Recent findings at the Large Hadron Collider (LHC) have provided significant insights into the behavior of particles resulting from heavy ion collisions. These discoveries reveal that a unique pattern of “flow” in the particles reflects collective behavior driven by pressure gradients created in extreme conditions that mimic the universe’s state just after the Big Bang.
Understanding Radial Flow and Viscosity
The new measurements, conducted by the ATLAS Collaboration, confirm the fluid-like nature of the quark-gluon plasma (QGP) while introducing a novel aspect of particle dynamics known as “radial flow.” This type of flow originates from a geometric perspective distinct from the previously observed “elliptic flow,” which has implications for understanding different types of viscosity within this fluid system.
Jiangyong Jia, a physicist at Stony Brook University and Brookhaven National Laboratory, emphasizes the importance of these findings. He stated, “The new results from ATLAS, while confirming the fluid-like nature of the QGP, also reveal something new because the type of flow we studied, ‘radial’ flow, has a different geometric origin from the ‘elliptic’ flow studied previously.” Jia, who also led the ATLAS analysis, noted that these insights enhance the understanding of particle interactions under extreme conditions.
The results from ATLAS are further validated by complementary measurements from the ALICE detector at the LHC, which have also been published in the same issue of Physical Review Letters. These findings offer a more comprehensive picture of the dynamics at play during these collisions.
A Historical Perspective on Particle Flow
The exploration of particle flow patterns began with the initial data from the Relativistic Heavy Ion Collider (RHIC), which started operation in 2001. Early observations indicated directional differences in the flow of particles following gold ion collisions. Scientists noted an elliptical pattern, where more particles were observed along the reaction plane—defined by the direction of the colliding ions—compared to the transverse direction.
Researchers hypothesized that this elliptic flow was influenced by the shape of the overlap region between colliding gold ions. The resulting pressure gradients within this oblong fireball would push particles outward more vigorously along the waistband of the football-like shape than towards its pointed ends. This discovery was pivotal as it suggested that quarks and gluons maintain strong interactions even when separated from their usual configurations within protons and neutrons.
According to Peter Steinberg, a physicist at Brookhaven Lab and co-author of the ATLAS paper, these radial flow measurements are “completing a story that started the minute RHIC turned on.” The extreme nature of the elliptic flow led physicists to conclude that it originated from a nearly frictionless liquid, characterized by extremely low shear viscosity.
The implications of these findings are profound, as they enhance our understanding of the early universe and the fundamental forces that govern particle interactions. The ongoing research at both the LHC and RHIC continues to shed light on the conditions that existed moments after the Big Bang, contributing to the broader field of nuclear physics.
As scientists continue to analyze these complex phenomena, the collective behavior of particles in high-energy collisions remains a key area of exploration, promising to uncover more about the universe’s origins and the fundamental nature of matter.
