Nonhydrodynamic quasinormal modes and equilibration of a baryon dense holographic QGP with a critical point

We compute the homogeneous limit of nonhydrodynamic quasinormal modes (QNMs) of a phenomenologically realistic Einstein-Maxwell-Dilaton (EMD) holographic model for the quark-gluon plasma (QGP) that is able to i) quantitatively describe state-of-the-art lattice results for the QCD equation of state and higher-order baryon susceptibilities with 2 + 1 flavors and physical quark masses up to the highest values of the baryon chemical potential currently reached in lattice simulations, ii) describe the nearly perfect fluidity of the strongly coupled QGP produced in ultrarelativistic heavy-ion collisions, and iii) give a very good description of the bulk viscosity extracted via some recent Bayesian analyses of hydrodynamical descriptions of heavy-ion experimental data. This EMD model has been recently used to predict the location of the QCD critical point in the QCD phase diagram, which was found to be within the reach of upcoming low-energy heavy-ion collisions. The lowest quasinormal modes of the SO(3) rotationally invariant quintuplet, triplet, and singlet channels evaluated in the present work provide upper bounds for characteristic equilibration times describing how fast the dense medium returns to thermal equilibrium after being subjected to small disturbances. We find that the equilibration times in the different channels approach each other at high temperatures, but they are well separated at the critical point. Moreover, in most cases, these equilibration times decrease with increasing baryon chemical potential while keeping the temperature fixed.