University of Minnesota
School of Physics & Astronomy

Spotlight

Heavy Ion Collisions

Clint Young
Clint Young
Annie Bartels
                                                       

Clint Young is a Research Associate at the School of Physics and Astronomy who studies ultrarelativistic heavy ion collisions. In these experiments, atomic nuclei are accelerated to speeds exceeding 99% of the speed of light and then collide.

The facilities for making and examining these collisions are huge; the most recent examples are Brookhaven’s Relativistic Heavy Ion Collider and the Large Hadron Collider at CERN. "Although the heavy ion program at the Large Hadron Collider sounds similar to the particle physics experiments there as well, the experiments have very different goals." The heavy ion collisions create the hottest man-made substance in existence: about 5*1012 Kelvin, 300,000 times hotter than the center of the sun. By reaching these temperatures, heavy-ion collisions recreate conditions in the early universe only about 1 microsecond after the Big Bang.

Quantum Chromo Dynamics (QCD) is the fundamental theory of nuclear physics. At its heart is the property that constituents of protons and neutrons, called quarks, carry not one but three charges known as “colors.” “There are certain properties that we don’t fully understand, such as confinement: the phenomenon that color charged particles (such as quarks) cannot be observed in isolation, and in fact all stable particles interacting with the nuclear force are composite, color-neutral objects.” Numerical simulations of lattice QCD verify the property of confinement and also predict that at high temperatures, the quarks and gluons of QCD behave as if they are nearly free, in a fluid known as quark-gluon plasma. Heavy-ion collisions give physicists the opportunity to study this material, both verifying the results of lattice QCD calculations as well as studying the transport coefficients of this material that are not otherwise easily determined. One of these results is quite surprising: that the quark-gluon plasma produced in these experiments is a “nearly-perfect fluid”, with viscosities about as low as the proposed theoretical lower limit.

Much of Young’s work focuses on penetrating probes: photons, dileptons, heavy quarks, and quarkonia which are produced in numbers and at momenta in heavy-ion collisions that differ from what would be naively expected from simple proton collisions. “Electromagnetic probes give literally an X-ray image of these events, however an X-ray that is overexposed of a moving object. Still, a lot has already been learned from these probes and some unexpected results have raised as many questions as have been answered.”

Recently, Young has been working with Joseph Kapusta, a professor of the School of Physics and Astronomy, in studying how hydrodynamics, a technique ordinarily reserved for large systems, is being pushed to its limits in describing heavy-ion collisions. “Hydrodynamical descriptions, in order to be accurate, have often relied on a few conditions to be true, among them, that the system is sufficiently large, and that the system is far from a critical point so that scale-invariant fluctuations do not dominate the system’s dynamics. Hydrodynamics has successfully described the particle yields in heavy-ion collisions and yet some heavy-ion collisions might also break both of these conditions.” Thermal noise becomes important for explaining the dynamics of sufficiently small samples of viscous fluid. “Far from being a nuisance, the thermal noise in heavy-ion collisions is closely related to the fluid’s transport coefficients and offers the chance to make independent measurements of these quantities. Heavy ion experiments, while having the original goal of recreating a phase transition in the early universe, have become important for understanding the dynamics of quarks and gluons.”