Orientifold limits of singular F-theory vacua

Abstract

We construct global orientifold limits of singular F-theory vacua whose associated gauge groups are SO(3), SO(5), SO(6), F4, SU(4), and Spin(7). For each limit we show a universal tadpole relation is satisfied, which is a homological identity whose dimension-zero component encodes the matching of the D3 charge between each F-theory compactification and its orientifold limit. While for smooth F-theory compactifications which admit global orientifold limits the contribution to the associated universal tadpole relation comes from its Chern class, we show that for all singular F-theory compactifications under consideration, the contribution to the universal tadpole relation comes from its stringy Chern class. Moreover, the brane spectrum associated with each of our limits consists solely of smooth branes, as opposed to the Sen limit which necessarily involves singular branes.