collisions Ca + Ca at E/A = 400 MeV and 1050 MeV and Nb + Nb at E/A = 400 MeV and 650 MeV have been measured with the Plastic Ball/Wall 4π spectrometer. The degree of isotropy expressed by the ratio of the mean transverse to the mean longitudinal momentum in each event is determined as a function of the charged particle multiplicity. Also discussed are proton energy distributions at θ_cm = 90° and their multiplicity dependences.     

Models for new particle production in nuclear collisions

Following up our earlier work and also stimulated by recent experimental results on positron production in heavy ion collisions we investigate various mechanisms for the production of a light, previously unknown particle in such collisions. As more conventional mechanisms seem to be inadequate we concentrate on processes directly or indirectly connected with the strong electric or magnetic fields and quark degrees of freedom in nuclei. We also consider modifications of the color confinement scheme. Our analysis is confined to schemes staying within the framework of the standardSU(3)_c ×SU(2)×U(1) model. Since most of the possibilities discussed can be rejected, severe modifications of basic concepts of particle physics seem to be required if the production of a new particle is confirmed.     

Transport coefficients in high energy nuclear collisions

Starting from the Boltzmann equation for a fast test nucleon moving in the background medium provided by the other nucleons we derive an analytic expression for the transition kernel. From its first and second moments the friction and diffusion coefficients of a Fokker-Planck equation are obtained. These are compared with the results of a phenomenological transport model.     

Particle production in high energy nucleus-nucleus collisions

Recent data on the production of pions and strange particles at the Bevalac and Synchrophasotron accelerators are reviewed, covering pion spectra and multiplicity distributions, λ, K⁺ and K⁻ yields and spectra, and Λ polarization. Emphasis is placed on recent progress in determining the equation of state of compressed fireball nuclear matter from the observed pion yield in central collisions. Further, the information derived from apparent spectral temperatures is critically examined, along with a discussion of thermal and chemical equilibrium attainment in the reactions, as revealed by particle spectra and yields.     

Study of neutral-charged particle correlations in high energy collisions

Recent experiments at Serpukhov, NAL, and CERN indicate a strong correlation between neutral and charged pions produced in high energy collisions, in contrast to the trend shown by data at lower energies. This study of the energy and charge dependence of these correlations indicates that they do not depend upon the initial state particles and that they are in reasonable agreement with the critical fluid gas model. These high energy correlation data are also studied in terms of a semi-inclusive Koba-Nielsen-Olesen scaling relation.     

Baryon-to-entropy ratio in very high energy nuclear collisions

We compute as a function of rapidity y the baryon number carried by quarks and antiquarks with p _T > p ₀ ≈ 2 GeV produced in Pb+Pb collisions at TeV energies. The computation is carried out in lowest order QCD perturbation theory using structure functions compatible with HERA results. At p ₀ = 2 GeV the initial gluon density is both transversally saturated and thermalised in the sense that the energy/gluon equals to that of an ideal gas with the same energy density. Even at these high energies the initial net baryon number density at y = 0 at τ = 0.1 fin will be more than the normal nuclear matter density but the baryon-toentropy ratio is only (B-B?)/S ～ 1/5000. Further evolution of the system is discussed and the final baryon-to-entropy ratio is estimated.     

A model for multiparticle production in high energy hadronic collisions

A model for multiparticle production process in high-energy hadronic collisions is proposed. In the centre of mass (CM) system of colliding particles the target and the projectile are assumed to pass through each other sharing energies allowed by kinematical constraints. Thus in app collision the energy associated with each is √S/2 (S being the square of the CM energy) which is taken to be the real variable that governs the number of particles produced. In the case of hadronnucleus collisions the projectile and the target ofv nucleons lying in a (Lorentz contracted) tube pass through each other sharing energies ⋍ √S _A2, whereS _A ⋍vS. Before the final state particles emerge from these systems, the constituents of the target, i.e.,v nucleons share equally (= √S _A2v) the total energy associated with the target and become the centres from which final state particles stem out. Several results have been discussed.     

Characteristics of charged particle production in relativistic heavy-ion collisions

Inclusive spectra of charged particles at midrapidity in Au+Au collisions at $\sqrt {s_{NN} } = 130$ GeV and 200 GeV were measured with the STAR detector at RHIC. The measured mean transverse momentum 〈p _T 〉 shows a characteristic dependence on charged particle multiplicity and beam energy in Au+Au collisions that is distinctly different from pp, $p\bar p$ and e⁺e^? collisions. A 32%±3%(syst) increase in 〈p _T 〉 from pp to Au+Au collisions was observed at 200 GeV. While the charged multiplicity was found to increase by 19%±5%(syst) from $\sqrt {s_{NN} } = 130$ GeV to 200 GeV, no significant difference in 〈p _T 〉 was found between the two energies. A comparison with model predictions is discussed.     

Study of charged multiplicity distributions in high energy particle collisions

Data on hadron-proton collisions ranging from 1 to 300 GeV/c in incident momentum show evidence for an energy-dependent approach to a single semi-inclusive scaling curve for the charged multiplicity cross sections as predicted by Koba, Nielsen and Olesen. The onset of this scaling behavior is shown to depend on the initial state hadrons. The relation between the onset of this apparent scaling and the approach to a constant value of 〈n〉/D is suggestive of a two-component process.     

Azimuthal asymmetry of direct photons in high energy nuclear collisions

We show that a sizable azimuthal asymmetry, characterized by a coefficient nu2, is to be expected for large pT direct photons produced in noncentral high energy nuclear collisions. This signal is generated by photons radiated by jets interacting with the surrounding hot plasma. The anisotropy is out of phase by an angle pi/2 with respect to that associated with the elliptic anisotropy of hadrons, leading to negative values of nu2. Such an asymmetry, if observed, could be a signature for the presence of a quark gluon plasma and would establish the importance of jet-plasma interactions as a source of electromagnetic radiation.     

Strange particle production in nuclear collisions at 200 GeV per nucleon

Multiplicities and spectra of strange particles ( andK ^– produced in central³²S+S,³²S+Ag and³²S+Au collisions at 200 GeV per nucleon are presented and compared with data on strange particle production in protonnucleus and nucleon-nucleon interactions. It is shown that strangeness production in³²S+Ag collisions is enhanced by a factor of two, similar to that found previously in central³²S+S collisions.     

Simple models for transverse energy production in high energy proton-nucleus collisions

A simple model which includes the effects of primary degradation is shown to be able to reproduce FERMILAB data for transverse energy production in pA collisions of 800 GeV protons on all targets from protons to ²⁰⁸Pb.