Bremsstrahlung in heavy-ion collisions

The bremsstrahlung background emission of γ-rays in equal-mass heavy-ion collisions is calculated. The cross section is found to be about an order of magnitude smaller than preliminary upper limits of ¹²C-¹²C experiments in progress.     

Determining pion source parameters in relativistic heavy-ion collisions

Two-pion inclusive spectra from relativistic heavy-ion collisions are related to single-particle inclusive measurements in a Lorentz-invariant formulation of the Hanbury-Brown-Twiss effect. A thermal pion source with a gaussian space-time distribution is assumed. The angular anisotropy and moments of the correlation function are computed to facilitate the determination of pion source parameters from the ratios of the two-pion to single-pion inclusive data.     

Universal pion freeze-out in heavy-ion collisions

Based on an evaluation of data on pion interferometry and on particle yields at midrapidity, we propose a universal condition for thermal freeze-out of pions in heavy-ion collisions. We show that freeze-out occurs when the mean free path of pions lambda(f) reaches a value of about 1 fm, which is much smaller than the spatial extent of the system at freeze-out. This critical mean free path is independent of the centrality of the collision and beam energy from the Alternating Gradient Synchrotron to the Relativistic Heavy Ion Collider.     

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.     

A particle-hole calculation for pion production in relativistic heavy-ion collisions

A differential cross section for π-meson production in peripheral heavy-ion collisions is formulated within the context of a particle-hole model in the Tamm-Dancoff approximation. This is the first attempt at a fully quantum-mechanical particle-hole calculation for pion production in relativistic heavy-ion collisions. The particular reaction studied is an ¹⁶O projectile colliding with a ¹²C target at rest. In the projectile we form a linear combination of isobar-hole states, with the possibility of a coherent isobar giant resonance. The target can be excited to its giant M1 resonance (J^π = 1⁺, T = 1) at 15.11 MeV, or to its isobar analog neighbours, ¹²B at 13.4 MeV and ¹²N at 17.5 MeV. The theory is compared to recent experimental results.     

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.     

Estimating the particle-antiparticle correlation effect for pion production in heavy-ion collisions

Back-to-back ππ correlations arising owing to the evolution of the pionic field in the course of the pion-production process are estimated for central heavy-ion collisions at moderate energies.     

Bremsstrahlung from colour charges as a source of soft particle production in hadronic collisions

It is proposed that soft particle production in hadronic collisions is dominated by multiple gluon exchanges between partons from the colliding hadrons, followed by radiation of hadronic clusters from the coloured partons distributed uniformly in rapidity. This explains naturally two dominant features of the data: (a) the linear increase of rapidity spectra in the regions of limiting fragmentation and, (b) the proportionality between the increasing width of the limiting fragmentation region and the height of the central plateau.     

Azimuthal asymmetry of direct photons in intermediate energy heavy-ion collisions

Hard photon emitted from energetic heavy ion collisions is of very interesting since it does not experience the late-stage nuclear interaction, therefore it is useful to explore the early-stage information of matter phase. In this work, we have presented a first calculation of azimuthal asymmetry, characterized by directed transverse flow parameter F   and elliptic asymmetry coefficient v₂$v_2$, for proton–neutron bremsstrahlung hard photons in intermediate energy heavy-ion collisions. The positive F   and negative v₂$v_2$ of direct photons are illustrated and they seem to be anti-correlated to the corresponding free proton's flow.     

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.     

High-energy photons as a thermometer for ultra-relativistic nuclear collisions

I calculate the transverse momentum distribution for high-energy (p _T =1–3 GeV) photons produced in ultra-relativistic nuclear collisions. I assume a strong first-order deconfinement phase transition, and consider transition temperatures in the range 150–200 MeV. For simplicity, I also assume thermal and chemical equilibrium throughout the collision. I then fit the transverse momentum distribution in the range 1–2 GeV to a thermal distribution. The fitting temperature depends only on the transition temperature, so this provides an accurate (although theory-dependent) measure of the QCD transition temperature.