Jet fragmentation function moments in heavy ion collisions

The nature of a jet’s fragmentation in heavy-ion collisions has the potential to cast light on the mechanism of jet quenching. However, the presence of the huge underlying event complicates the reconstruction of the jet fragmentation function as a function of the momentum fraction z of hadrons in the jet. Here we propose the use of moments of the fragmentation function. These quantities appear to be as sensitive to quenching modifications as the fragmentation function directly in z. We show that they are amenable to background subtraction using the same jet-area-based techniques proposed in the past for jet p _t ’s. Furthermore, complications due to correlations between background-fluctuation contributions to the jet’s p _t and to its particle content are easily corrected for.     

Equilibrium in heavy ion collisions

We discuss the question of equilibration in heavy ion collisions and how it can be addressed in experiment     

Femtoscopy in heavy ion collisions

Non-trivial space-time geometrical effects are at the core of bulk-sector heavy ion physics, and two-particle correlations at low relative velocity are the most direct probe of this geometry at the femtometer scale. I present a brief overview of the wealth of femtoscopic measurements from the past two decades of heavy ion experiments. Essentially every conceivable “knob” at our disposal has been turned; the response of two-particle correlations to these variations has revealed much about the space-momentum substructure of the hot source created in the collisions. I discuss the present status of the femtoscopic program and questions which remain, and point to new efforts which aim to resolve them. PACS 25.75.Ld; 25.75.Gz; 24.10.Nz     

Kaon flow in heavy ion collisions

Using transport models that include explicitly the kaon degree of freedom, we have studied kaon flow, i.e., the average kaon transverse momentum as a function of rapidity, and the associated flow parameter in heavy ion collisions at both SIS and AGS energies. It is found that the pattern of kaon flow in heavy ion collisions is sensitive to the kaon potential used in the model and thus provides a useful means for studying the kaon properties in nuclear medium.     

Event-by-event fluctuations in heavy ion collisions

We discuss the physics underlying event-by-event fluctuations in relativistic heavy ion collision. We will emphasize how the fluctuations of particle ratios can be utilzed to explore the properties of the matter created in these collisions. In particular we will argue that the fluctutions of the ratio of positively over negatively charged particles may serve as a unique signature for the Quark-Gluon Plasma.     

Nuclear flow in heavy ion collisions

Short overview of the flow phenomenon from the low upto relativistic heavy ion energies is given. Strict difference between spectator and participant flow is emphased. The latter is described by the modification of the nuclear blast model. Thermal source expanding in a plane perpendicular to the line connecting the centers of projectile and target is assumed. Observed dependences of squeeze-out, side-splash and of mean transverse-energy in relativistic heavy-ion collisions on the particle's mass are reproduced. Partioning of initial projectile energy into thermal and compressional parts in²⁰⁹Bi(1 GeV/u)+²⁰⁸Pb and¹⁹⁷Au(150–800 MeV/u)+¹⁹⁷Au collisions is discussed.Invited lecture given at the International School-Workshop Relativistic Heavy-Ion Physics, Prague (Czech Republic), 19–23 September 1994.Experimental data obtained at Bi+Pb collisions [6] are the result of the group effort. The contribution of V. Wagner, M. Pachr, Yu. G. Sobolev, R. Simon, S. Hlava, M. umbera as well as of my student R. Pleska, who carried out most of the fits, is gratefully acknowledged. This work was supported by the Granting Agency of the Czech Republic under contract No. 202/93/1144. A.K. gratefully acknowledges support from EC during his stay at KVI Groningen, where part of this work was done.     

Hadron chemistry in heavy ion collisions

The relaxation times necessary to establish chemical equilibrium among different hadrons in hot, dense hadronic matter are investigated in a statistical model. Consequences for heavy ion collisions are exploited in the framework of a simple reaction model. The possibility of Bose-Einstein pion condensation around the break-up time of the nuclear fireball is pointed out.     

Collective phenomena in heavy ion collisions

Collective flow from ultra-relativistic heavy ion collisions is an important hadronic observable sensitive to the early stages of system evolution. We have calculated flow components from a tilted, ellipsoidally expanding source. We also reviewed the recent experimental techniques used for calculation of the v _nFourier coefficients and pointed out a few possible problems connected to these techniques, which may lead to serious inaccuracy in the flow analysis.     

Pion condensation in heavy ion collisions

We calculate the pion spectrum in dense nuclear matter for finite temperatures. The critical temperatureT _c(ρ) that marks the beginning of a second order phase transition due to pion condensation is given in a phase diagram. We show that in heavy ion collisions, pion condensation should occur, leading to an enhancement in the formation of nuclear shock waves.     

Strangeness in relativistic heavy ion collisions

Abundances of strange antibaryons formed in nuclear collisions at above 10 GeV/A are considered as a most accessible diagnostic tool for the study of the possible formation and physical properties of the quark-gluon plasma phase of hadronic matter. In this report we describe the current status and develop a dynamical approach in order to describe strange particle formation in nuclear collisions at high energy.     

Charmed exotics in heavy ion collisions

Based on the color–spin interaction in diquarks, we argue that charmed multiquark hadrons are likely to exist. Because of the appreciable number of charm quarks produced in central nucleus–nucleus collisions at ultrarelativistic energies, the production of charmed multiquark hadrons is expected to be enhanced in these collisions. Using both the quark coalescence model and the statistical hadronization model, we estimate the yield of charmed tetraquark mesons, T_cc, and pentaquark baryons, Θ_cs, in heavy ion collisions at RHIC and LHC. We further discuss the decay modes of these charmed exotic hadrons in order to facilitate their detections in experiments. PACS 25.75.Dw; 14.20.Lq; 14.40.Lb     

Processes with heavy ion collisions

Some Coulomb effects in heavy ion collisions are considered. Among them the process of muon and lepton pair creation, Coulomb and unitary corrections, statistics of multiple pair production. Effects of multiple photon exchange in process of lepton pair production by linearly polarized photon on nuclei are considered. This process is used for measuring the polarization of initial photon. Relativistic muon energy loss due to the light lepton pair production in the Coulomb field are calculated. Also we consider the effects of multiple photon exchange in elastic lepton scattering on unscreened atomic field and discuss the possible experimental testing.     

Auto-ionization of positrons in heavy ion collisions

Autoionization of positrons occurs as a fundamentally new process of quantum electrodynamics, if empty 1s- or 2p _1/2? etc. electronic shells obtain binding energies larger than 2m _e c ². This effect should be experimentally observable in the scattering of very heavy ions (Z≧80) on each other since in such collisions superheavy electronic molecules are formed (superheavy quasi-molecules). The scattering mechanism and the distribution of autoionization positrons are discussed. The adiabaticity of the heavy ion collision is studied and the electron-positron pair production background to the ionization problem is estimated. Analytic solutions are obtained for 1/r-potentials for the caseZα≧1. The phase shifts of negative energy solutions in the case of cutoff Coulomb potentials reveal the accuracy of the autoionization formalism.     

System size in relativistic heavy ion collisions

System size is more than a geometrical quantity in relativistic heavy ion collisions; it is closely related to evolution process, i.e. a different system size corresponds to a different evolution process, and whether QGP is produced depends on the system size. We propose that the system size should be under the same level when comparing the measurements from different colliding nuclei. The equivalence of the peripheral collisions of Au-Au and the central collisions of smaller nuclei is studied using the Monte Carlo method. Comparing the transverse overlapping area of the colliding nuclei, the number of participant nucleons and the number of nucleon-nucleon binary collisions in various colliding nuclei, we give an estimate of the correspondence in system size. This is helpful in the experimental comparison of the measurements from different colliding nuclei.     

Multiphonon excitations in heavy ion grazing collisions

A semiclassical model is used to study the excitation of giant resonances in heavy ion grazing collisions. The projectile is described as a moving Woods-Saxon potential with a fixed shape, and the evolution of the target state is calculated by time-dependent perturbation theory. Using random phase approximation wave functions, probabilities to excite various resonances are obtained. Multiple phonon excitations appear as the possible mechanism for the structures observed in heavy ion collisions.     

Projectile fluorescence yields in heavy ion collisions

Fluorescence yields of highly ionized projectiles entering thin solid foils are determined from a target-thickness analysis of projectile and target K X-ray cross sections. Values are obtained for 20–80 MeV Cl ions on Cu and compared with scaling calculations.     

Chemical thermalization in relativistic heavy ion collisions

We compute by numerical integration of the Dirac equation the number of quark-antiquark pairs initially produced in the classical color fields of colliding ultrarelativistic nuclei. While the number of pairs is parametrically suppressed in the coupling constant, we find that in this classical field model their production rate is comparable to the thermal ratio of gluons/pairs=9Nf/32. After isotropization one thus would have a quark-gluon plasma in chemical equilibrium.     

Multifragmentation and dynamics in heavy ion collisions

A midrapidity zone formed in heavy-ion collisions has been investigated through special selections of light particles and intermediate mass fragments detected in the reaction ³⁵Cl on ¹²C at 43 MeV/nucleon and the reactions ⁵⁸Ni on ¹²C, ²⁴Mg, and ¹⁹⁷Au at 34.5 MeV/nucleon, and of neutron energy spectra measured in the reaction ³⁵Cl on ^natTa. Properties of the observables have been examined to characterize the neck-like structure formed between the two reaction partners. For the HERACLES collaboration.