Nuclear Physics and Astrophysics Constraints on the High Density Matter Equation of State

Status and Perspectives of Continuous Gravitational Wave Searches

The birth of gravitational wave astronomy was triggered by the first detection of a signal produced by the merger of two compact objects (also known as a compact binary coalescence event). The following detections made by the Earth-based network of advanced interferometers had a significant impact in many fields of science: astrophysics, cosmology, nuclear physics and fundamental physics. However, compact binary coalescence signals are not the only type of gravitational waves potentially detectable by LIGO, Virgo, and KAGRA. An interesting family of still undetected signals, and the ones that are considered in this review, are the so-called continuous waves, paradigmatically exemplified by the gravitational radiation emitted by galactic, fast-spinning isolated neutron stars with a certain degree of asymmetry in their mass distribution. In this work, I will review the status and the latest results from the analyses of advanced detector data.

Translating Neutron Star Observations to Nuclear Symmetry Energy via Deep Neural Networks

One of the most significant challenges involved in efforts to understand the equation of state of dense neutron-rich matter is the uncertain density dependence of the nuclear symmetry energy. In particular, the nuclear symmetry energy is still rather poorly constrained, especially at high densities. On the other hand, detailed knowledge of the equation of state is critical for our understanding of many important phenomena in the nuclear terrestrial laboratories and the cosmos. Because of its broad impact, pinning down the density dependence of the nuclear symmetry energy has been a long-standing goal of both nuclear physics and astrophysics. Recent observations of neutron stars, in both electromagnetic and gravitational-wave spectra, have already constrained significantly the nuclear symmetry energy at high densities. The next generation of telescopes and gravitational-wave observatories will provide an unprecedented wealth of detailed observations of neutron stars, which will improve further our knowledge of the density dependence of nuclear symmetry energy, and the underlying equation of state of dense neutron-rich matter. Training deep neural networks to learn a computationally efficient representation of the mapping between astrophysical observables of neutron stars, such as masses, radii, and tidal deformabilities, and the nuclear symmetry energy allows its density dependence to be determined reliably and accurately. In this work, we use a deep learning approach to determine the nuclear symmetry energy as a function of density directly from observational neutron star data. We show, for the first time, that artificial neural networks can precisely reconstruct the nuclear symmetry energy from a set of available neutron star observables, such as masses and radii as measured by, e.g., the NICER mission, or masses and tidal deformabilities as measured by the LIGO/VIRGO/KAGRA gravitational-wave detectors. These results demonstrate the potential of artificial neural networks to reconstruct the symmetry energy and the equation of state directly from neutron star observational data, and emphasize the importance of the deep learning approach in the era of multi-messenger astrophysics.

Exploring the role of hyperons in high density matter in the Quark-Meson-Coupling model

Microscopic composition and properties of matter at super-saturation densities have been a subject of intense investigations for decades. Experimental and observational data and fundamental laws of physics indicate that heavy strange baryons and mesons are essential components of the matter. The Quark-Meson-Coupling-Model (QMC) is well suited for such a study. The model is based on interaction between quarks in individual baryons instead between the baryons as entities without internal structure. This approach significantly increases transparency and reduces the number of variable parameters of the model, thus offering deeper insight into the physics of high density hadronic matter. In this contribution, we review the effect of hyperons on neutron star (NS) properties, the speed of sound and the symmetry energy, both at zero and finite temperature. The QMC results are contrasted with the outcome of the traditional relativistic mean field DD2Y model. The (lack of) the so-called ‘hyperon puzzle’ in both models is discussed.