Periods of join algebraic cycles

Abstract: We determine the cycle class of join algebraic cycles inside smooth hypersurfaces by means of their periods. We show that being a join algebraic cycle is equivalent to have its associated Artin Gorenstein algebra isomorphic to the tensor product of the Artin Gorenstein algebras of each generating cycle. This decomposition allows us to relate the quadratic fundamental form associated to the Hodge loci of each algebraic cycle with the one associated to their join. As a first application we study at a second order approximation several Hodge loci of join algebraic cycles given as combinations of two linear cycles (previously studied by Movasati, Dan, Kloosterman and the second author), showing the non-smoothness in some new cases. Our main application is to show the existence of fake linear cycles inside hypersurfaces of any dimension and degree, given as join of $0$-dimensional fake linear cycles inside hypersurfaces of $\mathbb{P}^1$ with all their closed points defined over $\mathbb{Q}$. This shows that also for high degree hypersurfaces, there are infinitely many (non smooth) Hodge loci whose Zariski tangent space has the same codimension as the codimension of the component associated to a linear cycle.