Collective phenomena in jets: a new window into the ultimate quark-gluon droplet

• Wei Li (Rice University (US))

High-energy heavy-ion collisions produce a deconfined Quark-Gluon Plasma (QGP), which exhibits remarkable collective behavior as a nearly "perfect" fluid. Surprisingly, similar signatures of collectivity have emerged in small systems such as proton-proton (pp) and proton-nucleus (pA) collisions, challenging conventional wisdom on the minimum system size required for emergent QCD phenomena. In this talk, I will introduce a novel perspective: collective behavior may manifest even within individual high-energy jets—localized partonic cascades resulting from hard scatterings. Using new results from the CMS experiment at the LHC, I will discuss evidence that high-multiplicity jets can exhibit features reminiscent of QGP-like collectivity, potentially revealing new aspects of nonperturbative QCD at the smallest accessible scales. I will further explore how these findings connect to jet substructure studies, particularly in understanding parton shower evolution, hadronization, and the role of the QCD vacuum in shaping final-state interactions. These insights open an intriguing new direction in the search for the ultimate limit of partonic collectivity and the fundamental nature of QCD in extreme conditions.