
 Nuclear astrophysics is a field at the intersection of nuclear physics and astrophysics, which seeks to understand the nuclear engines of astronomical objects and the origin of the chemical elements. This white paper summarizes progress and status of the field, the new open questions that have emerged, and the tremendous scientific opportunities that have opened up with major advances in capabilities across an ever growing number of disciplines and subfields that need to be integrated. We take a holistic view of the field discussing the unique challenges and opportunities in nuclear astrophysics in regards to science, diversity, education, and the interdisciplinarity and breadth of the field. Clearly nuclear astrophysics is a dynamic field with a bright future that is entering a new era of discovery opportunities.

Horizons: nuclear astrophysics in the 2020s and beyond

Hanbury-Brown-Twiss (HBT) correlations for charged pions in central Au+Au collisions at $\sqrt{s_\mathrm{NN}}=2.4 - 7.7~\text{GeV}$ (corresponding to beam kinetic energies in the fixed target frame from $E_{\rm{lab}}=1.23~\text{to}~30~\text{GeV/nucleon}$) are calculated using the UrQMD model with different equations of state. The effects of a phase transition at high baryon densities are clearly observed in the HBT parameters that are explored. It is found that the available data on the HBT radii, $R_{O}/R_{S}$ and $R^{2}_{O}-R^{2}_{S}$, in the investigated energy region favors a relatively stiff equation of state at low beam energies which then turns into a soft equation of state at high collision energies consistent with astrophysical constraints on the high density equation of state of QCD. The specific effects of two different phase transition scenarios on the $R_{O}/R_{S}$ and $R^{2}_{O}-R^{2}_{S}$ are investigated. It is found that a phase transition with a significant softening of the equation of state below 4 times nuclear saturation density can be excluded using HBT data. Our results highlight that the pion's $R_{O}/R_{S}$ and $R^{2}_{O}-R^{2}_{S}$ are sensitive to the stiffness of the equation of state, and can be used to constrain and understand the QCD equation of state in the high baryon density region. 

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Effects of a phase transition on two-pion interferometry in heavy ion collisions at 
              
                
              
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To explain the production of light nuclei in heavy-ion collisions at extreme energies, we focus on the deuteron case. A Gibbs ensemble at chemical freeze-out is a prerequisite to investigate the non-equilibrium evolution of the expanding fireball. Quantum statistical approaches allow to describe correlations including bound state formation in the strongly interacting and hot system. We consider the virial approach to evaluate proton-neutron correlations. In generalization of the treatment of protons in pionic matter (pion-proton puzzle), the influence of the pion environment on deuteron-like correlations is evaluated using data for the pion-deuteron scattering phase shifts. Calculated yields for deuteron production are compared with the ones observed at the LHC. 

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Deuteron yields from heavy-ion collisions at energies available at the CERN Large Hadron Collider: Continuum correlations and in-medium effects

Recent beam energy scan experiments at the BNL Relativistic Heavy Ion Collider by the STAR Collaboration [Phys. Lett. B 827, 137003 (2022) and Phys. Rev. Lett. 128, 202303 (2022)] found that hadronic interactions dominate the collective flow and the proton cumulant ratios are driven by baryon number conservation in a region of high baryon density in $\sqrt{{s}_{NN}}=3$ GeV $\mathrm{Au}+\mathrm{Au}$ reactions, indicating that the dense medium formed in such collisions is likely hadronic matter. Within an updated a relativistic transport model with momentum dependent isoscalar and isovector single-nucleon mean-field potentials corresponding to different symmetry energies at suprasaturation densities, the $n/p, {\ensuremath{\pi}}^{\ensuremath{-}}/{\ensuremath{\pi}}^{+}, {K}_{s}^{0}/{K}^{+}, {\mathrm{\ensuremath{\Sigma}}}^{\ensuremath{-}}/{\mathrm{\ensuremath{\Sigma}}}^{+}$, and ${\mathrm{\ensuremath{\Xi}}}^{\ensuremath{-}}/{\mathrm{\ensuremath{\Xi}}}^{0}$ ratios are studied for central $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{{s}_{NN}}=3$ GeV, where the maximum central density reaches about $(3.6--4.0){\ensuremath{\rho}}_{0}$. The doubly strange ${\mathrm{\ensuremath{\Xi}}}^{\ensuremath{-}}/{\mathrm{\ensuremath{\Xi}}}^{0}$ ratio is found to have the strongest sensitivity to the variation of high-density nuclear symmetry energy. Thus, the ${\mathrm{\ensuremath{\Xi}}}^{\ensuremath{-}}/{\mathrm{\ensuremath{\Xi}}}^{0}$ ratio in relativistic heavy-ion reactions at $\sqrt{{s}_{NN}}\ensuremath{\sim}3$ GeV may help probe sensitively the poorly known symmetry energy of dense neutron-rich matter critically important for understanding various properties of neutron stars. 

/pdf/probing-the-high-density-nuclear-symmetry-energy-with-the-2qwk0ofx.pdf

Probing the high-density nuclear symmetry energy with the 
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http://manuscript.elsevier.com/S0146641022000230/pdf/S0146641022000230.pdf

Institute of Technology

Papers

Transport model comparison studies of intermediate-energy HI collisions