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In heavy-ion collisions, low relative-velocity two-particle correlations have been a tool for assessing space-time characteristics of particle emission. Those characteristics may be cast in the form of a relative emission source related to the correlation function through the Koonin-Pratt (KP) convolution formula that involves the relative wave-function for the particles in its kernel. In the literature, the source has been most commonly sought by parametrizing it in a Gaussian form and fitting to the correlation function. At times the source was more broadly imaged from the function, still employing a fitting. Here, we propose the use of the Richardson-Lucy (RL) optical deblurring algorithm for deducing the source from a correlation function. The RL algorithm originally follows from probabilistic Bayesian considerations and relies on the intensity distributions for the optical object and its image, as well as the convolution kernel, being positive definite, which is the case for the corresponding quantities of interest within the KP formula.

We systematically explore the relative impact of the Coulomb interaction and the nuclear symmetry energy on the proton and neutron density distributions in the 212Pb + 208Pb, 132Sn + 124Sn and 54Ca + 48Ca central collisions at beam energies below 800 MeV/A. The Boltzmann-Uhlenbeck-Uehling (pBUU) transport and the Time-Dependent-Hartree-Fock (TDHF) frameworks with SV-bas and SV-sym34 Skyrme parametrizations are employed. Maximum total particle number density and the proton-neutron (isospin) asymmetry have been calculated as a function of beam energy, system size and the Skyrme model, with and without the Coulomb force for all systems. The maximum total density, overall not exceeding 2.5-3.0ρ0 (ρ0=0.16fm−3), is observed to be lowered by the Coulomb interaction by less than 10%. The maximal asymmetries are not enhanced but decreased in the reaction as compared to the initial state in the majority of cases, and lowered by up to 45% due to the Coulomb effects. Furthermore, the proton and neutron density distributions in the plane transverse to the beam direction have been modelled. The distributions are found to vary throughout the reaction space, with the Coulomb force increasing the isospin differences closer to the center of the collision. The general conclusion of this work is that the Coulomb interaction plays a significant role in the collision dynamics, and enhances the rather weak response of the colliding systems to the nuclear symmetry energy and its density dependence. Thus, inferring symmetry energy from comparing theory and data requires careful modelling of Coulomb effects, in addition to nuclear, in any reaction simulation.

The binary neutron-star merger event, GW170817, has cast a new light on nuclear physics research. Using a neutron-star model that includes a crust equation of state (EoS), we calculate the properties of a 1.4 solar-mass neutron star. The model incorporates more than 200 Skyrme energy density functionals, which describe nuclear matter properties, in the outer liquid core region of the neutron star. We find a power-law relation between the neutron-star tidal deformability, Λ, and the neutron-star radius, R. Without an explicit crust EoS, the model predicts a smaller R and the difference becomes significant for stars with large radii. To connect the neutron star properties with nuclear matter properties, we confront the predicted values for Λ against the Taylor expansion coefficients of the Skyrme interactions. There is no pronounced correlation between Skyrme parameters relevant to symmetric nuclear matter and neutron star properties. However, we find the strongest correlation between Λ and Ksym, the curvature of the density dependence of the symmetry energy at saturation density. At twice the saturation density, our calculations show a strong correlation between Λ and total pressure, providing guidance to laboratory nucleus-nucleus collision experiments. Keywords: Equation of state, Neutron star merger, Skyrme interactions, Tidal polarizability

The LIGO-Virgo collaboration detection of the binary neutron-star merger event, GW170817, has expanded efforts to understand the Equation of State (EoS) of nuclear matter. These measurements provide new constraints on the overall pressure, but do not elucidate its origins, by not distinguishing the contribution to the pressure from symmetry energy which governs much of the internal structure of a neutron star. By combining the neutron star EoS extracted from the GW170817 event and the EoS of symmetric matter from nucleus-nucleus collision experiments, we extract the symmetry pressure, which is the difference in pressure between neutron and nuclear matter over the density region from 1.2ρ0 to 4.5ρ0. While the uncertainties in the symmetry pressure are large, they can be reduced with new experimental and astrophysical results. Keywords: Equation of state, Neutron star merger, Skyrme interactions, Tidal polarizability, Gravitational waves, Heavy ion collisions

In the past two decades, pions created in the high density regions of heavy ion collisions have been predicted to be sensitive at high densities to the symmetry energy term in the nuclear equation of state, a property that is key to our understanding of neutron stars. In a new experiment designed to study the symmetry energy, the multiplicities of negatively and positively charged pions have been measured with high accuracy for central 132Sn+124Sn, 112Sn+124Sn, and 108Sn+112Sn collisions at E/A=270 MeV with the SπRIT Time Projection Chamber. While individual pion multiplicities are measured to 4% accuracy, those of the charged pion multiplicity ratios are measured to 2% accuracy. We compare these data to predictions from seven major transport models. The calculations reproduce qualitatively the dependence of the multiplicities and their ratios on the total neutron and proton number in the colliding systems. However, the predictions of the transport models from different codes differ too much to allow extraction of reliable constraints on the symmetry energy from the data. This finding may explain previous contradictory conclusions on symmetry energy constraints obtained from pion data in Au+Au system. These new results call for still better understanding of the differences among transport codes, and new observables that are more sensitive to the density dependence of the symmetry energy.

Using transport theory to model central Au + Au collisions in the energy region of 20–110 MeV/u, at impact parameters b≤5 fm, we predict a measurable impact of the spinodal instability as the collective expansion sets in with energy. Two transport models are employed, the pBUU model, solving a Boltzmann-Uehling-Uhlenbeck equation, and the Brownian Motion (BM) model, solving a set of Langevin equations to describe the motion of individual nucleons in a noisy nuclear medium. We find without ambiguity, for the first time, that a combination of delayed equilibration, onset of collective expansion and the spinodal instability produces a pair of transient ring structures, made of the projectile and target remnants, with spectator nucleons predicted to end in the entities reminiscent of stones in jewelry, on the rings. The ring structures, calculated in the configuration space and mapped onto the velocity space, could be detected in experimental collective flow data.

Abstract: The interplay of the effects of geometry and collective motion on correlation functions is investigated for central collisions at . The data cannot be explained without collective motion, which could be partly along the beam axis. A semi-quantitative description of the data can be obtained using a Monte Carlo model, where thermal emission is superimposed on collective motion. Both the emission volume and the competition between the thermal and collective motion influence significantly the shape of the correlation function, motivating new strategies for extending intensity interferometry studies to massive particles. [Copyright &y& Elsevier]