Algebraic cycles and Fano threefolds of genus 7

Abstract

Let $Y$ be a very general prime Fano threefold of genus 7. We exhibit an explicit 2-cycle on $Y\times Y$ that is Abel-Jacobi trivial but non-torsion in the Chow group $A^4(Y\times Y)$. As a consequence, $Y$ does not admit a multiplicative Chow-Künneth decomposition, in the sense of Shen-Vial. We also show that any Fano threefold has a multiplicative Chow-Künneth decomposition modulo algebraic equivalence.