Nuclear physics research is focused on understanding the matter composed of quarks and gluons, which makes up 99% of the mass of the universe. Most of this matter is found at the core of atoms, the same atoms that comprise all we see around us (including ourselves). Researchers seek to answer questions such as how the universe evolved just after the Big Bang from a super-hot plasma of quarks and gluons, how the different elements of the universe were formed, and how a nucleus is made up of individual protons and neutrons interacting with each other with the strongest force in Nature. The protons and neutrons themselves are the basic bound states of quarks in the universe; how these states are formed from quarks interacting with the gluonic field described by Quantum Chromodynamics is still only poorly understood, and under active study.
Modern experimental research in this field uses high-energy acceleration of both protons and large nuclei, while much of modern theoretical research relies on high powered computational facilities to understand data and make detailed predictions. The University of Colorado has active groups in both theoretical and experimental research in nuclear physics.
Physicists in this fi eld explore the nature of the strong force by studying the theory of Quantum Chromodynamics. Unlike the quantum theory of electromagnetism, Quantum Chromodynamics has the property that the fundamental particles (quarks and gluons) interact more and more weakly when probed at higher and higher energy scales or temperatures. This property of the theory is called asymptotic freedom.
One interesting consequence of asymptotic freedom is that at some temper- ature, the interaction should be so weak that the fundamental particles no longer are bound (con ned) inside ordinary nuclei. Using state-of-the art computer simulations of Quantum Chromodynamics at nite temperature, it is possible to calculate this de-con nement temperature to be T≈170 MeV, or about 2 trillion Kelvin. Above this temperature, matter is in a new phase of matter, called the quark-gluon plasma. The properties of this quark-gluon plasma are currently investigated using experiments at the Relativistic Heavy-Ion Collider (RHIC) and the Large Hadron Collider (LHC).
The experimental data from RHIC and the LHC strongly indicate that the quark-gluon plasma is an exceptionally good liquid, with a very small viscosity. This motivates the theoretical study of the experimental results using fluid dynamic simulations. Since the energy involved in these experiments is very large, the fluid constituents are moving almost at the speed of light, making it necessary to use a fully relativistic version of hydrodynamics.
Very high densities are similar to very high temperatures in the sense that the interaction of Quantum Chromodynamic becomes weak. The centers of neutron stars are expected to reach the highest particle densities in the universe, so it is possible that neutron stars harbor quark matter in their inner cores. Studying the properties of neutron stars and comparing to observational data is also one of the research subject of theoretical nuclear physics.
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