Signatures of QCD conductivities in heavy-ion collisions

TL;DR

The paper investigates how diffusion of baryon, electric charge, and strangeness in QCD matter created in relativistic heavy-ion collisions shapes rapidity distributions. It develops a causal relativistic diffusive hydrodynamics framework with Grad's off-equilibrium corrections, uses a lattice-QCD based NEOS-4D equation of state, and performs (1+1)D simulations across Au+Au energies to connect conductivities to midrapidity yields. Baryon diffusion dominates diffusion toward midrapidity, off-diagonal conductivities are generally smaller, and delta f at particlization is essential for charge conservation; the authors also build response surfaces using linear, quadratic, and Gaussian process regression to extract conductivities, with global energy fits improving constraints, especially for cross terms like kappa_BQ. This work provides a practical route to constrain QCD transport coefficients from heavy-ion data and informs future 3+1D studies and broader observables.

Abstract

Dissipative processes are pivotal for understanding the hydrodynamic evolution of hot and dense QCD matter created in relativistic nuclear collisions. The interplay of multiple conserved charges -- net baryon, strangeness, and electric charge -- is of particular interest. We simulate the longitudinal hydrodynamic evolution with the three diffusion currents in a hydrodynamic model with a lattice-QCD-based equation of state, NEOS-4D, and estimate rapidity distributions including diffusive corrections to the phase-space distribution in the presence of multiple charges, which ensure charge conservation at particlization. We determine the response of particle yields at midrapidity to changes in the diagonal and off-diagonal conductivities. Inversely, we find that most components of the conductivity matrix can be constrained experimentally using identified particle multiplicities at different collision energies.

Figures (9)

• Figure 1: The coefficients in the deformation tensors, (a) $\mathcal{B}_{V_B}$, (b) $\mathcal{B}_{V_Q}$, (c) $\mathcal{B}_{V_S}$, (d) $\mathcal{D}^B_{V_B}$, (e) $\mathcal{D}^B_{V_Q}(=\mathcal{D}^Q_{V_B})$, (f) $\mathcal{D}^B_{V_S}(=\mathcal{D}^S_{V_B})$, (g) $\mathcal{D}^Q_{V_Q}$, (h) $\mathcal{D}^Q_{V_S}(=\mathcal{D}^S_{V_Q})$, and (i) $\mathcal{D}^S_{V_S}$ at $\mu_B = \mu_Q = \mu_S = 0$ GeV (solid line), $\mu_B = 0.6$ GeV and $\mu_Q = \mu_S = 0$ GeV (dashed line), $\mu_Q = 0.1$ GeV and $\mu_B = \mu_S = 0$ GeV (dashed-dotted line), and $\mu_S = 0.2$ GeV and $\mu_B = \mu_Q = 0$ GeV (dotted line).
• Figure 2: Spacetime rapidity distributions of net baryon density at the initial time (thin dotted line) and 10 fm/$c$ (thick lines) at (a) $\sqrt{s_\mathrm{NN}} = 19.6$ GeV, (b) $62.4$ GeV, and (c) $200$ GeV. The same for (d)-(f) net charge and (g)-(i) net strangeness densities, respectively. Dashed, dash-dotted, and solid lines denote ideal, diagonal diffusion, and full diffusion cases, respectively.
• Figure 3: Net baryon rapidity distributions for the (a) diagonal and (b) full diffusive cases, with and without $\delta f$ corrections (solid and dashed-dotted lines, respectively) compared to the ideal case (dashed lines) at $\sqrt{s_\mathrm{NN}} = 19.6$ GeV. The same at (c)(d) $\sqrt{s_\mathrm{NN}} = 62.4$ GeV and (e)(f) $200$ GeV. Initial net baryon distributions (dotted lines) are plotted for comparison assuming $\eta_s$ equals $y$.
• Figure 4: Net electric charge rapidity distributions for the (a) diagonal and (b) full diffusive cases, with and without $\delta f$ corrections (solid and dashed-dotted lines, respectively) compared to the ideal case (dashed lines) at $\sqrt{s_\mathrm{NN}} = 19.6$ GeV. The same at (c)(d) $\sqrt{s_\mathrm{NN}} = 62.4$ GeV and (e)(f) $200$ GeV. Initial net electric charge distributions (dotted lines) are plotted for comparison assuming $\eta_s$ equals $y$.
• Figure 5: Net strangeness rapidity distributions for the (a) diagonal and (b) full diffusive cases, with and without $\delta f$ corrections (solid and dashed-dotted lines, respectively) compared to the ideal case (dashed lines) at $\sqrt{s_\mathrm{NN}} = 19.6$ GeV. The same at (c)(d) $\sqrt{s_\mathrm{NN}} = 62.4$ GeV and (e)(f) $200$ GeV. Initial net strangeness distributions (dotted lines) are plotted for comparison assuming $\eta_s$ equals $y$.