Type I Superstrings

In this case the $L$ and $R$ degrees of freedom are identified. Type I and Type IIB theories have the same spectrum, except that in the former one the states which are not invariant under the change of orientation of the worldsheet, are projected out. This worldsheet parity $\Omega$ interchanges the left and right modes. Type I superstring theory is a theory of breakable closed strings, thus it incorporates also open strings. The $\Omega$ operation leave invariant only one half of the spacetime supersymmetry, thus the theory is ${\cal N}=1$. The spectrum of bosonic massless states in the NS-NS sector is: $G_{IJ}$ (spacetime metric) and $\Phi$ (dilaton) from the closed sector and $B_{IJ}$ is projected out. On the R-R sector there is an antisymmetric field $B_{IJ}$ of the closed sector. The open string sector is necessary in order to cancel tadpole diagrams. A contribution to the spectrum come from this sector. Chan-Paton factors can be added at the boundaries of open strings. Hence the cancellation of the tadpole are needed 32 labels at each end. Therefore in the NS-NS sector there are 496 gauge fields in the adjoint representation of SO(32). truecm