Orientifold planes play a crucial role in flux compactifications of string
theory, and we demonstrate their deep connection to achieving scale-separated
solutions. Specifically, we show that when an orientifold plane contributes at
leading order to the non-zero value of the scalar potential, then either the
weak coupling limit or the large volume limit implies scale separation, meaning
that the Kaluza-Klein tower mass decouples from the inverse length scale of the
lower-dimensional theory. Notably, in the supergravity limit such solutions are
inherently scale-separated. This result is independent of the spacetime
dimension and the dimensionality of the O$p$-plane as long as $p<7$. Similarly, we show that parametric scale separation is not possible for isotropic compactifications with a leading curvature term that generically arise in the AdS/CFT context. We classify all possible flux compactification setups in both type IIA and type IIB string theory for O$p$-planes with $2\leq p\leq 6$ and present their universal features. While the parametrically controlled scale-separated solutions are all AdS, we also find setups that allow for dS vacua. We prove that flux quantization prevents these dS vacua from arising in a regime of parametric control.

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