Vertex Operators for Closed Superstrings

We construct an iterative procedure to compute the vertex operators of theclosed superstring in the covariant formalism given a solution of IIA/IIBsupergravity. The manifest supersymmetry allows us to construct vertexoperators for any generic background in presence of Ramond-Ramond (RR) fields.We extend the procedure to all massive states of open and closed superstringsand we identify two new nilpotent charges which are used to impose the gaugefixing on the physical states. We solve iteratively the equations of the vertexfor linear x-dependent RR field strengths. This vertex plays a role in studyingnon-constant C-deformations of superspace. Finally, we construct an action forthe free massless sector of closed strings, and we propose a form for thekinetic term for closed string field theory in the pure spinor formalism.Comment: TeX, harvmac, amssym.tex, 41 pp; references adde