Title: Exploring the Origin of Collective Phenomena in Small Collision Systems at the Large Hadron Collider.
Speaker: Dr. Debojit Sarkar, CERN
Abstract: Observing collective behaviors—such as long-range two-particle correlations and elliptic flow—in relativistic proton-proton and proton-lead collisions at Large Hadron Collider (LHC) energies has sparked debate about the potential formation of quark-gluon plasma (QGP), a short-lived state of strongly interacting partonic matter thought to have existed a few microseconds after the Big Bang, in both heavy-ion collisions and small collision systems at the LHC.
In this talk, I’ll present our latest findings showing, for the first time, a distinctive baryon-meson elliptic-flow grouping and splitting with approximately 5σ significance at intermediate pT in high-multiplicity p–Pb and pp collisions—patterns remarkably similar to those in heavy-ion collisions. The Hydrodynamics+Coalescence+Fragmentation model, which includes partonic flow and quark coalescence, matches the data, while other models fall short, providing strong evidence of a collectively flowing partonic medium (QGP) in high-multiplicity collisions. We also investigate how far down in system size the dynamics of small systems and heavy-ion physics remain similar. The observed baryon-meson v2 grouping and splitting down to lower multiplicities in pp and p-Pb collisions suggests that partonic collectivity may persist even in smaller systems. However, recent observations of long-range correlations in e+e- collisions, differing from pp results at the LHC, indicate differences in underlying dynamics responsible for collective-like behaviors across various systems. To further explore the origins of collectivity in small systems, I’ll discuss our latest findings on the longest-range two-particle correlations (|∆η| > 5.0 and 6.5) ever measured, extending to or below minimum-bias multiplicities in pp and p–Pb collisions. These measurements push the boundaries of our understanding of collective medium formation in small systems. I’ll also address the significance and relevance of these measurements for the physics goals of current and upcoming collider facilities.