Fourier analysis of nonself-averaging quasiperiodic oscillations in the excitation functions of dissipative heavy-ion collisions: quantum chaos in dissipative heavy-ion collisions?

We employ stochastic modelling of statistical reactions with memory to study quasiperiodic oscillations in the excitation functions of dissipative heavy-ion collisions. The Fourier analysis of excitation function oscillations is presented. It suggests that S-matrix spin and parity decoherence, damping of the coherent nuclear rotation and quantum chaos are sufficient conditions to explain the nonself-averaging of quasiperiodic oscillations in the excitation functions of dissipative heavy-ion collisions.     

The effect of angular-momentum dissipation on fluctuations of excitation functions in heavy-ion collisions

We discuss here the effect of dissipation of relative angular momentum on fluctuations of excitation functions in dissipative heavy-ion collisions. Dissipation and fluctuation of relative angular momentum modify and smooth the time-angle localization of the rotating dinuclear system. The secondary maxima in the energy correlation function of the cross-section shift to smaller values of the energy difference, the shift depending on the relaxation time and the diffusion coefficient for angular-momentum dissipation. The results are illustrated for the collision²⁸Si(E _lab=130 MeV)+⁴⁸Ti.Partly supported by the Alexander von Humboldt Foundation     

Variational method for inner-shell excitation in heavy-ion collisions

In the framework of the monopole model we study a perturbation method for inner-shell excitation which covers the whole projectile velocity region. This is achieved by introducing a velocity-dependent variational parameter and minimizing the time-dependent perturbation. We apply the method to theK-excitation of nonrelativistic systems.     

Internal pair creation induced by nuclear coulomb excitation in heavy-ion collisions

Internal electron-positron pair creation caused by nuclear Coulomb excitation in heavy-ion collisions is compared with the spontaneous and induced positron production in overcritical quasimolecules with Z₁ + Z₂$?170. Using the rotation-vibration nuclear model in the calculation of the quadrupole Coulomb excitation and the conversion coefficient for electron-positron pair formation, the total cross section for the system ²³⁸₉₂U-²³⁸₉₂U was found to be ${\sigma }_C^{e^{+},e^{?}}=0.125mb$ at E_kin^c.m. = 797 MeV, which is four times smaller than the cross section ${\sigma }_{ind}^{e^{+},e^{?}}$ for the corresponding vacuum decay process if a K-shell vacancy production of L₀ = 10^? is assumed. Evaluation of the ratio $R=d{\sigma }_{ind}^{e^{+},e^{?}}({?}_{c.m.})/d{\sigma }_C^{e^{+},e^{?}}({?}_{c.m.})$ between the differential cross sections with respect to the ion angle ?_c.m leads to R = 60 at ?_c.m. = 160°. In contrast to the induced positron spectrum, the decay of excited nuclear levels in U is not followed by positrons with kinetic energy above 0.9 MeV. Therefore a unique determination of the decay of the vacuum in overcritical fields is experimentally possible provided that L₀ is not drastically reduced. In addition, similar calculations have been performed for pair creation resulting from nuclear Coulomb excitation for the systems U-Cf, Th-Th, Pb-Pb, Ni-Ni, Xe-Ba, Xe-Nd and Xe-Ce. The occupation of vacant bound states of superheavy electronic molecules by conversion electrons of γ-rays from nuclear transitions with E_γ < 1 MeV is also discussed.     

Power spectrum analysis to identify exotic events in ultra-relativistic heavy-ion collisions

The usefulness of the power spectrum method has been studied for identifying non-statistical fluctuations artificially injected in a small subset of normal simulated events in S + Au and Pb + Pb collisions at CERN SPS energy. Number of peaks in the power spectrum above a threshold is used as the selection criterion. In most cases the efficiency of selection is high and the impurity in the filtered sample due to the statistical fluctuation of normal events is quite low. Special treatment is suggested for weak non-statistical component at a level of admixture comparable to the statistical fluctuation.     

Cherenkov radiation from jets in heavy-ion collisions

The possibility of Cherenkov-like gluon bremsstrahlung in dense matter is studied. We point out that the occurrence of Cherenkov radiation in dense matter is sensitive to the presence of partonic bound states. This is illustrated by a calculation of the dispersion relation of a massless particle in a simple model in which it couples to two different massive resonance states. We further argue that detailed spectroscopy of jet correlations can directly probe the index of refraction of this matter, which in turn will provide information about the mass scale of these partonic bound states.     

Temperatures in heavy-ion collisions from pion multiplicities

A thermodynamically consistent treatment of the nuclear interaction is employed to study the dependence of pion production on the nuclear equation of state in heavy-ion collisions. Massive baryon resonances, heavy mesons and the Bose condensation of pions are incorporated into a macrocanonical relativistic quantum-statistical treatment of the highly excited system. The measured pion multiplicities, which vary over eight orders of magnitude in the bombarding energy range from 30 MeV/nucleon to 4 GeV/nucleon, are reproduced within a simple one-dimensional fluiddynamical model if it is assumed that nuclear matter is rather incompressible. The pion yields are in this model directly related to the compression energy, which amounts to one-half of the total center-of-mass energy at all BEVALAC energies. The maximum compression derived is uncertain by about 10% and 30% at E_lab = 0.4 and 1.8 GeV/nucleon, respectively. The temperatures of the system in the moment of the chemical freeze-out of the pion/delta degree of freedom are determined from the measured pion yields and range from 10 MeV to 100 MeV. An extrapolation to CERN/BNL energies, i.e. E_lab > 10 GeV/nucleon, yields T = 150–200 MeV. A strong energy dependence of the cross sections and the slopes of hard γ's is predicted by this model. The calculated photon yields are in surprising agreement with the data on γ- production at intermediate energies.     

Electrons from quasi-atoms formed in heavy-ion collisions

Electrons emitted in heavy-ion collisions of oxygen and sulfur projectiles with heavy targets were measured. Electron energies up to five times the K-shell binding energy of the target atom were observed. The high-energy part can be explained well within the framework of the Born approximation using relativistic electron wave functions for the united atoms. It is argued that the high-energy shape of the spectra is directly related to the form factor of inner shell electrons of the united atoms.     

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.     

Light from cascading partons in relativistic heavy-ion collisions

We calculate the production of high energy photons from Compton and annihilation processes as well as fragmentation off quarks in the parton cascade model. The multiple scattering of partons is seen to lead to a substantial production of high energy photons, which rises further when parton multiplication due to final state radiation is included. The photon yield is found to be directly proportional to the number of hard collisions and thus provides valuable information on the preequilibrium reaction dynamics.     

Charge-dependent correlations in heavy-ion collisions from stochastic hydrodynamics

Charge-dependent correlations from both background and charge separation contribute to experimental observables in heavy-ion collisions. In this paper, we use stochastic hydrodynamics to study background charge asymmetry due to fluctuations. Using the rapidity-dependent correlation and a simple ansatz for particle distributions, we find a fluctuation-induced correlation to provide a type of background F -correlation. Experimental data for Au+Au collisions at $\sqrt{{s}_{\mathrm{NN}}}=200\,\mathrm{GeV}$ are compared. We also make predictions for F -correlations in isobar collisions. Combining this with our previous chiral magnetic effect results, we obtain δ -correlations for collisions in the three types of system. Computations from our model show an almost identical background with less than 2% difference for isobars, but roughly 10% difference for their charge separations. In combination with our earlier works, we provide a consistent method of calculating both the chiral magnetic effect and the charged background in the context of stochastic hydrodynamics.     

Directed flow of baryons from high-energy heavy-ion collisions

The collective motion of nucleons from high-energy heavy-ion collisions is analyzed within a relativistic two-fluid model for different equations of state (EoS). As function of beam energy the theoretical slope parameter F _y of the differential directed flow is in good agreement with experimental data, when calculated for the QCD-consistent EoS described by the statistical mixed-phase model. Within this model, which takes the deconfinement phase transition into account, the excitation function of the directed flow (P _x ) turns out to be a smooth function in the whole range from SIS till SPS energies. This function is close to that for pure hadronic EoS and exhibits no minimum predicted earlier for a two-phase bag-model EoS. Attention is also called to a possible formation of nucleon antiflow (F _y < 0) at energies ? 100 A·GeV.     

Generation and division of excitation energy in heavy-ion collisions studied by measuring charged-particle survival fractions

Charged-particle survival fractions of primary projectile-like fragments from the ⁴⁰Ar + ¹⁹⁷Au reaction at 450 MeV were measured by using a large array of 32 phoswich detectors operating in coincidence with a detector of projectile-like fragments. Differential survival fractions of the primary pickup and stripping reaction products indicate a dependence of the average excitation energy generated in the primary fragments on the direction of the mass transfer.     

Probing meson spectral functions with double differential dilepton spectra in heavy-ion collisions

The double differential dilepton spectrum d/(d ² d _⊥ ²) at fixed transverse mass M _⊥ allows a direct access to the vector meson spectral functions. Within a fireball model the sensitivity of d/(d ² d _⊥ ²) against variations of both the in-medium properties of mesons and the dynamics of the fireball is investigated. In contrast to the integrated invariant-mass spectrum d/d ², in the spectrum d/(d ² d _⊥ ²) with fixed M _⊥ the ω signal is clearly seen as bump riding on the ρ background even in case of strong in-medium modifications.[3mm] Received: 16 November 2000 / Accepted: 16 January 2001     

Charmonium and bottomonium in heavy-ion collisions

We review the present status in the theoretical and phenomenological understanding of charmonium and bottomonium production in heavy-ion collisions. We start by recapitulating the basic notion of “anomalous quarkonium suppression” in heavy-ion collisions and its recent amendments involving regeneration reactions. We then survey in some detail concepts and ingredients needed for a comprehensive approach to utilize heavy quarkonia as a probe of hot and dense matter. The theoretical discussion encompasses recent lattice QCD computations of quarkonium properties in the Quark-Gluon Plasma, their interpretations using effective potential models, inelastic rate calculations and insights from analyses of electromagnetic plasmas. We illustrate the powerful techniques of thermodynamic Green functions (T-matrices) to provide a general framework for implementing microscopic properties of heavy quarkonia into a kinetic theory of suppression and regeneration reactions. The theoretical concepts are tested in applications to heavy-ion reactions at SPS, RHIC and LHC. We outline perspectives for future experiments on charmonium and bottomonium production in heavy-ion collisions over a large range of energies (FAIR, RHIC-II and LHC). These are expected to provide key insights into hadronic matter under extreme conditions using quarkonium observables.     

Shadowing effect on pion spectra from intermediate-energy heavy-ion collisions

Pion spectra from asymmetric heavy-ion collisions at intermediate energies are studied in the slab geometry. The effect of pion absorption is treated in a phenomenological way. The shadowing of outgoing pions explains the apparent velocity of “the pion emitting source”.     

Hydrodynamic models for heavy-ion collisions and beyond

A generic property of a first-order phase transition in equilibrium, and in the limit of large entropy per unit of conserved charge, is the smallness of the isentropic speed of sound in the “mixed phase”. A specific prediction is that this should lead to a non-isotropic momentum distribution of nucleons in the reaction plane (for energies ≈ 40 A GeV in our model calculation). On the other hand, we show that from present effective theories for low-energy QCD one does not expect the thermal transition rate between various states of the effective potential to be much larger than the expansion rate, questioning the applicability of the idealized Maxwell/Gibbs construction. Experimental data could soon provide essential information on the dynamics of the phase transition.     

Neutron and proton diffusion in heavy-ion collisions

A statistical theory for the exchange of protons and neutrons in heavy-ion collisions is formulated and applied to recent data. No systematic deviations are observed which could be evidence for (additional) quantal effects in the $\text{N}\text{Z}$ equilibration.