Observation of Universal Expansion Anisotropy from Cold Atoms to Hot Quark-Gluon Plasma

Azimuthal anisotropy has been ubiquitously observed in high-energy proton-proton, proton-nucleus, and nucleus-nucleus (heavy-ion) collisions, shaking the early belief that those anisotropies must stem from utterly strong interactions. This work reports a study of anisotropic expansion of cold ^6Li Fermi gases, initially trapped in an anisotropic potential, as a function of the interaction strength that can be readily tuned by an external magnetic field. It is found that the expansion anisotropy builds up quickly at small interaction strength, without the need of utterly strong interactions. A universal behavior of the expansion anisotropy is quantitatively observed between cold-atom and heavy-ion systems, despite their vast differences in physics. This universality will potentially unify a variety of disciplines in nature, from the weakly interacting dilute systems of gases to the strongly interacting quark-gluon plasma of the early universe.