M-theory Potential from the $G_2$ Hitchin Functional in Superspace

We embed the component fields of eleven-dimensional supergravity into a superspace of the form $X\times Y$ where $X$ is the standard 4D, $N=1$ superspace and $Y$ is a smooth 7-manifold. The eleven-dimensional 3-form gives rise to a tensor hierarchy of superfields gauged by the diffeomorphisms of $Y$. It contains a natural candidate for a $G_2$ structure on $Y$, and being a complex of superforms, defines a superspace Chern-Simons invariant. Adding to this a natural generalization of the Riemannian volume on $X\times Y$ and freezing the (superspin-$\frac32$ and 1) supergravity fields on $X$, we obtain an approximation to the eleven-dimensional supergravity action that suffices to compute the scalar potential. In this approximation the action is the sum of the superspace Chern-Simons term and a superspace generalization of the Hitchin functional for $Y$ as a $G_2$-structure manifold. Integrating out auxiliary fields, we obtain the conditions for unbroken supersymmetry and the scalar potential. The latter reproduces the Einstein-Hilbert term on $Y$ in a form due to Bryant.