Neutron stars—compact objects with masses similar to that of our Sun but radii comparable to the size of a city—contain the densest form of matter in the universe that can be probed in terrestrial laboratories as well as in earth- and space-based observatories. The historical detection of gravitational waves from a binary neutron star merger has opened the new era of multimessenger astronomy and has propelled neutron stars to the center of a variety of disciplines, such as astrophysics, general relativity, nuclear physics, and particle physics. The main input required to study the structure of neutron stars is the pressure support generated by its constituents against gravitational collapse. These include neutrons, protons, electrons, and perhaps even more exotic constituents. As such, nuclear physics plays a prominent role in elucidating the fascinating structure, dynamics, and composition of neutron stars.
The Nuclear Physics of Neutron Stars
Nucleon Clustering in Light Nuclei
The ability to model the nature of the strong interaction at the nuclear scale using ab initio approaches and the development of high-performance computing is allowing a greater understanding of the details of the structure of light nuclei. The nature of the nucleon–nucleon interaction is such that it promotes the creation of clusters, mainly α-particles, inside the nuclear medium. The emergence of these clusters and understanding the resultant structures they create has been a long-standing area of study. At low excitation energies, close to the ground state, there is a strong connection between symmetries associated with mean-field, single-particle behavior and the geometric arrangement of the clusters, while at higher excitation energies, when the cluster decay threshold is reached, there is a transition to a more gas-like cluster behavior. State-of-the-art calculations now guide the thinking in these two regimes, but there are some key underpinning principles that they reflect. Building from the simple ideas to the state of the art, a thread is created by which the more complex calculations have a foundation, developing a description of the evolution of clustering from α-particle to 16O clusters.
Progress in Gamma Detection for Basic Nuclear Science and Applications
J. Simpson and A. J. Boston
The atomic nucleus, consisting of protons and neutrons, is a unique strongly interacting quantum mechanical system that makes up 99.9% of all visible matter. From the inception of gamma-ray detectors to the early 21st century, advances in gamma detection have allowed researchers to broaden their understanding of the fundamental properties of all nuclei and their interactions. Key technical advances have enabled the development of state-of-the art instruments that are expected to address a wide range of nuclear science at the extremes of the nuclear landscape, excitation energy, spin, stability, and mass. The realisation of efficient gamma detection systems has impact in many applications such as medical imaging environmental radiation monitoring, and security. Even though the technical advances made so far are remarkable, further improvements are continually being implemented or planned.
Role of Quarks in Nuclear Structure
A. W. Thomas
The strong force that binds atomic nuclei is governed by the rules of Quantum Chromodynamics. Here we consider the suggestion the internal quark structure of a nucleon will adjust self-consistently to the local mean scalar field in a nuclear medium and that this may play a profound role in nuclear structure. We show that one can derive an energy density functional based on this idea, which successfully describes the properties of atomic nuclei across the periodic table in terms of a small number of physically motivated parameters. Because this approach amounts to a new paradigm for nuclear theory, it is vital to find ways to test it experimentally and we review a number of the most promising possibilities.
The Partonic Content of Nucleons and Nuclei
Deepening our knowledge of the partonic content of nucleons and nuclei represents a central endeavor of modern high-energy and nuclear physics, with ramifications in related disciplines, such as astroparticle physics. There are two main scientific drivers motivating these investigations of the partonic structure of hadrons. On the one hand, addressing fundamental open issues in our understanding of the strong interaction, such as the origin of the nucleon mass, spin, and transverse structure; the presence of heavy quarks in the nucleon wave function; and the possible onset of novel gluon-dominated dynamical regimes. On the other hand, pinning down with the highest possible precision the substructure of nucleons and nuclei is a central component for theoretical predictions in a wide range of experiments, from proton and heavy-ion collisions at the Large Hadron Collider to ultra-high-energy neutrino interactions at neutrino telescopes.