The flow of particles in the collisions of heavy ions

Scientists ran a comprehensive analysis of data from collisions of heavy ions (actually, nuclei of atoms) to determine which factors most influence fluctuations in the flow of particles 1. They found that the conditions that are established just as the ions collide have the greatest impact. These conditions are related to the size of the nuclei and the degree to which the collisions are off-centre.

The matter created in these collisions is called a quark-gluon plasma (QGP). This is a trillion-degree soup of quarks and gluons that replicates what the universe was like a microsecond after the Big Bang. Understanding its properties has been a central goal of nuclear physics.

heavy ions
Conditions set up in the earliest stage of collisions of gold ions play the greatest role in causing fluctuations in the flow of particles, with more particles emerging along the reaction plane than perpendicular to it. Image courtesy of Brookhaven National Laboratory

Nuclear physicists study QGP by colliding atomic nuclei at the Relativistic Heavy Ion Collider. The collisions melt protons and neutrons, setting free their constituent quarks and gluons. Off-centre collisions preferentially push more particles out along the reaction plane of the colliding nuclei than perpendicular to it.

But the flow is not quite as smooth as that simplistic picture suggests. There are lots of fluctuations. This study by the STAR Collaboration aimed to nail down the source of those fluctuations. The scientists assessed the influence of initial state effects by looking at collisions of different size nuclei (gold-gold, uranium-uranium, and copper-gold) and how head-on vs. glancing the collisions were.

They also looked at the effect of collision energy to reveal whether flow patterns were influenced by the QGP’s evolution over time. In addition, they analysed whether the fluctuations varied among different types of particles that emerge when quarks and gluons recombine to become composite particles called hadrons.

The researchers found that the degree to which the collisions were head-on and the size of the colliding nuclei had the greatest impact on flow fluctuations. Later stages including the evolution of the matter created in the collisions and its transformation into other particles did not have an effect.

Identifying that initial-state conditions are primarily responsible for fluctuations in the flow of particles from the QGP will help physicists make constrain key variables essential for making higher precision calculations of QGP properties such as viscosity, and for understanding phase changes in nuclear matter. That, in turn, will help them understand how the collision transforms nuclei from a collection of protons and neutrons into a plasma of quarks and gluons.

More on the subject:

The role of core electrons in the electronic stopping power of heavy ions


  1. M. S. Abdallah et al. (STAR Collaboration) (2022) Collision-System and Beam-Energy Dependence of Anisotropic Flow Fluctuations Phys. Rev. Lett. doi: 10.1103/PhysRevLett.129.252301

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