Fluid Acceleration in Heavy-Ion Collisions

Abstract: We study the generation and space-time evolution of fluid acceleration in heavy-ion collisions using AMPT and UrQMD transport models combined with a Gaussian smearing method. The peak proper acceleration reaches several hundred MeV, with mild model dependence. Transverse acceleration points outward and is strongest at the fireball boundary due to steep pressure gradients and low enthalpy density—a persistent feature even at early times and low energies. Longitudinal acceleration shows strong collision-energy dependence: low-energy collisions exhibit early deceleration from nuclear stopping, while ultra-relativistic collisions produce sharp acceleration pulses from passing nuclei. The volume-averaged acceleration is nearly centrality independent, as extreme acceleration localizes at boundaries. These strong acceleration fields may have important implications for QGP physics, including the Unruh effect mimicking a thermal bath, potential influences on the chiral phase transition and deconfinement, and contributions to spin polarization beyond vorticity.