Curvature squared invariants in six-dimensional $N$ = (1, 0) supergravity

We describe the supersymmetric completion of several curvature-squared invariants for $N$ = (1, 0) supergravity in six dimensions. The construction of the invariants is based on a close interplay between superconformal tensor calculus and recently developed superspace techniques to study general off-shell supergravity-matter couplings. In the case of minimal off-shell Poincaré supergravity based on the dilaton-Weyl multiplet coupled to a linear multiplet as a conformal compensator, we describe off-shell supersymmetric completions for all the three possible purely gravitational curvature-squared terms in six dimensions: Riemann, Ricci, and scalar curvature squared. A linear combination of these invariants describes the off-shell completion of the Gauss-Bonnet term, recently presented in arXiv:1706.09330. We study properties of the Einstein-Gauss-Bonnet super-gravity, which plays a central role in the effective low-energy description of α′-corrected string theory compactified to six dimensions, including a detailed analysis of the spectrum about the AdS₃ × S³ solution. We also present a novel locally superconformal invariant based on a higher-derivative action for the linear multiplet. This invariant, which includes gravitational curvature-squared terms, can be defined both coupled to the standard-Weyl or dilaton-Weyl multiplet for conformal supergravity. In the first case, we show how the addition of this invariant to the supersymmetric Einstein-Hilbert term leads to a dynamically generated cosmological constant and non-supersymmetric (A)dS₆ solutions. In the dilaton-Weyl multiplet, the new off-shell invariant includes Ricci and scalar curvaturesquared terms and possesses a nontrivial dependence on the dilaton field.