Thermal dileptons in high-energy nuclear collisions

Clear signs of excess dileptons above the known sources have been found at the SPS for a long time. However, a real clarification of these observations was only recently achieved by NA60, measuring dimuons with unprecedented precision in 158A GeV In-In collisions. The excess mass spectrum in the region is consistent with a dominant contribution from π⁺π⁻→ρ→μ⁺μ⁻ annihilation. The associated ρ spectral function shows a strong broadening, but essentially no shift in mass. In the region , the excess is found to be prompt, not due to enhanced charm production. The inverse slope parameter associated with the transverse momentum spectra rises with mass up to the ρ, followed by a sudden decline above. While the initial rise, coupled to a hierarchy in hadron freeze-out, points to radial flow of a hadronic decay source, the decline above signals a transition to a low-flow source, presumably of partonic origin. The mass spectra show a steep rise towards low masses characteristic of Planck-like radiation. The polarization of the excess referred to the Collins Soper frame is found to be isotropic. All observations are consistent with a global interpretation of the excess as thermal radiation. We conclude with a short discussion of a possible link to direct photons.     

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.     

Isotope distribution in nuclear multifragmentation

A statistical multifragmentation model is used to analyze the isotope distribution of medium sized nuclei produced in p + Kr reactions at high bombarding energies. The results show that the evaporation processes narrow significantly the primordial hot fragment distributions and washes out almost completely the patterns associated with the isotope distributions in the primary fragments. Thus the calculation supports a scenario in which the formation of hot primary fragments is assumed. From adjustments to the experimental data an excitation energy per nucleon of the initial system is found to range from 3.5 to 7 MeV.     

Isospin fractionation in nuclear multifragmentation

Isotopic distributions for light particles and intermediate mass fragments have been measured for 112Sn+112Sn, 112Sn+124Sn, 124Sn+112Sn, and 124Sn+124Sn collisions at E/A = 50 MeV. Isotope, isotone, and isobar yield ratios are utilized to estimate the isotopic composition of the gas phase at freeze-out. Analyses within the equilibrium limit imply that the gas phase is enriched in neutrons relative to the liquid phase represented by bound nuclei. These observations suggest that neutron diffusion is commensurate with or more rapid than fragment production.     

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.     

Surveying the role of excitation energy in probing nuclear dissipation

A dynamical Langevin model is employed to calculate the excess of the evaporation residue cross sections of the 194pb nucleus over that predicted by the standard statistical model as a function of nuclear dissipation strength. It is shown that large excitation energy can increase the effects of nuclear dissipation on the excess of the evaporation residues and the sensitivity of this excess to the dissipation strength, and that more higher excitation energies have little contribution to further raising this sensitivity. These results suggest that on the experimental side, producing those compound systems with moderate excitation energy is sufficient for a good determination of the pre-saddle nuclear dissipation strength by measuring the evaporation residue cross section, and that forming an extremely highly excited system does not considerably improve the sensitivity of evaporation residues to the dissipation strength.     

Surveying the role of excitation energy in probing nuclear dissipation

A dynamical Langevin model is employed to calculate the excess of the evaporation residue cross sections of the ^194Pb nucleus over that predicted by the standard statistical model as a function of nuclear dissipation strength. It is shown that large excitation energy can increase the effects of nuclear dissipation on the excess of the evaporation residues and the sensitivity of this excess to the dissipation strength,and that more higher excitation energies have little contribution to further raising this sensitivity. These results suggest that on the experimental side,producing those compound systems with moderate excitation energy is sucient for a good determination of the pre-saddle nuclear dissipation strength by measuring the evaporation residue cross section,and that forming an extremely highly excited system does not considerably improve the sensitivity of evaporation residues to the dissipation strength.     

Generators of nuclear collisions at the LHC energy

Current versions of the most popular generators of ultrarelativistic heavy-ion collisions are used to simulate central (b<3 fm) Pb + Pb collisions at the LHC energy, $\sqrt s = 6A$ TeV. The charged-particle density predicted by the generators for the mid-rapidity range, dN _ch/dη, varies in a wide range, from about 2000 to about 6000. Moreover, even for a given generator, it depends strongly on model parameters and can vary from 10 to 100%.     

Dynamical analysis of dissipative collisions between Ar and Ag nuclei in the fermi energy domain

⁴⁰Ar+¹⁰⁷ Ag reactions at 27 AMeV and 44AMeV have been studied within the Landau Vlasov microscopic transport model. In this framework the main characteristics of primary partners of binary collisions are shown to be well described, and information aboutσ _nn can be extracted as well as the angular momentum associated with the heavy fragment. Different contributions to the total energy are studied as a function of the reaction time. Our numerical method is compared with the results obtained using experimental techniques like proton spectra. Their impact parameter and energy dependences have been studied and the estimated “temperature parameter” is in agreement with the experimental results.     

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.     

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.     

Resonant spin-changing collisions in spinor fermi gases

The compensation of quadratic Zeeman effect and trap energy in high-spin fermions is shown to lead to resonances in the spin-changing collisions that are typically absent in spinor condensates and spin-1/2 fermions. We study these resonances in lattice fermions, showing that they permit the targeting of a particular spin-changing channel while suppressing the rest and the creation of magnetically insensitive superpositions of many-body states with entangled spin and trap degrees of freedom. Finally, the intersite tunneling may lead to a quantum phase transition described by a quantum Ising model.     

High energy nuclear collisions: Theory overview

We review some basic concepts of relativistic heavy-ion physics and discuss our understanding of some key results from the experimental program at the relativistic heavy-ion collider (RHIC). We focus in particular on the early time dynamics of nuclear collisions, some result from lattice QCD, hard probes and photons.     

New mechanism for the production of extremely fast light particles in heavy-ion collisions in the fermi energy domain

Employing a four-body classical model, various mechanisms responsible for the production of fast light particles in heavy-ion collisions at low and intermediate energies have been studied. It has been shown that, at energies lower than 50 A MeV, light particles of velocities of more than two times the projectile velocities are produced due to the acceleration of the target light particles by the mean field of the incident nucleus. It has also been shown that precision experimental reaction research in normal and inverse kinematics is likely to provide vital information about which mechanism is dominant in the production of fast light particles.     

Reducibility and thermal scaling in nuclear multifragmentation

Recent studies have revealed the existence of a number of reducibility and thermal scaling properties in nuclear multifragmentation. The probability of emitting n-fragments is found to be reducible to the probability of emitting a single fragment through the binomial expression. The resulting one fragment probability shows thermal scaling by producing linear Arrhenius plots. Similarly, the charge distributions associated with n-fragment emission are reducible to the one-fragment charge distribution. Thermal scaling is also observed. The reducibility equation contains a constant whose value, zero or positive, can be related to a univariant (two phase) or bivariant (one-phase) regime. The light fragment particle-particle angular correlations also show reducibility to the single-particle angular distributions as well as thermal scaling. A mass scaling associated with the angular correlations suggests emission from several small sources (A ≈ 20). The limits of applicability of scaling and reducibility are discussed as well as their implications for the mechanism of multifragmentation.     

Evidence for spinodal decomposition in nuclear multifragmentation

Multifragmentation of a "fused system" was observed for central collisions between 32 MeV/nucleon 129Xe and (nat)Sn. Most of the resulting charged products were well identified due to the high performances of the INDRA 4pi array. Experimental higher-order charge correlations for fragments show a weak but nonambiguous enhancement of events with nearly equal-sized fragments. Supported by dynamical calculations in which spinodal decomposition is simulated, this observed enhancement is interpreted as a "fossil" signal of spinodal instabilities in finite nuclear systems.     

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.