  Since the b ghost in the pure spinor formalism is a composite operator
depending on non-minimal variables, it is not trivial to impose the Siegel
gauge condition b_0 V=0 on BRST-invariant vertex operators. Using the antifield
vertex operator V* of ghost-number +2, we show that Siegel gauge unintegrated
vertex operators can be constructed as b_0 V* and Siegel gauge integrated
vertex operators as \int dz b_{-1} b_0 V*.
  These Siegel gauge vertex operators depend on the non-minimal variables, so
scattering amplitudes involving these operators need to be regularized using
the prescription developed previously with Nekrasov. As an example of this
regularization prescription, we compute the four-point one-loop amplitude with
four Siegel gauge integrated vertex operators. This is the first one-loop
computation in the pure spinor formalism that does not require unintegrated
vertex operators.
