TheType 0 string theory is a less well-known model ofstring theory. It is asuperstring theory in the sense that theworldsheet theory issupersymmetric. However, thespacetime spectrum is not supersymmetric and, in fact, does not contain anyfermions at all. In dimensions greater than two, the ground state is atachyon so the theory is unstable. These properties make it similar to thebosonic string and an unsuitable proposal for describing the world as we observe it, although a GSO projection does get rid of the tachyon and the evenG-parity sector of the theory defines a stable string theory. The theory is used sometimes as atoy model for exploring concepts in string theory, notablyclosed stringtachyon condensation. Some other recent interest has involved thetwo-dimensional Type 0 string which has a non-perturbatively stable matrix model description.
Like theType II string, differentGSO projections result in slightly different theories, Type 0A and Type 0B. The difference lies in which types ofRamond–Ramond fields lie in the massless spectrum.
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