No-Ghost Theorem for Neveu-Schwarz String in 0-Picture via Similarity Transformation

A proof of the no-ghost theorem for Neveu-Schwarz string in 0-picture is outlined in a manner using a similarity transformation. It is shown that a nontrivial metric consistent with the BRST cohomology is needed to define a positive semidefinite norm in the physical Hilbert space. The one-to-one correspondence between physical states in 0-picture and those in the conventional (−1)-picture are confirmed. As a by-product, we find a new inverse picture-changing operator, which is noncovariant but has nonsingular operator product with itself. A possibility to construct a new gauge-invariant superstring field theory is discussed.