Real Moment Map and Hyperkähler Geometry Techniques: The Cotangent Bundle to the 2-Sphere and SL(2, ℂ)-Adjoint Orbits

Abstract

Going back to Kirwan and others, there is an established theory that uses moment map techniques to study actions by complex reductive groups on Kähler manifolds. Work of P. Heinzner, G. Schwarz, and H. Stötzel has extended this theory to actions by real reductive groups. In this paper, we apply these techniques to actions of the real group SU(1,1) ⊂ SL(2, ℂ) on a certain complex manifold of dimension two. More precisely, because of the SU(2)-invariant hyperkähler structure on this manifold, we are able to study a family of actions which includes and “interpolates” two well-known actions of SL(2, ℂ): the adjoint action on the orbit of a semisimple element of 𝔰𝔩(2, ℂ), and the action of SL(2, ℂ) on the cotangent bundle of the flag variety of SL(2, ℂ).