Pion production mechanism in high energy heavy ion collisions

We study the mechanism of pion production in high energy heavy ion collisions with the nuclear cascade model developed in a previous paper. We describe time-dependently the reaction rates of the processes N+N→N+Δ, N+Δ→N+N, Δ→N+π, and N+π→Δ, to discuss the achievement of the chemical equilibrium. The importance of final Δ−N and π−N interactions is pointed out. The present cascade model reproduces well the observed pion yields in Ar+KCl high multiplicity events, without introducing the nuclear compression effect. The reason of the agreement is discussed by decomposing a variety of conditions employed in this model, and by reproducing the situations of Cugnon's cascade model and others.     

Subthreshold K production in high energy heavy ion collisions

We show that an important contribution to subthreshold K⁻ production in high energy heavy ion collision is the strangeness exchange reactions Yπ→K---N between the hyperons and pions initially produced in the collision. Estimates are given for the number of K⁻ produced via this mechanism and it accounts for a large fraction of the experimental observation.     

Thermal hadron production in high energy heavy ion collisions

We provide a method to test if hadrons produced in high energy heavy ion collisions were emitted at freeze-out from an equilibrium hadron gas. Our considerations are based on an ideal gas at fixed temperatureT _f , baryon number densityn _B , and vanishing total strangeness. The constituents of this gas are all hadron resonances up to a mass of 2 GeV; they are taken to decay according to the experimentally observed branching ratios. The ratios of the various resulting hadron production rates are tabulated as functions ofT _f andn _B . These tables can be used for the equilibration analysis of any heavy ion data; we illustrate this for some specific cases.     

Photoproduction of massive gauge bosons in high energy electron-proton collisions

A self-consistent renormalization scheme at finite temperature and zero momentum is used together with the finite temperature renormalization group to study the temperature dependence of the mass and the coupling to one-loop order in the (φ ³)₆- and (φ ⁴)₄-models. It is found that the critical temperature is shifted relative to the naive one-loop result and the coupling constants at the critical temperature get large corrections. In the high temperature limit of the (φ ⁴)₄-model the coupling decreases.     

On the production of slow particles in high energy heavy ion collisions

The mechanism for producing slow, target associated particles in high energy heavy ion collisions is investigated. It is shown that the same mechanism which was proposed for hadron induced interactions is likely to appear also for heavy-ion reactions. The results indicate however a much weaker correlation between impact parameter and yield of slow particles.     

Shadowing effects on quark and gluon production in ultrarelativistic heavy ion collisions

A framework for including shadowing effects to quark and gluon production in very high energy nucleus-nucleus collisions is given. A formalism for impact parameter dependent nuclear structure functions of partons is introduced and, using two models to describe the average shadowing corrections to the parton number densities, the average numbers and the transverse energy spectrum of hard partons produced inU+U collisions at RHIC and LHC energies are computed. We conclude that shadowing and also the impact parameter dependence of structure functions should be taken into account at very high energies.     

Some QCD estimates of the production of heavy quark flavours in hadron-hadron collisions

Quantum chromodynamic perturbation theory is used to investigate the hadronic production of heavy quark flavours. The theory is applied principally to the data on charmed meson production. Particular emphasis is placed on comparing data with this calculation in the specific region of phase space in which the data are taken. The general results of this approach are presented so that comparisons with data from future experiments on charm and heavier flavour production should be possible.     

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.     

Correlations between transverse energy production and the Drell-Yan process in heavy ion collisions at high energy

The expression for the Drell-Yan cross section is presented which takes into account the geometry of heavy ion collisions. The Drell-Yan cross section is shown to be proportional to the transverse energy produced within a fixed rapidity interval. A dependence steeper than linear of the cross section for production of dileptons with masses larger than about 2 GeV on the transverse energy in a given rapidity bin would be a clear-cut signal for the formation of the quark-gluon plasma.     

Isosinglet neutral heavy lepton production in high energy e-γ collisions

We evaluate the potential for searching for isosinglet neutral heavy leptons (N), such as right-handed neutrinos, in the next generation of e⁺e⁻ linear colliders, paying special attention to contributions from the various γe→WN initiated by photons from beamstrahlung and laser back-scattering. We find that these mechanisms are both competitive and complementary to the standard e⁺e⁻→νN annihilation processe for producing neutral heavy leptons in these machines and greatly extends the search range over HERA and LEP200.     

Heavy quark energy loss in high multiplicity proton-proton collisions at the LHC

One of the most promising probes to study deconfined matter created in high energy nuclear collisions is the energy loss of (heavy) quarks. It has been shown in experiments at the Relativistic Heavy Ion Collider that even charm and bottom quarks, despite their high mass, experience a remarkable medium suppression in the quark gluon plasma. In this exploratory investigation we study the energy loss of heavy quarks in high multiplicity proton-proton collisions at LHC energies. Although the colliding systems are smaller than compared to those at the Relativistic Heavy Ion Collider (p+p vs Au+Au), the higher energy might lead to multiplicities comparable to Cu+Cu collisions at the Relativistic Heavy Ion Collider. The interaction of charm quarks with this environment gives rise to a non-negligible suppression of high momentum heavy quarks in elementary collisions.     

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.     

The QCD pomeron in ultraperipheral heavy ion collisions: II. Heavy quark production

The heavy quark production in ultraperipheral heavy ion collisions is investigated, with particular emphasis on the results from the coherent interactions given by the two-photon process. One addresses the heavy quark total cross sections at photon level considering the saturation model and the BFKL dynamics in the color dipole picture. The corresponding cross sections at nuclear level are presented. It is verified that the QCD dynamics implies an enhancement of the cross section in comparison with previous calculations. Received: 30 January 2003 / Revised version: 26 March 2003 / Published online: 13 May 2003 RID="a" ID="a" e-mail: barros@ufpel.tche.br RID="b" ID="b" e-mail: magnus@if.ufrgs.br     

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.