Speaker
Description
The observation of long-range azimuthal anisotropies in small collision systems triggers pressing questions concerning the applicability of fluid dynamics and whether ensembles of only a small number of particles may be effectively treated as fluids. We propose to use experiments on expanding clouds of cold atoms to answer the latter question. We consider strongly-interacting fermions confined in a two-dimensional trap with an elliptical deformation, and, upon release of the trap, we use the emergence of elliptic flow, $v_2$, as an indicator of collective behavior. Borrowing a well-known method from heavy-ion collisions, we quantify the collectivity of the system through the ratio $v_2/\varepsilon_2$, where $\varepsilon_2$ is the anisotropy of the trapping potential. By experimentally controlling both the value of $\varepsilon_2$ and the number of trapped atoms (down to a single particle), one can investigate what particle number is needed for the system to yield a $v_2/\varepsilon_2$ as large as in the fluid dynamic regime. Interestingly, in a quantum formulation, we find that a small number of particles can show elliptic flow even in the absence of interactions, and we discuss how this quantum effect disappears quickly with increasing particle number.
Based on: S. Floerchinger, G. Giacalone, L. H. Heyen, L. Tharwat, https://arxiv.org/pdf/2111.13591.pdf
