BCS/BEC crossover in quark matter and evolution of its static and dynamic properties from the atomic unitary gas to color superconductivity

We study the evolution of dynamic properties of the BCS/BEC (Bose–Einstein condensate) crossover in a relativistic superfluid as well as its thermodynamics. We put particular focus on the change in the soft mode dynamics throughout the crossover, and find that three different effective theories describe it; these are, the time-dependent Ginzburg–Landau (TDGL) theory in the BCS regime, the Gross–Pitaevskii (GP) theory in the BEC regime, and the relativistic Gross–Pitaevskii (RGP) equation in the relativistic BEC (RBEC) regime. Based on these effective theories, we discuss how the physical nature of soft mode changes in the crossover. We also discuss some fluid-dynamic aspects of the crossover using these effective theories with particular focus on the shear viscosity. In addition to the study of soft modes, we show that the “quantum fluctuation” is present in the relativistic fermion system, which is in contrast to the usual Nozières–Schmitt-Rink (NSR) theory. We clarify the physical meaning of the quantum fluctuation, and find that it drastically increases the critical temperature in the weak coupling BCS regime.