Sasaki structures on general contact manifolds

Abstract: We extend the concept of a Sasakian structure on a cooriented contact manifold, given by a compatibility between the contact form $\eta$ and a Riemannian metric $g_M$ on $M$, to the case of a general contact structure understood as a contact distribution. Traditionally, the compatibility can be expressed as the fact that the symplectic form $\omega=\mathrm{d}(s^2\eta)$ and the metric $g(x,s)=\mathrm{d} s\otimes\mathrm{d} s+s^2g_M(x)$ define on the cone $\mathcal{M}=M\times\mathbb{R}_+$ a K\"ahler structure. Since general contact structures can be realized as homogeneous symplectic structures $\omega$ on $\mathrm{GL}(1;\mathbb{R})$-principal bundles $P\to M$, it is natural to understand Sasakian structures in full generality as related to `homogeneous K\"ahler structures' on $P$. The difficulty is that, even locally, contact distributions do not provide any preferred contact form, so the standard approach cannot be directly applied. However, we succeeded in characterizing homogeneous K\"ahler structures on $(P,\omega)$ and discovering a canonical lift of Riemannian metrics from the contact manifold $M$ to $P$, which allowed us to define Sasakian structures for general contact manifolds. This approach is completely conceptual and avoids ad hoc choices. Moreover, it provides a natural concept of Sasakian manifold products which we develop in detail.