Fluid Acceleration in Heavy-Ion Collisions

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.

💡 Research Summary

The paper presents a systematic study of fluid acceleration in relativistic heavy‑ion collisions using two widely employed transport models, AMPT and UrQMD, combined with a Gaussian smearing procedure to construct continuous fields from discrete particles. By defining both the Landau‑Lifshitz (energy‑flow) and Eckart (particle‑flow) four‑velocities and extracting the four‑acceleration a^μ = u^ν∂_ν u^μ from the smoothed velocity field, the authors obtain the full space‑time distribution of proper acceleration for a range of collision systems (Au+Au at √s_NN = 27–200 GeV and Pb+Pb at 2.76 TeV) and centralities.

The main findings are: (i) the peak proper acceleration reaches several hundred MeV (≈250 MeV at 62 GeV, ≈350 MeV at 2.76 TeV), with only modest dependence on the underlying transport model; (ii) transverse acceleration points outward and is strongest at the fireball edge, where steep pressure gradients coincide with low enthalpy density, a feature that persists from early times (t ≈ 0.5 fm/c) down to low collision energies; (iii) longitudinal acceleration exhibits a pronounced energy dependence: at low √s_NN the system experiences early deceleration due to nuclear stopping, whereas at ultra‑relativistic energies a sharp, short‑lived acceleration pulse appears as the two nuclei pass through each other; (iv) the volume‑averaged acceleration is nearly independent of centrality because the extreme acceleration is confined to a thin peripheral layer; (v) the magnitude of the acceleration is comparable to the Unruh temperature (T_U = a/2π), suggesting that partons may experience an effective thermal bath purely from non‑inertial motion.

Beyond these quantitative results, the authors discuss several theoretical implications. The acceleration term enters the mean spin vector S ∝ p × a, providing a natural source of spin polarization that complements the conventional vorticity‑driven contribution and may help resolve the “local spin polarization puzzle.” Acceleration also appears in the relativistic Euler equation a^μ = ∇^μP/(ε+P), linking it directly to pressure gradients and thus to the dynamics of the QCD phase transition. Model calculations in the literature show that acceleration can either restore chiral symmetry (acting like temperature) or enhance symmetry breaking (acting as a “refrigerator”), and the present work supplies realistic space‑time profiles for such studies. Moreover, acceleration contributes to non‑dissipative transport phenomena such as the inertial Nernst effect and curvature‑induced heat currents, and it modifies the fluid helicity evolution via ∂_μ h^μ = –2 a·ω, an analogue of the chiral anomaly.

In summary, the study demonstrates that fluid acceleration in heavy‑ion collisions is a robust, sizable field with distinct transverse and longitudinal characteristics, largely independent of centrality and only mildly model‑dependent. Its magnitude is sufficient to generate observable non‑inertial effects, ranging from Unruh‑like thermalization to spin polarization and anomalous transport. The results provide a concrete foundation for future phenomenological and experimental investigations of acceleration‑driven phenomena in the quark‑gluon plasma.