Disc amplitudes, picture changing and space-time actions

TL;DR

This paper details the calculation of D-brane couplings to closed string fields via disc amplitudes, ensuring gauge invariance and matching supergravity predictions.

Contribution

It provides a careful construction of vertex operators, addresses BRST-exact state decoupling, and explicitly computes two-point amplitudes for NS-NS and R-R fields.

Findings

Amplitudes are gauge-invariant and independent of picture charge distribution.

Explicit two-point amplitude calculations match supergravity predictions.

The boundary state formalism effectively encodes D-brane effects.

Abstract

We study in detail the procedure for obtaining couplings of D-branes to closed string fields by evaluating string theory disc amplitudes. We perform a careful construction of the relevant vertex operators and discuss the effects of inserting the boundary state which encodes the presence of the D-brane. We confront the issue of non-decoupling of BRST-exact states and prove that the problem is evaded for the computations we need, thus demonstrating that our amplitudes are automatically gauge-invariant and independent of the distribution of picture charge. Finally, we compute explicitly the two-point amplitudes of two NS-NS fields or one NS-NS and one R-R field on the disc, and we carefully compare all the lowest order terms with predictions from supergravity.