The affine closure of cotangent bundles of horospherical spaces

For a smooth quasi-affine variety $X$, the affine closure $\overline{T^{\ast }X}:=\text{Spec}(\mathbb{K}[T^{\ast }X])$ contains $T^{\ast }X$ as an open subset, and its smooth locus carries a symplectic structure. A natural question is whether $\overline{T^{\ast }X}$ itself is a symplectic variety. A notable example is the conjecture of Ginzburg and Kazhdan, which predicts that $\overline{T^{\ast }(G\mathrm{/}U)}$ is symplectic for a maximal unipotent subgroup $U$ in a reductive linear algebraic group $G$. This conjecture was recently proved by Gannon using representation-theoretic methods. In this paper, we provide a new geometric approach to this conjecture. Our method allows us to prove a more general result: $\overline{T^{\ast }(G\mathrm{/}H)}$ is symplectic for any horospherical subgroup $H$ in $G$ such that $G\mathrm{/}H$ is quasi-affine. In particular, this implies that the affine closure $\overline{T^{\ast }(G\mathrm{/}[P,P])}$ is a symplectic variety for any parabolic subgroup $P$ in $G$.