When atomic nuclei collide at nearly the speed of light, they can create quark-gluon plasma (QGP)—an extremely hot state of matter in which quarks and gluons move freely. This exotic substance behaves like an almost perfect fluid, giving scientists an opportunity to study conditions that existed moments after the Big Bang.
Scientists have extensively studied the powerful vorticity, or swirling motion, and intense electromagnetic fields generated in quark-gluon plasma. However, the plasma’s acceleration has received considerably less attention, despite playing a central role in the rapid expansion of the collision fireball. In relativistic hydrodynamics, acceleration is as fundamental as vorticity, just as electric and magnetic fields are equally important in electromagnetism.
Mapping Quark-Gluon Plasma Acceleration Across Collision Energies
A research team led by Fudan University physicists Yu-Gang Ma and Xu-Guang Huang investigated how acceleration develops and changes inside quark-gluon plasma.
The researchers combined two widely used particle transport models, AMPT and UrQMD, with a Gaussian smearing technique. This approach converted the distributions of individual particles into continuous energy, momentum, and velocity fields, allowing the team to study the plasma as an evolving relativistic fluid.
Using this method, the researchers tracked acceleration across a broad range of collision energies, from 3.5 GeV to 2.76 TeV.
“Acceleration is not merely a kinematic detail—it may act as a thermodynamic control parameter of QCD matter,” explains Professor Huang.
Extreme Acceleration Concentrated at the Fireball’s Edge
The simulations revealed that peak proper acceleration can reach several hundred MeV at both low and high collision energies. The strongest transverse acceleration consistently points outward and is concentrated near the outer boundary of the quark-gluon plasma fireball.
This boundary becomes an acceleration hotspot because pressure decreases sharply in that region while the enthalpy density remains low. According to the relativistic Euler equation, these conditions combine to produce a substantial increase in acceleration.
The plasma responds differently at different collision energies. At lower energies, nuclear stopping initially slows the colliding matter, generating deceleration of up to approximately 500 MeV. At ultrarelativistic energies, the nuclei pass through one another so rapidly that they create brief, intense acceleration pulses in the newly formed plasma.
Because the strongest acceleration remains concentrated near the fireball’s boundary, its overall magnitude changes only slightly between central collisions and collisions occurring at an angle, known as noncentral collisions.
Acceleration May Become a New Control Parameter for Extreme Matter
Acceleration may affect more than the movement of quark-gluon plasma. Through the Unruh effect, an accelerating observer would perceive empty space as a thermal environment. Accelerations reaching several hundred MeV could therefore correspond to temperatures near the QCD transition temperature.
This possibility suggests that acceleration could introduce a new “acceleration axis” into the phase diagram of quantum chromodynamics (QCD) matter. It may influence both the chiral transition and the transition associated with quark confinement.
Acceleration could also produce previously unexplored transport phenomena and affect the alignment of particle spins. These effects would complement those caused by vorticity and could help scientists investigate unresolved spin behavior observed in experiments at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC).
Searching for Observable Acceleration Signals at Particle Colliders
The research team plans to incorporate more realistic hydrodynamic evolution into future calculations. The scientists also aim to identify measurable experimental signals of quark-gluon plasma acceleration, including distinctive patterns in hyperon spin polarization.
By connecting non-inertial quantum effects with observable particle behavior, the study opens a new avenue for exploring matter governed by the strong nuclear interaction.
“Just as temperature and density define the phase diagram of matter, acceleration may open a new axis of that diagram,” stated Professor Huang. “By mapping this hidden dimension of the quark-gluon plasma, we hope to turn non-inertial quantum effects into signatures that experiments can actually measure.”
Source: www.sciencedaily.com