[Paper Review] Probing neutron-star matter in the lab: similarities and differences between binary mergers and heavy-ion collisions

Elias R. Most, Anton Motornenko+5 more|arXiv (Cornell University)|Jan 31, 2022

Pulsars and Gravitational Waves Research4 citations

TL;DR

This paper performs a direct numerical comparison of thermodynamic conditions in low-energy heavy-ion collisions (HICs) and binary neutron-star mergers (BNSMs) using a finite-temperature equation of state. It finds that HICs with beam energies of 450–600 MeV per nucleon probe thermodynamic states—specifically entropy per baryon S/A ≈ 1.8–2.2—very similar to those in BNSMs with total masses of 2.6–2.8 M⊙, suggesting HICs can effectively simulate BNSM-like hot and dense matter in the lab.

ABSTRACT

Binary neutron-star mergers and heavy-ion collisions are related through the properties of the hot and dense nuclear matter formed during these extreme events. In particular, low-energy heavy-ion collisions offer exciting prospects to recreate such {extreme} conditions in the laboratory. However, it remains unexplored to what degree those collisions can actually reproduce hot and dense matter formed in binary neutron star mergers. As a way to understand similarities and differences between these systems, we {discuss their geometry and }perform a direct numerical comparison of the thermodynamic conditions probed in both collisions. To enable a direct comparison, we employ a finite-temperature equation of state able to describe the entire high-energy phase diagram of Quantum Chromodynamics. Putting side by side the evolution of both systems, we find that laboratory heavy-ion collisions at the energy range of $E_{\mathrm{lab}}=0.4 - 0.6\ A$ MeV probe (thermodynamic) states of matter that are very similar to those created in binary neutron-star mergers. These results can inform future low-energy heavy-ion collisions probing this regime.

Motivation & Objective

• To determine to what extent low-energy heavy-ion collisions can reproduce the thermodynamic conditions of hot and dense matter formed in binary neutron-star mergers.
• To establish a direct, quantitative comparison between HICs and BNSMs using a common equation of state and numerical framework.
• To identify the beam energy range in HICs that matches the entropy per baryon and density-temperature trajectories observed in BNSMs.
• To assess the validity of using HICs as a laboratory probe for neutron-star matter, particularly in the context of future gravitational wave and heavy-ion experiments.

Proposed method

• Numerical simulations of both low-energy HICs and BNSMs are performed using the same relativistic hydrodynamics framework and a finite-temperature equation of state covering the full QCD phase diagram.
• The comparison focuses on local thermodynamic conditions—specifically temperature, baryon density, and entropy per baryon—rather than global flow or geometry.
• Shock heating at the initial impact is used as a figure of merit to equate the thermodynamic evolution of both systems.
• A mapping is constructed between the total gravitational mass of BNSMs (2.6–2.8 M⊙) and the beam energy per nucleon in HICs (450–600 MeV/A) via matching entropy per baryon (S/A ≈ 1.8–2.2).
• The study uses a single, consistent equation of state to ensure comparability across both systems, minimizing model dependence.
• The analysis accounts for differences in isospin asymmetry (Y_iso ≈ -0.1 in HICs vs. -0.4 to -0.5 in neutron-star matter), but shows that thermodynamic similarity persists despite this.

Experimental results

Research questions

• RQ1 Can low-energy heavy-ion collisions at beam energies of 450–600 MeV/A reproduce the thermodynamic conditions observed in binary neutron-star mergers?
• RQ2 What is the quantitative mapping between the total mass of merging neutron stars and the beam energy in heavy-ion collisions that yields comparable entropy per baryon?
• RQ3 How similar are the temperature, density, and entropy trajectories in HICs and BNSMs when using the same equation of state and numerical methods?
• RQ4 To what extent do differences in isospin asymmetry and system size affect the comparability of thermodynamic states in HICs and BNSMs?
• RQ5 Can HICs serve as a viable laboratory probe for the hot and dense matter formed in neutron-star mergers, particularly in light of future gravitational wave and heavy-ion data?

Key findings

• HICs with beam energies of 450–600 MeV per nucleon produce thermodynamic conditions—specifically entropy per baryon S/A ≈ 1.8–2.2—that closely match those in binary neutron-star mergers with total masses of 2.6–2.8 M⊙.
• The comparison reveals that despite differences in isospin asymmetry and system size, the thermodynamic evolution of HICs and BNSMs follows remarkably similar trajectories in the QCD phase diagram.
• The study establishes a direct mapping between BNSM total mass and HIC beam energy, providing a benchmark for future low-energy HIC experiments such as those at the HADES facility.
• The results suggest that low-energy HICs can probe the same regime of hot and dense matter as BNSMs, offering a complementary path to study the QCD phase diagram.
• The findings are robust against uncertainties in viscous effects and cold equation of state, though future studies incorporating viscosity in both systems would improve precision.
• The work supports the use of HICs as a laboratory probe for neutron-star matter, particularly in conjunction with multi-messenger data from gravitational wave detectors.