Collision edges reveal hidden acceleration patterns
When atomic nuclei collide at nearly light speed, the intense forces involved produce the hottest known form of matter. This substance, called quark–gluon plasma, exists only for a fleeting moment. During this time, it behaves like a fluid, with quarks and gluons moving freely instead of being tightly bound. While scientists have studied the plasma’s rotation and electromagnetic fields, the acceleration behind its expansion has been largely overlooked. Now, a team of physicists has uncovered that the strongest accelerations occur at the edges of the plasma, or fireball.
Yu-Gang Ma and Xu-Guang Huang of Fudan University led a detailed study, combining two well-established models for simulating particle transport—AMPT and UrQMD—with a technique called Gaussian smearing. This method converts the specific positions of individual particles into smooth, continuous maps of energy, momentum, and fluid velocity. With these maps, the researchers were able to trace how acceleration formed and evolved as the collision energy changed from 3.5 GeV to 2.76 TeV.
The simulations showed that the proper acceleration, or the acceleration experienced by the moving fluid itself, could reach hundreds of MeV at both ends of the energy spectrum. This acceleration consistently pointed outward and reached its maximum strength near the outer boundary of the fireball. At that edge, the pressure drops dramatically while the enthalpy density, a measure of the fluid’s overall energy content, remains low. These two factors, according to the relativistic Euler equation, combine to produce the observed acceleration effects.
The specifics of the acceleration changed depending on the energy of the collisions. At lower energies, the process known as nuclear stopping caused an early deceleration, peaking around 500 MeV. In contrast, at ultra-relativistic energies, the nuclei passed through each other almost instantly, causing the newly formed plasma to experience short but intense bursts of acceleration. Despite these differences, the strongest acceleration was always most prominent at the fireball’s edge. The overall strength of the effect remained largely unaffected by the geometry of the collision, whether the nuclei hit directly or off center.
Acceleration may redefine QCD phase structure
The implications of these findings stretch beyond the movement of the plasma. According to the Unruh effect, an observer undergoing strong acceleration can perceive empty space as if it were a hot environment. In this context, an acceleration of several hundred MeV could mimic a temperature close to the transition point in quantum chromodynamics (QCD), the theory that describes the strong nuclear force. This suggests that acceleration might serve as a new parameter in the phase diagram of QCD matter. Such a phase diagram typically maps how matter transitions between different states based on temperature and density.
Professor Huang highlights the potential of acceleration to expand the phase diagram of matter, offering a new dimension to the understanding of quantum chromodynamics. The team now aims to refine their simulation methods by including more realistic hydrodynamic evolution. Their immediate goal is to identify measurable signals that could be confirmed through experiments. These signals might reveal the presence of non-inertial quantum effects that are typically difficult to observe. By capturing these effects, the research could help move theoretical models into the realm of testable predictions, opening new frontiers in high-energy physics. This work could ultimately reshape the way scientists interpret the quark–gluon plasma and its role in the evolution of the universe as a whole. The next step for the team is to refine their models further and look for direct experimental evidence that could validate their simulations. If successful, their work may provide a new framework for exploring the complex interactions that govern the behavior of matter under extreme conditions, bringing us closer to a deeper understanding of the fundamental laws of nature.
These findings were published in Nuclear Science and Techniques (DOI: 10.1007/s41365-026-02044-8). The research was supported by the National Natural Science Foundation of China, the Natural Science Foundation of Shanghai, and the National Key Research and Development Program of China. Follow us on Google and Google News for updates on the latest scientific discoveries and developments in physics and beyond. Stay connected for more insights into how fundamental forces shape the universe around us.