On the b-antighost in the pure spinor quantization of superstrings

Recently Berkovits has constructed a picture raised, compound field bB which is used to compute higher loop amplitudes in the pure spinor approach of superstrings. On the other hand, in the twisted and gauge-fixed, superembedding approach with n=2 world-sheet (w.s.) supersymmetry that reproduces the pure spinor formulation, a field b appears quite naturally as the current of one of the two twisted charges of the w.s. supersymmetry, the other being the BRST charge. In this Letter we study the relation between b and bB. We shall show that bZ, where Z is a picture raising operator, and bB belong to the same BRST cohomological class. This result is of importance since it implies that the cumbersome singularity which is present in b, is in fact harmless if b is combined with Z.