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The $\mathrm{AdS}_5 \times \mathrm{S}^5$ mirror model as a string

Doing a double Wick rotation in the worldsheet theory of the light cone $\mathrm{AdS}_5 \times \mathrm{S}^5$ superstring results in an inequivalent, so-called mirror theory that plays a central role in the field of integrability in AdS/CFT. We show that this mirror theory can be interpreted as the light cone theory of a free string on a different background. This background is related to $\mathrm{dS}_5 \times \mathrm{H}^5$ by a double T duality, and has hidden supersymmetry. The geometry can also be extracted from an integrable deformation of the $\mathrm{AdS}_5 \times \mathrm{S}^5$ sigma model, and we prove the observed mirror duality of these deformed models at the bosonic level as a byproduct. While we focus on $\mathrm{AdS}_5 \times \mathrm{S}^5$, our results apply more generally.