A note on obstinate tachyons in classical dS solutions

TL;DR

The paper tackles the pervasive tachyon problem in classical de Sitter vacua from flux compactifications in massive IIA string theory. It identifies a semi-universal orientifold-volume modulus σ, in addition to the universal dilaton- and volume-like moduli ρ and τ, whose scaling in the scalar potential constrains stability and often signals instability. Through a detailed analysis of O6 orientifolds on SU(2)×SU(2) and broader scans across SU(3)-structure manifolds, the authors show that all known tachyons lie in the subspace spanned by ρ, τ, and the σ's, and, in a SUSY Minkowski limit of the isotropic case, the tachyon aligns with the sgoldstino. The results offer a practical diagnostic for metastability and suggest strategies to achieve more stable constructions by limiting moduli or examining lower-dimensional compactifications, while clarifying the connection between sgoldstino dynamics and orientifold-volume fluctuations.

Abstract

The stabilisation of the dilaton and volume in tree-level flux compactifications leads to model independent and thus very powerful existence and stability criteria for dS solutions. In this paper we show that the sizes of cycles wrapped by orientifold planes are scalars whose scalings in the potential are not entirely model independent, but enough to entail strong stability constraints. For all known dS solutions arising from massive IIA supergravity flux compactifications on SU(3)-structure manifolds the tachyons are exactly within the subspace spanned by the dilaton, the total volume and the volumes of the orientifold cycles. We illustrate this in detail for the well-studied case of the O6 plane compactification on SU(2)xSU(2)/Z_2xZ_2. For that example we uncover another novel structure in the tachyon spectrum: the dS solutions have a singular, but supersymmetric, Minkowski limit, in which the tachyon exactly aligns with the sgoldstino.