Dilatonic states near holographic phase transitions

Abstract

The spectrum of bound states of special strongly coupled confining field theories might include a parametrically light dilaton, associated with the formation of enhanced condensates that break (approximate) scale invariance spontaneously. It has been suggested in the literature that such a state may arise in connection with the theory being close to the unitarity bound in holographic models. We extend these ideas to cases where the background geometry is non-AdS, and the gravity description of the dual confining field theory has a top-down origin in supergravity.We exemplify this programme by studying the circle compactification of Romans sixdimensional half-maximal supergravity. We uncover a rich space of solutions, many of which were previously unknown in the literature. We compute the bosonic spectrum of excitations, and identify a tachyonic instability in a region of parameter space for a class of regular background solutions. A tachyon only exists along an energetically disfavoured (unphysical) branch of solutions of the gravity theory; we find evidence of a first-order phase transition that separates this region of parameter space from the physical one. Along the physical branch of regular solutions, one of the lightest scalar particles is approximately a dilaton, and it is associated with a condensate in the underlying theory. Yet, because of the location of the phase transition, its mass is not parametrically small, and it is, coincidentally, the next-to-lightest scalar bound state, rather than the lightest one.

The Coulomb branch of N=4 SYM and dilatonic scions in supergravity

“…The gravity duals of the confining theories studied in Refs. [1,2] exhibit large departures from AdS geometry. What renders one of the scalar fluctuations parametrically light along the metastable branch is the interplay between the presence of a vacuum expectation value (VEV) breaking spontaneously scale invariance, the explicit breaking due to relevant deformations, and the effects of the nearby instability.…”

“…In Refs. [1] and [2], we proposed a mechanism giving rise to an approximate dilaton in the spectrum of composite states of special classes of confining theories, in four dimensions, that admit a higher-dimensional gravity dual. We provided two explicit, calculable realisations of this mechanism, which generalises the ideas proposed in Ref.…”

“…Ref. [1] studies the half-maximal supergravity in D = 6 dimensions, due to Romans [28][29][30][31] (see also Refs. [32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49]), compactified on a circle [50][51][52][53], while Ref.…”

“…In Refs. [1,2], we explored regions of the admissible parameter space overlooked in the earlier literature.…”

“…The significance of Refs. [1,2] extends beyond producing calculable examples of the ideas exposed in Refs. [3][4][5][6].…”