In this talk I will review the recent developments on the phenomenology and experimental searches for collectivity in small collision systems. For the phenomenology part I will focus on the hybrid approach based on the Color Glass Condensate (CGC) and Hydrodynamics (hydro) simulation . For the experimental part my focus will be on the RHIC small system scan program . I will discuss how the RHIC results indicate a dominant role of hydrodynamic final state effects on the observability of collectivity in small collision systems. However, the sub-dominant role of CGC driven correlations with an effect increasing with decreasing multiplicity is expected based on hybrid CGC+hydro calculations. I’ll briefly touch on an outstanding puzzle related to the measurements of triangular anisotropy coefficients from RHIC small system scan program.
Where do we go from here ? Several promising directions have been identified. STAR with its extended pseudorapidity acceptance has just collected data on Oxygen-Oxygen collisions at RHIC. This exploratory program promises a better understanding of the role of geometry that drives collectivity in small systems. Recently, hints of collectivity has been observed in photo-nuclear processes by studying ultra-peripheral Pb+Pb collisions . Interestingly, similar processes can be studied in photo-production limits of DIS e+p/A collisions at the future Electron Ion Collider.
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