The Chow motive of LSV hyper-Kälher manifolds

Abstract

Let $X$ be a smooth cubic fourfold over $\C$ and let $π: \sJ_U \to U$, with $U\subset ({\P }^5{)}^{\ast }$, be the Lagrangian fibration whose fibres are the smooth hyperplane sections $Y_H=X\cap H$, with $H\in U$. There always exists a (not unique) smooth compactification $\bar \sJ \to (¶^5)^*$ which is a hyper-Kälher manifold of OG10 type. Since two different compactifications are birationally equivalent their Chow motives are isomorphic. For a general $X$ a geometrical construction of a smooth compactification $\sJ(X)$ with irreducible fibres has been described in [LSV]. In this note we prove that the Chow motive $h(\sJ(X)) $ is a direct summand of the (twisted) motive of $X^5$ and therefore is is of abelian type if $h(X)$ is of abelian type.We describe a 10 -dimensional family $\sF$ of cubics $X$ such that the compactification $\sJ(X)$ is unique, smooth, with irreducible fibres, and the Chow motive $h( \sJ(X) )$ is of abelian type.

13 pages