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.

Figures (4)

• Figure 1: Spatial profiles of the $x, y$, and $z$ components of $a^\mu$ in the transverse plane at midrapidity for $\sqrt{s}=7.7$ GeV (upper panel, UrQMD) and $\sqrt{s}=200$ GeV (lower panel, AMPT) for Au + Au collisions.
• Figure 2: Impact parameter dependence of the proper acceleration averaged over the whole volume of the fireball (left panels) and over the transverse plane at midrapidity ($z=0$) (right panels) for lower collision energies (upper panel, UrQMD) and for higher collision energies (lower panel, AMPT). The collision system is Pb + Pb at $\sqrt{s}=2.76$ TeV and Au + Au for other collision energies; the same applies to the figures below.
• Figure 3: Time evolution of the proper acceleration averaged over the whole fireball for lower collision energies (upper panels, UrQMD) and higher collision energies (lower panels, AMPT) for two centralities, $b=0$ (left panles) and $b=5$ fm (right panels).
• Figure 4: Time evolution of different components of the proper acceleration averaged over the $x>0$ half plane at midrapidity ($z=0$) for $\sqrt{s}=7.7$ GeV (upper panels, UrQMD) and 200 GeV (lower panels, AMPT) for two centralities, $b=0$ (left panels) and $b=5$ fm (right panels).