Hosted by: Jin Huang

In recent years, there been rapid progresses in our understanding of the long-range ridge in small collision system at RHIC and LHC. I will discuss the nature of collectivity (flow) driving the ridge, as well as the dominating non-collective (or non-flow) background that complicates the extraction of the ridge. I shows that the standard multi-particle cumulant method, often used to defined collectivity in heavy ion collisions, is overwhelmed by non-collective background in pp and low multiplicity pPb collisions. This problem is resolved with an alternative method based on two or more subevents separated in pseudorapidity (η), and therefore offers a robust data-driven definition of collectivity based on the existence of long-range azimuthal correlations between multiple distinct η ranges. With this new cumulant method, we are able to probe reliably the event-by-event fluctuation of collectivity in small collision systems.