Novel collective phenomena in high-energy proton–proton and proton–nucleus collisions

The observation of long-range rapidity correlations among particles in high-multiplicity p–p and p–Pb collisions has created new opportunities for investigating novel high-density QCD phenomena in small colliding systems. We review experimental results related to the study of collective phenomena in small systems at RHIC and the LHC along with the related developments in theory and phenomenology. Perspectives on possible future directions for research are discussed with the aim of exploring emergent QCD phenomena.