Towards a characterization of toric hyperkähler varieties among symplectic singularities

Abstract

Let $(X,\omega )$ be a conical symplectic variety of dimension $2n$ which has a projective symplectic resolution. Assume that $X$ admits an effective Hamiltonian action of an $n$-dimensional algebraic torus $T^n$, compatible with the conical ${\mathbf{C}}^{\ast }$-action. A typical example of $X$ is a toric hyperkahler variety $Y(A,0)$. In this article, we prove that this property characterizes $Y(A,0)$ with $A$ unimodular. More precisely, if $(X,\omega )$ is such a conical symplectic variety, then there is a $T^n$-equivariant (complex analytic) isomorphism $\phi :(X,\omega )\to (Y(A,0),{\omega }_{Y(A,0)})$ under which both moment maps are identified. Moreover $\phi$ sends the center $0_X$ of $X$ to the center $0_{Y(A,0)}$ of $Y(A,0)$.

Ver 2: 40 pages: Minor corrections, Ver 3: We add Question in the introduction. Moreover, we add Proposition (4.6) which makes the argument more precise in the description of the local structure of the relative moment map. Ver 4: Obvious mistakes at the beginning of the proof of Prop (3.4) are corrected. Ver5: Final version (To appear in Selecta Math.)