QUANTUM OPEN STRING THEORY WITH MANIFEST CLOSED STRING FACTORIZATION

We present a quantum theory of covariant open and closed strings that satisfies manifestly the full BV master equation. The classical action includes interacting open strings and free closed strings. All open-closed and closed string self-interactions appear at the quantum level. The string diagrams, arising from a new minimal area problem, make all amplitudes manifestly factorizable in every open and closed string channel. As by-products we obtain a global closed string symmetry of classical open string theory, and a gauge invariant action for open strings propagating in closed string backgrounds.