Regular homomorphisms, with a twist

Abstract

Let $X/K$ be a variety over a field, and $A/K$ an abelian variety. A regular homomorphism to $A$ (in codimension $i$) induces, for every smooth geometrically connected pointed $K$-scheme $(T,t_0)$ and every cycle class $Z\in C{H}^i(T\times X)$, a morphism $T\to A$ of varieties over $K$. In this note we show that, even if $T$ admits no $K$-point, the data $(T,Z)$ determines a torsor $A^{(T,Z)}$ over $K$ under $A$ and a $K$-morphism $T\to A^{(T,Z)}$. This can be used to provide an obstruction to the existence of algebraic cycles defined over $K$. We then connect this obstruction to some recent results of Hassett--Tschinkel and Benoist--Wittenberg on rationality of threefolds.

To appear in La Matematica