Finite generation of Noether-Lefschetz divisors and the slope of the moduli space of cubic fourfolds

TL;DR

This paper investigates the structure of divisors on moduli spaces of cubic fourfolds and K3 surfaces, providing bounds, presentations, and explicit formulas related to their effective cones and Picard groups.

Contribution

It introduces a slope bound for the effective cone of cubic fourfolds and provides a finite presentation of the Picard group for K3 moduli spaces using Noether-Lefschetz divisors.

Findings

Established an explicit slope bound for cubic fourfolds.

Provided a finite generating set for the Picard group of K3 moduli spaces.

Derived explicit formulas for the Hodge class in terms of Noether-Lefschetz divisors.

Abstract

We study divisors on moduli spaces of cubic fourfolds with simple singularities and of quasi-polarized K3 surfaces of degree $2d$. For the moduli space of cubic fourfolds, we introduce a slope quantity to characterize the effective cone and prove an explicit bound for it. For the K3 moduli spaces, we give an explicit finite presentation of the rational Picard group by showing that it is generated by Noether-Lefschetz divisors of discriminant less than or equal to $4d$. As a byproduct, we obtain two explicit expressions for the Hodge class in terms of Noether-Lefschetz divisors, and we indicate analogous results for higher-codimension Noether-Lefschetz cycles.