Petersen graph and monodromy of the 27 lines on the Clebsch surface

Abstract

Let $G$ be the orbifold fundamental group of the moduli space of smooth cubic surfaces $\mathcal{M}_{\mathsf{sm}}$ in $\mathbb{P}^3_{\mathbb{C}}$ with base point at the Clebsch surface $X_{\mathbf{1}}$. The image of the monodromy action $G \to \lbrace \text{Permutations of $27$ lines on $X_{\mathbf{1}}$} \rbrace$ is famously the Weyl group of type $E_6$. Here we give a description of this monodromy action in terms of the Petersen graph by working out the action of ten explicit generators of $G$ by elementary calculation. These ten generators were found in joint work with Allcock and Looijenga while studying the description of $\mathcal{M}_{\mathsf{sm}}$ as a discriminant complement in a complex $4$-ball quotient.