The axial charge relaxation coefficient in the Schwinger-Keldysh effective theory of QCD must be of second order in the quark mass, Γ_A = γ_A m_q^2, subleading in the broken phase and leading in the restored phase.
Pseudo-spontaneous $U(1)$ Symmetry Breaking in Hydrodynamics and Holography
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abstract
We investigate the low-energy dynamics of systems with pseudo-spontaneously broken $U(1)$ symmetry and Goldstone phase relaxation. We construct a hydrodynamic framework which is able to capture these, in principle independent, effects. We consider two generalisations of the standard holographic superfluid model by adding an explicit breaking of the $U(1)$ symmetry by either sourcing the charged bulk scalar or by introducing an explicit mass term for the bulk gauge field. We find agreement between the hydrodynamic dispersion relations and the quasi-normal modes of both holographic models. We verify that phase relaxation arises only due to the breaking of the inherent Goldstone shift symmetry. The interplay of a weak explicit breaking of the $U(1)$ and phase relaxation renders the DC electric conductivity finite but does not result in a Drude-like peak. In this scenario we show the validity of a universal relation, found in the context of translational symmetry breaking, between the phase relaxation rate, the mass of the pseudo-Goldstone and the Goldstone diffusivity.
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Schwinger-Keldysh effective action for hydrodynamics with approximate symmetries
The axial charge relaxation coefficient in the Schwinger-Keldysh effective theory of QCD must be of second order in the quark mass, Γ_A = γ_A m_q^2, subleading in the broken phase and leading in the restored phase.