M-theory interpretation of the real topological string

Understand

We describe the type IIA physical realization of the unoriented topological string introduced by Walcher, describe its M-theory lift, and show that it allows to compute the open and unoriented topological amplitude in terms of one-loop diagram of BPS M2-brane states.

• This confirms and allows to generalize the conjectured BPS integer expansion of the topological amplitude.
• The M-theory lift of the orientifold is freely acting on the M-theory circle, so that integer multiplicities are a weighted version of the (equivariant subsector of the) original closed oriented Gopakumar-Vafa invariants.
• The M-theory lift also provides new perspective on the topological tadpole cancellation conditions.