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 six-dimensional 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.
Highlights
The Standard Model of particle physics is likely to be replaced by a more complete theory above some unknown new physics scale Λ
A tachyon only exists along an energetically disfavored 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
When we analyze the region with φ 2 > 0, we find the existence of a critical choice φc2 for which a phase transition takes place, with the physically realized background minimizing the free energy density being given by confining solutions when φ 2 < φc2, and singular domain-wall solutions for φ 2 > φc2
Summary
The Standard Model of particle physics is likely to be replaced by a more complete theory above some unknown new physics scale Λ. We choose this class mostly on the grounds of simplicity— the model is a simple example of a gravity theory which provides the dual of a confining field theory, within the topdown approach to holography To this end, we broaden the classes of backgrounds studied in earlier publications about this same special system [92,93,98,99]. We uncover evidence of a first-order phase transition in the gravity theory and show that the physically realized solutions do not come immediately close to the tachyonic ones, undermining the chain of implications from the previous paragraph This is illustrated, which sketches the free energy as a function of the source of a relevant deformation in the field theory for two of the different branches of the classical solutions.
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