Axion monodromy in a model of holographic gluodynamics

The low energy field theory for N type IIA D4-branes at strong ’t Hooft coupling, wrapped on a circle with antiperiodic boundary conditions for fermions, is known to have a vacuum energy which depends on the θ angle for the gauge fields, and which is a multivalued function of this angle. This gives a field-theoretic realization of “axion monodromy” for a nondynamical axion. We construct the supergravity solution dual to the field theory in the metastable state which is the adiabatic continuation of the vacuum to large values of θ. We compute the energy of this state and show that it initially rises quadratically and then flattens out. We show that the glueball mass decreases with θ, becoming much lower than the 5d KK scale governing the UV completion of this model. We construct two different classes of domain walls interpolating between adjacent vacua. We identify a number of instability modes — nucleation of domain walls, bulk Casimir forces, and condensation of tachyonic winding modes in the bulk — which indicate that the metastable branch eventually becomes unstable. Finally, we discuss two phenomena which can arise when the axion is dynamical; axion-driven inflation, and axion strings.