Vortical effects in Dirac fluids with vector, chiral and helical charges

TL;DR

This paper investigates the role of helicity, chiral, and helical charges in Dirac fluids, revealing new transport phenomena, a novel hydrodynamic wave, and potential triangle anomalies involving helicity currents.

Contribution

It introduces the concept of helical vortical effects, derives the pressure for fermions with multiple chemical potentials, and explores the implications for anomalies and transport in Dirac fluids.

Findings

Helical vortical effects induce helicity currents in vorticity-rich media.

A new hydrodynamic excitation called the helical vortical wave is identified.

Finite helical chemical potential characterizes fermion ensembles independently of vector charge.

Abstract

Helicity of free massless Dirac fermions is a conserved, Lorentz-invariant quantity at the level of the classical equations of motion. For a generic ensemble consisting of particles and antiparticles, the helical and chiral charges are different conserved quantities. The flow of helicity can be modelled by the helicity current, which is again conserved in the absence of interactions. Similar to the axial vortical effect which generates an axial (chiral) current, the helicity current is induced by vorticity in a finite temperature medium with vector (electrical) charge imbalance via the helical vortical effects, leading to new nondissipative transport phenomena. These phenomena lead to the appearance of a new hydrodynamic excitation, the helical vortical wave. Our results suggest the existence of a new type of triangle anomalies in QED which involve the helicity currents in addition to the standard vector and axial currents. Further exploiting the conservation of the helical current, we show that a finite helical chemical potential may be used to characterise thermodynamic ensembles of fermions similarly to, but independently of, the vector charge and chirality. We derive the pressure $P$ for fermions at finite vector, axial and helical chemical potentials and show that the quantities arising in anomalous transport, including various vortical and circular conductivities and the shear-stress coefficients, can be obtained by differentiation of $P$ with respect to the appropriate chemical potentials. Finally, we calculate the helicity relaxation time in the quark-gluon plasma above the crossover and show that it is similar to that for the axial charge.