Chiral Vortical Instability
We revisit the collective modes of chiral matter described by the second-order chiral hydrodynamics, noticing that chiral shear waves (CSWs) may become unstable for momenta above a characteristic scale. In the absence of sufficient dissipation, this instability emerges within the hydrodynamic regime, depending on the interplay between shear viscosity and the anomalous vortical contribution to the stress-energy tensor at second order in hydrodynamic expansion. We show that this instability generates helical flows and name it the {\it chiral vortical instability} (CVI). Alongside the chiral plasma and magnetovortical instabilities, CVI tends to transfer initial microscopic chirality into macroscopic helicities, which combine into a generalized axial charge. We further find that an elementary static Gromeka-Arnold-Beltrami-Childress flow, corresponding to a CSW at a specific momentum, solves the full nonlinear equations of second-order chiral hydrodynamics, whereas global rotation of a chiral medium is not a solution. This observation supports the relevance of CVI beyond the hydrodynamic regime. Finally, we briefly note that CVI may have multiple phenomenological implications across various systems, including QCD matter produced in heavy-ion collisions and primordial plasma in the early Universe.