On Modified Diagonal Cycles and the Beauville Decomposition of the Ceresa Cycle

Abstract

Let $C$ be a curve of genus $g\ge 2$, and let $J$ be its Jacobian. The choice of a degree 1 divisor $e$ on $C$ gives an embedding of $C$ into $J$; we denote by $[C{]}_{}^e\in \mathrm{CH}\left(J;\mathbb{Q}\right)$ the class in the Chow group of $J$ defined by its image. It is known that the vanishing of the Ceresa cycle $\mathrm{Cer}(C,e):=[C{]}^e-[-1{]}_{\ast }[C{]}^e$ is equivalent to both the vanishing of the 1st Beauville component $[C{]}_{(1)}^e$ and the vanishing of the 3rd Gross--Kudla--Schoen modified diagonal cycle ${\Gamma }^3(C,e)\in \mathrm{CH}(C^3;\mathbb{Q})$. We extend this result to show that the vanishing of the $s$-th Beauville component $[C{]}_{(s)}^e$ for $s\ge 1$ is equivalent to the vanishing of the $(s+2)$-nd modified diagonal cycle ${\Gamma }^{s+2}(C,e)\in \mathrm{CH}(C^{s+2};\mathbb{Q})$. Moreover, we establish "successive vanishing" results for these cycles. We apply our results to study the rational (non)-triviality of $[C{]}_{(s)}^e$ in the special case $s=2$. Finally in the $s=1$ case, we show an integral refinement to the original statement, relating the order of torsion of $\mathrm{Cer}(C,e)\in \mathrm{CH}(J;\mathbb{Z})$ to that of ${\Gamma }^3(C,e)\in \mathrm{CH}(C^3;\mathbb{Z})$.

34 pages, comments welcome. Final version, to appear in Épijournal de Géométrie Algébrique