N-loop superstring amplitude from Feynman-like rules; Spin structures including Ramond sector, handle operator and g-vacuum

Using the operator approach we construct the operator part of the N-loop amplitude of the superstring out of the three-string vertices and propagators. To include the Ramond sector straightforwardly into our formulation, the fermions are bosonized. As a result we obtain the g-vacuum expressed by the θ{symbol}-function for the corresponding Riemann surface of genus N with specified spin structure. This operator construction has the advantage of providing us with a handle operator connecting the two g-vacua for genus N and genus (N + 1) in a compact form.