Smallest QCD Droplet and Flow Fluctuations

by Seyed Farid Taghavi (Technische Universitaet Muenchen (DE))

Thursday 19 Sept 2019, 11:00 → 12:00 Europe/Zurich

4/2-037 - TH meeting room (CERN)

According to the standard picture of heavy ion collision, the final momentum distribution is a manifestation of the initial energy density after the collective evolution.  This picture has been confirmed for large systems over the past years. However, peculiar flow-like signals in the small systems have been observed at LHC and RHIC recently. This observation brings us to the ongoing debate about the smallest system size for collective evolution. In this talk, we first discuss the cumulants of multi-dimensional flow fluctuations and non-Bessel-Gaussianity in one-dimensional flow distributions that are applicable for large and small systems. After that by employing Gubser flow, we show that the existence of a consistent hydrodynamic solution translates the initiation time of hydrodynamic evolution to a lower bound on the system size. We then introduce a simple and rather generic model for the initial state fluctuation, and together with Gubser flow, we explain the observed flow harmonics for pp collisions by LHC. We also discuss bounds on multiplicity at which hydrodynamics is relevant and has a clear signal in the experiment.