Gso Projection, BRST Cohomology and Picture-Changing in N=2 String Theory
Abstract
We investigate in detail the critical N=2 fermionic string with and without a global Z2 twist. An analysis of BRST cohomology shows that twisted sectors contain massless `spacetime' fermions which are non-local\/ with respect to the standard massless boson. However, two distinct GSO projections exist, one (untwisted) retaining merely the usual boson and its spectral-flow partner, the other (twisted) yielding two fermions and one boson, on the massless level. The corresponding chiral BRST-invariant vertex operators are constructed in certain pictures, and their fusion and picture-changing are investigated, including the construction of inverse picture-changing operators. The N=2 `spacetime supersymmetry' generators are null\/ operators, since the twisted massless states fail to interact. The untwisted three- and four-point functions are recalculated at tree-level.
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