Equivariant principal bundles over toric Deligne--Mumford stacks

Abstract

Let $\mathscr{X}(\boldsymbolΣ)$ be the toric Deligne--Mumford stack with stacky torus $\mathscr{T}$ associated to the stacky fan $\boldsymbolΣ = (N,Σ, β)$. A toric principal $H$-bundle over $\mathscr{X}(\boldsymbolΣ)$ is a principal $H$-bundle $\mathscr{P}$ equipped with a $\mathscr{T}$-action lifting the $\mathscr{T}$-action on $\mathscr{X}(\boldsymbolΣ)$, such that the $\mathscr{T}$-action and the $H$-action on $\mathscr{P}$ commute. For a connected reductive algebraic group $H$ over $\mathbb{C}$, we classify the isomorphism classes of framed toric principal $H$-bundles over $\mathscr{X}(\boldsymbolΣ)$ in terms of piecewise linear maps from $|Σ|$ to the cone over the Tits building of $H$. Using this classification, we describe the equivariant automorphism group of a toric principal $H$-bundle, and give a necessary and sufficient condition for an equivariant reduction of the structure group. We also show that every toric principal $\mathrm{GL}(r)$-bundle over the weighted projective stack $\mathbb{P}(w_0,\dots,w_n)$ splits equivariantly whenever $r < n$, and that every toric principal $H$-bundle over the weighted stacky projective line $\mathbb{P}(a,b)$ splits equivariantly when $H$ is a connected reductive algebraic group.