The Mori cone of certain Hassett spaces

Abstract

In this paper, we prove that the Mori cone of Hassett spaces whose universal family is a $\mathbf{P}^1$-bundle is generated by 1-dimensional strata. This extends the case of the symmetric GIT quotient $(\mathbf{P}^1)^n//PGL_2$ established by Bolognesi and Massarenti. Along the way, we review how these spaces are naturally isomorphic to certain GIT quotients $(\mathbf{P}^1)^n//PGL_2$, characterize them as the targets of the birational contractions of the blow-up of $\mathbf{P}^{n-3}$ at $n-1$ general points with $\mathbf{Q}$-factorial image, and deduce that their effective cone is likewise generated by strata.