Freedom In Heavy Ion Collisions Research Articles

Isospin Dynamics and Dipolar degrees of freedom in Heavy Ion collisions

In this contribution the connection between the Dipolar degree of freedom excited in Heavy Ion Collisions through pre-equilibrium dipolar modes and the Isospin-equilibration processes is discussed. The connection leads to the definition of a new observable the so called “dipolar signal”, able to describe the equilibration process in the momentum space related to the final fragmentation produced by the HIC. Preliminary results on this observable measured on the system 48Ca +27 Al at 40 MeV/nucleon and corresponding to different degrees of dissipation/centrality are presented. The beam was accelerated through the Superconductor Cyclotron at the Laboratori Nazionali del Sud di Catania (Italy) and fragments were detected and identified with the 4π multi-detector CHIMERA.

Long-time drift of the isospin degree of freedom in heavy ion collisions

The drift of the isospin degree of freedom (IDOF) is experimentally visualized through the wide-range angular distribution and the beam energy dependence of the isospin composition of light charged particles (LCP) produced in heavy ion collisions. It is evidenced that the net isospin transport is presented in the whole process from early dynamic emission to the statistical evaporation lasting to very late stage. Due to the long-time accumulation of the isospin effect, the angular distribution of the relative neutron excess summing over the LCPs is utilized as a sensitive probe to the symmetry energy. With S fixed at 28.3 MeV, a soft density dependence with L=33–61 MeV (0.95 confidential level) at ρ0 is circumstantially deduced using an improved quantum molecular dynamics transport model attached with a statistical emission afterburner.

Effects of the kinetic symmetry energy reduced by short-range correlations in heavy-ion collisions at intermediate energies

Besides earlier predictions based on both phenomenological models and modern microscopic many-body theories, circumstantial evidence was recently found for a reduced kinetic symmetry energy of isospin-asymmetric nucleonic matter compared to the free Fermi gas model prediction due to the short-range correlation of high-momentum neutron-proton pairs. While keeping the total symmetry energy near the saturation density of nuclear matter consistent with existing experimental constraints, we examine the correspondingly enhanced role of the isospin degree of freedom in heavy-ion collisions at intermediate energies due to the reduced (enhanced) kinetic (potential) symmetry energy. Important observable consequences are investigated.

Isospin effects on pt-differential flow in heavy ion collisions at intermediate energies

This paper aims to study the role of isospin degree of freedom in heavy-ion collisions through the transverse momentum (pt), neutron to proton ratio and system mass dependence of pt-differential transverse flow. Our study shows that (pt)-differential transverse flow dependence can act as sensitive probe to study symmetry energy and its density dependence compared to the energy of vanishing flow. Symmetry energy and its density dependence play a dominant role over the isospin-dependence of nucleon–nucleon cross-section at Fermi energy.

Energy dependence of potential barriers and its effect on fusion cross sections

Couplings between relative motion and internal structures are known to affect fusion barriers by dynamically modifying the densities of the colliding nuclei. The effect is expected to be stronger at energies near the barrier top, where changes in density have longer time to develop than at higher energies. Quantitatively, modern TDHF calculations are able to predict realistic fusion thresholds. However, the evolution of the potential barrier with bombarding energy remains to be confronted with the experimental data. The aim is to find signatures of the energy dependence of the barrier by comparing fusion cross-sections calculated from potentials obtained at different bombarding energies with the experimental data. This comparison is made for the $^{40}$Ca+$^{40}$Ca and $^{16}$O+$^{208}$Pb systems. Fusion cross-sections are computed from potentials calculated with the density-constrained TDHF method. The couplings decrease the barrier at low-energy in both cases. A deviation from the Woods-Saxon nuclear potential is also observed at the lowest energies. In general, fusion cross-sections around a given energy are better reproduced by the potential calculated at this energy. The coordinate-dependent mass plays a crucial role for the reproduction of sub-barrier fusion cross-sections. Effects of the energy dependence of the potential can be found in experimental barrier distributions only if the variation of the barrier is significant in the energy-range spanned by the distribution. It appears to be the case for $^{16}$O+$^{208}$Pb but not for $^{40}$Ca+$^{40}$Ca. These results show that the energy dependence of the barrier predicted in TDHF calculations is realistic. This confirms that the TDHF approach can be used to study the couplings between relative motion and internal degrees of freedom in heavy-ion collisions.

NIMROD–ISiS, a versatile tool for studying the isotopic degree of freedom in heavy ion collisions

The upgraded NIMROD–ISiS 4 π detector array is described. The array is composed of 228 detector modules annularly arranged into 14 rings. Telescopes of Si–CsI and Si–Si–CsI provide excellent isotopic resolution throughout the detector array. To enhance event characterization, the entire charged particle array is housed inside the TAMU Neutron Ball.

Evidence for non-hadronic interactions of charm degrees of freedom in heavy-ion collisions at relativistic energies

Within the Hadron-String Dynamics (HSD) transport approach we study the suppression pattern of charmonia at RHIC with respect to centrality and rapidity employing various model concepts, such as variants of the ‘comover absorption’ model or the ‘charmonium melting’ scenario. We find that especially the ratio of the forward to mid-rapidity nuclear modification factors of J / Ψ ( R A A forward ( J / Ψ ) / R A A mid ( J / Ψ ) ) cannot be explained by the interactions with ‘formed’ comoving mesons or by the ‘color screening mechanism’ alone. Only when incorporating interactions of the c or c ¯ quark with a pre-hadronic medium satisfactory results are obtained. A detailed comparison to the PHENIX data demonstrates that non-hadronic interactions are mandatory to describe the narrowing of the J / Ψ rapidity distribution from pp to central Au + Au collisions. The Ψ ′ to J / Ψ ratio is found to be crucial in disentangling the different charmonium absorption scenarios, especially in the RHIC energy range. Furthermore, a comparison of the transport calculations to the statistical model of Gorenstein and Gazdzicki as well as the statistical hadronization model of Andronic et al. shows differences in the energy dependence as well as centrality dependence of the J / Ψ to pion ratio which may be exploited experimentally to disentangle different concepts. We find additionally that the collective flow of charm in the HSD transport appears compatible with the data at SPS energies but substantially underestimates the data at top RHIC energies, such that the large elliptic flow v 2 of charm seen experimentally has to be attributed to early interactions of non-hadronic degrees of freedom.

A Novel Algorithm to the Transient State of Nuclear Matter with Isospin Asymmetry, Thermal Excitation, and Compression

A dynamical algorithm which includes the isospin degrees of freedom in heavy ion collisions is introduced. A description of this algorithm. is given in detail.

Isospin equilibrium and non-equilibrium in heavy-ion collisions at intermediate energies

The equilibrium and non-equilibrium of the isospin degree of freedom are studied in terms of an isospin-dependent QMD model, which includes isospin-dependent symmetry energy, Coulomb energy, N - N cross sections and Pauli blocking. It is shown that there exists a transition from the isospin equilibrium to non-equilibrium as the incident energy from below to above a threshold energy in central, asymmetric heavy-ion collisions. Meanwhile, it is found that the phenomenon results from the co-existence and competition of different reaction mechanisms, namely, the isospin degree of freedom reaches an equilibrium if the incomplete fusion (ICF) component is dominant and does not reach equilibrium if the fragmentation component is dominant. Moreover, it is also found that the isospin-dependent N - N cross sections and symmetry energy are crucial for the equilibrium of the isospin degree of freedom in heavy-ion collisions around the Fermi energy.

The Role of Rotational Degrees of Freedom in Heavy-Ion Collisions

Following a major shortage of 99Mo in the 2009–2010 period, concern grew that the aging reactor production facilities needed to be replaced. Most producers were using highly enriched 235U (HEU) as the target material. The Organisation for Economic Co-...Read More

The Role of Rotational Degrees of Freedom in Heavy-Ion Collisions

The Role of Rotational Degrees of Freedom in Heavy-Ion Collisions

The role of rotational degrees of freedom in heavy ion collisions

The degrees of freedom affected by the angular momentum are identified. The relevance of the equilibrium fluctuations in a diffusive evolution of the system is discussed. The statistical limit is described and chosen as a reference for comparing with experiment. The rigid rotation regime is shown to be reached in a variety of reactions. The fragment spin alignment is measured from γ-ray multiplicities and anisotropies as well as from sequential fission angular distributions. Good agreement is obtained with the statistical model for the P zz component of the polarization tensor. The P xy component seems also to reach the statistical limit at large Q-value. The effect of shells on the angular momentum transferred to the fragments and on its misalignment is discussed theoretically and specific predictions are made.

Collective degrees of freedom and one-body dissipation in heavy ion collisions

We employ a three-parameter family of volume conserving nuclear shapes to couple relative motion and collective degrees of freedom in heavy ion collisions. Damping is achieved through the one-body dissipation mechanism. Results are compared with experimental data.