QFT In the literature: open

Nonperturbative content of closed superstring field theory

In plain words

String field theory writes string theory like an ordinary field theory and fixes many subtle quantities. Whether it defines string theory beyond the weak-coupling series is unknown.

Precise statement

Closed superstring field theory gives a consistent perturbative definition, including D-instanton contributions with fixed normalizations (Sen 2020-2024). Determine whether it admits classical solutions describing distant backgrounds (full background independence) and a nonperturbative formulation beyond the $g_{s}$ expansion.

What would settle it

Construction of a classical solution of closed string field theory describing a background at finite distance in moduli space, or a nonperturbative definition of its path integral.

Status in the literature

Background independence is shown only for nearby backgrounds (Sen and Zwiebach, 1993-1994); D-instanton normalizations computed since 2020 match dual predictions.