Graviton vertex operators in the de Donder gauge and BRST descent

Abstract

We revisit the role of the trace sector of the graviton in covariant string amplitudes by constructing the ghost-number-three graviton vertex operator via BRST descent, imposing only the de Donder gauge condition without requiring tracelessness. The resulting operator is well defined even at zero momentum, while for nonzero momentum the trace-dependent sector of the descent chain forms a trivial BRST descent chain. Evaluating its disk one-point function in the presence of a D$p$-brane, we show that the trace-dependent term is essential for reproducing the correct graviton-brane coupling, canceling the Dirichlet-direction contributions and leaving the expected coupling to the world-volume components.