HIR refer to collisions between large nuclei, e.g. one nucleus of gold colliding with another one of uranium. We study these type of collisions at intermediate energies, that is, when the speed of the moving nucleus is about 10% that of the speed of light. These collisions are performed, for instance, at the Texas A&M Cyclotron, at the National Superconducting Cyclotron Lab of Michigan State University, at Japan's Riken, etc; although these labs also study collisions at much higher speeds (a basic introduction to nuclear physics can be found at this page of the National Superconducting Cyclotron Lab).
UTEP's Nuclear Physics group has studied these type of reactions using several techniques, namely, statistical methods, transition state treatment, and a computational technique known as molecular dynamics. The main focus of the investigations have been on understanding the fragmentation of the nuclei during these collisions. These reactions are believed to produce a liquid-to-gas phase change, i.e. nuclei are drops of liquid like matter that -when heated in the collision- boil smaller droplets off much like water producing vapor bubbles.
Nuclei are small clusters of up to 300 nucleons (protons and neutrons) that behave as drops of liquid with the usual properties of matter, such as compressibility, heat capacity, etc. Being so small, however, many of these properties are difficult to extract from the reactions which are extremely brief (10-15 s) and it is instructive to study larger systems, which are generically known as "Nuclear Matter". Nuclear matter does not exist in our corner of the universe but it is known that there are stars composed of it.
UTEP's Nuclear Physics group studies nuclear matter using the computational technique known as molecular dynamics in which a large collection of "nucleons" are positioned in a large cube and are allowed to interact with one another through nuclear forces. Varying the density and temperature of the nuclear systems, it is possible to extract a lot of information such as how the binding energy or pressure depend on the density and temperature; such information is generically known as the "Nuclear Equation of State".
A specific application of these studies has focused on the structures that nuclear matter achieve at very low temperatures and low densities. These structures are generically known by the catchy name "Nuclear Pasta" due to their similarity with Italian delicacies such as spaghetti, lasagna, gnocchi, etc. In particular, these structures are believed to exist on Neutron Star Crusts; the 2012 study "Topological characterization of neutron star crusts was selected to appear in Physics-Spotlighting Exceptional research: Italian Delicacies Served Up in a Neutron Star Crust.
Nowadays it is possible to produce two nuclear reactions in sequence, that is, one reaction in which a projectile nucleus is broken into a smaller nucleus, which is then accelerated as a secondary projectile for a second collision. The advantage of this is that the second projectile is not a 'normal' nucleus in the sense of being a stable nucleus as those found in nature, but one with more neutrons than protons which are unstable; these nuclei are knows as "Rare Isotopes". These type of reactions are currently being performed in several labs around the world, and in the USA a new dedicated facility, Facility for Rare Isotope Beams, is under construction at Michigan State U.
A topic of interest in neutron-rich nuclei is how these nuclei compare to "normal" ones. For this purpose reactions both with rare isotopes are performed and compared to reactions carried out with normal nuclei. Such comparison yielded a power law known as Isoscaling. Thanks to one of our isoscaling studies, "Probabilistic aspects of isoscaling", J.A. Muñoz and J.A. López received the Hyer Award to from the Texas Section of the American Physical Society in San Marcos meeting in October 23, 2009, TX. for Research with undergraduate student. See information here.
The variation of the nuclear forces with the excess neutrons is believed to be through a component of the nuclear force known as the "Nuclear Symmetry Energy"; this was the focus of the article "Isospin-asymmetric nuclear matter", by J.A. López, E. Ramírez-Homs, R. González and R. Ravelo, Phys. Rev. C 89, 024611 (2014); see preprint here.