Researchers Tsuyoshi Miyatsu, Myung-Ki Cheoun, Kyungsik Kim, and Koichi Saito from the Department of Physics at the University of Tokyo have published a study that delves into the nuclear equation of state (EoS) for matter with unequal numbers of neutrons and protons, known as isospin-asymmetric matter. Their work, titled “Constraining the nuclear equation of state from terrestrial experiments and neutron star observations using relativistic mean-field models,” was published in the journal Physical Review C.
The nuclear equation of state describes the relationship between various properties of nuclear matter, such as density, pressure, temperature, and energy. Understanding this relationship is crucial for both nuclear physics and astrophysics, particularly in the study of neutron stars, which are incredibly dense remnants of massive stars. The researchers focused on a new set of interactions within a framework called relativistic mean-field (RMF) models, specifically the OMEG family, which incorporates certain mixings of nuclear forces.
The OMEG family of interactions is designed to reproduce both terrestrial nuclear measurements and astrophysical constraints. Terrestrial measurements include data from nuclear experiments conducted on Earth, while astrophysical constraints come from observations of neutron stars, such as those from the Neutron star Interior Composition Explorer (NICER) and the gravitational wave event GW170817. The researchers found that the mixing of certain nuclear forces, specifically the sigma and delta mesons (σ-δ mixing), softens the nuclear symmetry energy and pressure at densities around twice that of normal nuclear matter.
This softening allows for relatively small neutron-star radii and tidal deformabilities, which are measures of how easily a neutron star can be deformed by an external gravitational field. At the same time, the nuclear EoS remains stiff enough at higher densities to support neutron stars with masses up to twice that of the Sun (2M⊙). The study highlights the importance of the curvature parameter, Ksym, in achieving this soft-to-hard behavior of the nuclear EoS. Astrophysical data favor small or even negative values of Ksym, which has implications for our understanding of neutron star structure and the behavior of nuclear matter under extreme conditions.
For the energy sector, particularly in nuclear energy, understanding the nuclear equation of state is fundamental. It helps in modeling nuclear reactions and processes, which can inform the development of advanced nuclear reactors and fusion energy technologies. The insights gained from this research can contribute to the design of safer, more efficient nuclear power systems and the exploration of alternative energy sources.
This article is based on research available at arXiv.