The science of Nuclear Physics concerns itself with the properties of "nuclear" matter. Such matter constitutes the massive centers of the atoms that account for 99.9 % of the world we see around us. Nuclear matter is within the proton and neutron building blocks of these nuclei, and appears in bulk form in neutron stars and in the matter that arose in the Big Bang. Nuclear physicists study the structure and properties of such matter in its various forms, from the soup of quarks and gluons present at the birth of our universe to the nuclear reactions in our Sun that make life possible on Earth.
Nuclear physics both contributes to and benefits from other fields—for instance, from atomic physics for intricate table-top experiments, to high energy physics for hall-size collider detectors. High Energy Physics is concerned with the elementary particles and their interactions; it is the goal of nuclear physics to understand and explain why and how these particles, through their interactions, group themselves together to form matter.
Nuclear physics has expanded in recent years. It now encompasses topics formerly considered the domain of particle physics, including exotic mesons, multi-GeV reaction studies, and the quark-gluon plasma. Even the more traditional nuclear structure studies now explore regions of nuclei and nuclear excitation, angular momenta, and stability that were not previously accessible, because of the advent of more advanced detectors and accelerators.
Traditional nuclear physics describes nuclear properties in terms of protons and neutrons. Most nuclei are well described as nucleons interacting either through empirical interactions in the shell model, or through a force derived from nucleon-nucleon scattering.
Modern nuclear physics has much greater ambitions. New insights are gained into conventional descriptions by studying nuclei with much larger neutron, or proton excesses, and by investigating nuclei with high spin. Reactions with intermediate-energy mesons and electromagnetic probes provide insight into the quark substructure of the individual composite particles, the high momentum structure of nuclei, and the transition between current-quark, constituent-quark, and meson-baryon degrees of freedom. Relativistic-heavy-ion experiments are recreating the quark-gluon plasma, not present since the first seconds after the creation of the universe many billions of years ago.