When two atomic nuclei collide at nearly the speed of light, they create one of the hottest and most energetic forms of matter known to science: quark–gluon plasma. In this extreme state, protons and neutrons dissolve, allowing quarks and gluons to move collectively as a nearly perfect fluid. For decades, researchers have focused on the plasma’s enormous vorticity and intense electromagnetic fields. A new study now draws attention to another fundamental feature that may be just as important: fluid acceleration, the force that drives the plasma’s explosive expansion.
The research, led by physicists Yu-Gang Ma and Xu-Guang Huang of Fudan University, provides the most detailed mapping yet of how acceleration develops inside the fireball produced by heavy-ion collisions. The team combined two established transport models, AMPT and UrQMD, with a Gaussian smearing method that transforms the discrete particles generated in simulations into continuous energy-density, momentum, and velocity fields. This approach enabled the researchers to calculate how the plasma accelerates across collision energies ranging from 3.5 gigaelectronvolts to 2.76 teraelectronvolts.
In relativistic hydrodynamics, acceleration is not simply a description of how fast matter moves. It is a dynamical response to pressure gradients and energy distribution, and it helps determine how the plasma evolves in space and time. The researchers describe it as a quantity that stands alongside vorticity in the relativistic description of fluid motion. In an analogy with electromagnetism, acceleration can be viewed as having a role comparable to the electric field, while vorticity resembles the magnetic field. “Acceleration is not merely a kinematic detail—it may act as a thermodynamic control parameter of QCD matter,” Professor Huang explains.
The simulations reveal that the strongest proper acceleration can reach several hundred megaelectronvolts at both low and ultra-high collision energies. These values are not temperatures in the conventional sense, but they characterize the scale of the force acting on the fluid in natural units used by particle physicists. The transverse component of the acceleration consistently points outward from the collision zone. Rather than being distributed evenly throughout the fireball, it becomes especially intense near the outer boundary, where the plasma encounters a rapidly changing environment.
This concentration at the edge arises from two effects described by the relativistic Euler equation. First, the pressure falls sharply near the boundary, creating a strong pressure gradient that pushes the plasma outward. Second, the enthalpy density—the relativistic quantity that measures the energy available to drive fluid motion—is lower in the peripheral region. A given pressure gradient can therefore generate a larger acceleration where the enthalpy is small. The edge of the fireball acts as an acceleration hotspot, producing a violent outward flow as the plasma expands and cools.
The origin of the acceleration also changes with collision energy. At lower energies, the incoming nuclei lose a substantial fraction of their longitudinal momentum through nuclear stopping. This process can generate early deceleration signals reaching roughly 500 megaelectronvolts. At ultra-relativistic energies, however, the two nuclei pass through one another more rapidly. Their fleeting interaction with the newly created plasma can pull the fluid into sharp acceleration pulses, producing a more abrupt dynamical response even though the collision takes place at a much higher energy.
One surprising result is that the overall acceleration changes only weakly with the impact geometry of the collision. Heavy-ion collisions can be nearly head-on or substantially off-center, producing fireballs with very different shapes and angular momentum. Yet the most extreme acceleration remains concentrated at the boundary in both cases. Because the edge is always where pressure gradients and low enthalpy combine most effectively, the local acceleration appears to be governed more by the structure of the fireball’s surface than by the precise degree of overlap between the colliding nuclei.
The findings may also connect fluid acceleration to the thermodynamics of quantum chromodynamics, the theory describing quarks and gluons. Through the Unruh effect, an observer undergoing constant acceleration perceives the quantum vacuum as a thermal bath. If the acceleration in the quark–gluon plasma reaches a few hundred megaelectronvolts, the associated Unruh temperature could become comparable to the temperature scale of the QCD transition, where strongly interacting matter changes between confined and deconfined phases. This raises the possibility that acceleration could influence chiral symmetry restoration and quark deconfinement in ways not captured by temperature and density alone.
The researchers propose that acceleration may eventually become a new axis in the QCD phase diagram, alongside temperature and baryon density. It could also contribute to transport phenomena and spin polarization, offering a complementary explanation for some of the spin patterns observed in relativistic heavy-ion experiments at facilities such as the Relativistic Heavy Ion Collider and the Large Hadron Collider. Since accelerated fluid elements can affect how particles are emitted and polarized, the study points toward experimental signatures that may be measurable in the final distribution of particles, particularly hyperons.
The work remains based on transport-model simulations rather than a complete hydrodynamic description of the evolving plasma. The team’s next step is to incorporate realistic hydrodynamic evolution and identify observables capable of separating acceleration effects from those caused by vorticity, electromagnetic fields, and other collective phenomena. If those signatures can be confirmed experimentally, the results would expand the understanding of the quark–gluon plasma from a hot, rapidly rotating fluid to a non-inertial quantum medium whose acceleration may help determine its phase structure and observable behavior.
Subject of Research: Computational simulation/modeling of fluid acceleration in heavy-ion collisions
Article Title: Fluid acceleration in heavy-ion collisions
Web References: https://doi.org/10.1007/s41365-026-02044-8
References: Nuclear Science and Techniques, DOI: 10.1007/s41365-026-02044-8
Image Credits: Xu-Guang Huang
Keywords
Quark–gluon plasma, heavy-ion collisions, fluid acceleration, relativistic hydrodynamics, quantum chromodynamics, nuclear physics, particle physics, Unruh effect, spin polarization, QCD phase diagram
