Second-order spin hydrodynamics from Zubarev's nonequilibrium statistical operator formalism

Using the Zubarev's nonequilibrium statistical operator formalism, we derive the second-order expression for the dissipative tensors in relativistic spin hydrodynamics, {\em viz.} rotational stress tensor ($τ_{μν}$), boost heat vector ($q_μ$), shear stress tensor ($π_{μν}$), and bulk viscous pressure ($Π$). The first two ($τ_{μν}$ and $q_μ$) emerge due to the inclusion of the antisymmetric part in the energy-momentum tensor, which, in turn, governs the conservation of spin angular momentum ($Σ^{αμν}$). As a result, new thermodynamic forces, generated due to the antisymmetric part of $T_{μν}$, contain the spin chemical potential. In this work, we have also taken the spin density ($S^{μν}$) as an independent thermodynamic variable, in addition to the energy density and particle density, thereby resulting in two novel transport coefficients given by the correlation between spin density tensor and rotational stress tensor and vice versa. Additionally, the newly found terms in $π_{μν}$ and $Π$ are the artifacts of the new thermodynamic forces that arise due to the antisymmetric part of $T^{μν}$. Finally, we have derived the evolution equations for the aforesaid tensors: $τ_{μν}$, $q_μ$, $π_{μν}$, and $Π$.