The closed string tadpole in open string field theory

Understand

We compute a class of gauge invariant observables for marginal solutions and the tachyon vacuum.

• In each case we find that the observables are related in a simple way to the closed-string tadpole on a disk with appropriate boundary conditions.
• We give a sketch of an argument that this result should hold in general using the BRST invariance of the closed string two-point function.
• Finally, we discuss the analogous set of invariants in the Berkovits superstring field theory.