The intersection theory of the moduli stack of vector bundles on ${\mathbb{P}}^1$

Abstract

We determine the integral Chow and cohomology rings of the moduli stack ${\mathcal{B}}_{r,d}$ of rank $r$, degree $d$ vector bundles on ${\mathbb{P}}^1$ bundles. We first show that the rational Chow ring $A_{\mathbb{Q}}^{\ast }({\mathcal{B}}_{r,d})$ is a free $\mathbb{Q}$-algebra on $2r+1$ generators. The isomorphism class of this ring happens to be independent of $d$. Then, we prove that the integral Chow ring $A^{\ast }({\mathcal{B}}_{r,d})$ is torsion-free and provide multiplicative generators for $A^{\ast }({\mathcal{B}}_{r,d})$ as a subring of $A_{\mathbb{Q}}^{\ast }({\mathcal{B}}_{r,d})$. From this description, we see that $A^{\ast }({\mathcal{B}}_{r,d})$ is not finitely generated as a $\mathbb{Z}$-algebra. Finally, the cohomology ring of ${\mathcal{B}}_{r,d}$ is isomorphic to its Chow ring.