A new analysis supports the idea that particles of light (photons) colliding with heavy ions create a fluid of "strongly interacting" particles. The calculations are based on the hydrodynamic particle flow seen in collisions of various types of ions at both the Large Hadron Collider (LHC) and the Relativistic Heavy Ion Collider (RHIC). With only modest changes, these calculations also describe flow patterns seen in near-miss collisions at the LHC. In these collisions, photons that form a cloud around the speeding ions collide with the ions in the opposite beam.
The results indicate that photon-heavy ion collisions can create a strongly interacting fluid that responds to the initial collision geometry, exhibiting hydrodynamic behavior. This further means that these collisions can form a quark-gluon plasma, a system of quarks and gluons liberated from the protons and neutrons that make up the ions. These findings will help guide future experiments at the Electron-Ion Collider (EIC), a facility planned to be built at Brookhaven National Laboratory over the next decade.
It may seem surprising that photon-heavy ion collisions can produce a hot and dense fluid. But it's possible because a photon can undergo quantum fluctuations to become another particle with the same quantum numbers. A likely example is a rho meson, made of a quark and antiquark held together by gluons. When a rho meson collides with a nucleus, it forms a collision system very similar to a proton-nucleus collision, which also exhibits flow-like signals.
The current analysis by theorists at Brookhaven National Laboratory and Wayne State University sought to explain data from the ATLAS experiment at the LHC. The theorists found that accounting for the energy difference between the rho meson and the much higher energy of the incoming nucleus was the most important ingredient for their calculations' ability to reproduce the experimental results. In the most energetic heavy ion collisions, the pattern of particles emerging transverse to the colliding beams generally persists no matter how far you look from the collision point along the beamline. But in lower-energy photon-lead collisions, the model showed that the geometry of the particle distributions changes rapidly with increasing longitudinal distance. This decorrelation had a large effect on the observed flow pattern, showing that 3D hydrodynamic modeling is essential for simulating these low energy photon-lead collisions.
This research was funded by Department of Energy Office of Science, Office of Nuclear Physics and the National Science Foundation. The research used computational resources of the Open Science Grid, supported by the National Science Foundation.