The Road to Precision: Extraction of the Specific Shear Viscosity of the Quark-Gluon Plasma

For a few fleeting moments after the Big Bang the universe was filled with an astonishingly hot and dense soup known as the Quark-Gluon Plasma (QGP), a precursor to the matter we observe today that consisted of elementary particles. While this Quark-Gluon Plasma also contained leptons and weak gauge bosons, its transport properties were dominated by the strong interaction between quarks and gluons. Utilizing the most powerful particle accelerators, physicists now conduct head-on collisions between heavy ions, such as gold or lead nuclei, to recreate conditions that existed at the birth of the universe. The most striking discovery in relativistic heavy-ion collisions is that the hot and dense matter created during the collisions behaves like an almost perfect (inviscid) liquid, meaning that it can be characterized by a very small shear viscosity (η) to entropy density (s) ratio (or, “specific shear viscosity”). It turns out that the η/s for the QGP is smaller than that of any known substance, including that of superfluid liquid helium. In Figure 1, we highlight schematically the specific shear viscosity η/s normalized by 1/4π ħkB, the minimum bound in a large class of theories with infinitely strong coupling [5], for four different types of fluids. The QGP at high temperature reflects the smallest value of η/s of any fluids occurring in nature.