Soft nuclear equation-of-state from heavy-ion data and implications for compact stars

Measurements of kaon production at subthreshold energies in heavy-ion collisions point to a soft nuclear equation-of-state for densities up to 2-3 times nuclear matter saturation density. We apply these results to study the implications on compact star properties, especially in the context of the recent measurement of the two solar mass pulsar PSR J1614-2230. The implications are two-fold: Firstly, the heavy-ion results constrain nuclear matter at densities relevant to light neutron stars. Hence, a radius measurement could provide information about the density dependence of the symmetry energy which is a crucial quantity in nuclear physics. Secondly, the information on the nucleon potential obtained from the analysis of the heavy-ion data can be combined with restrictions from causality on the nuclear equation-of-state. From this we can derive a limit for the highest allowed compact star mass of three solar masses.

💡 Research Summary

The paper investigates the implications of a soft nuclear equation of state (EoS) inferred from sub‑threshold kaon (K⁺) production in heavy‑ion collisions measured by the KaoS spectrometer at GSI. Transport‑model analyses (IQMD and RQMD) of Au+Au and C+C collisions show that the observed K⁺ multiplicity ratios can only be reproduced if the nucleon potential is relatively attractive, corresponding to a compressibility (stiffness parameter) K≈200 MeV for isospin‑symmetric matter at densities of 2–3 n₀ (where n₀≈0.17 fm⁻³ is nuclear saturation density). This “soft” EoS is distinct from the compression modulus K₀≈235 MeV that characterizes matter exactly at saturation.

The authors first explore how this experimentally constrained soft EoS influences the structure of low‑mass neutron stars (≈1.1–1.6 M☉). Using a Skyrme‑type parametrization, they vary the symmetry‑energy magnitude S₀ (28–32 MeV) and its density dependence (power‑law exponent γ=0.5–1.1). For a fixed K≈200 MeV, the resulting stellar radii and moments of inertia are highly sensitive to the slope parameter L=3n₀(dS/dn)ₙ₀. For a 1.25 M☉ star, the radius can differ by up to ~1.5 km and the moment of inertia by ~2.5×10⁴³ g cm² between stiff (large L) and soft (small L) symmetry‑energy scenarios. Because stars of this mass have central densities ≤3 n₀, their interiors are fully described by the KaoS‑validated EoS. Consequently, precise radius measurements (e.g., with the proposed LOFT X‑ray timing mission, aiming at ~5 % accuracy) could directly discriminate between different symmetry‑energy behaviors, providing a valuable cross‑check between nuclear‑physics experiments and astrophysical observations.

In the second part, the paper addresses the maximum possible neutron‑star mass. Following the method of Ref.