Representations of braid groups via cyclic covers of the sphere: Zariski closure and arithmeticity

Gabrielle Menet, Duc-Manh Nguyen

Published 2023-10-16Version 1

Let $d \geq 2$ and $n\geq 3$ be two natural numbers. Given any sequence $\kappa=(k_1,\dots,k_n) \in \mathbb{Z}^n$ such that $\gcd(k_1,\dots,k_n,d)=1$, we consider the family of Riemann surfaces obtained from the plane curves defined by $y^d=\prod_{i=1}^n(x-b_i)^{k_i}$, where $\{b_1,\dots,b_n\}$ are $n$ distinct points in $\mathbb{C}$. The monodromy of the fiber cohomology of this family provides us with a representation of the pure braid group $\mathrm{PB}_n$ into some symplectic group. By restricting to a specific subspace in the cohomology of the fiber, we obtain a representation $\rho_d$ of $\mathrm{PB}_n$ into a linear algebraic group defined over $\mathbb{Q}$. The first main result of this paper is a criterion for the Zariski closure of the image of $\rho_d$ to be maximal, and the second main result is a criterion for the image to be an arithmetic lattice in the target group. The latter generalizes previous results of Venkataramana and gives an answer to a question by McMullen.