GSO projection


The GSO projection is an ingredient used in constructing a consistent model in superstring theory. The projection is a selection of a subset of possible vertex operators in the worldsheet conformal field theory —usually those with specific worldsheet fermion number and periodicity conditions. Such a projection is necessary to obtain a consistent worldsheet CFT. For the projection to be consistent, the set A of operators retained by the projection must satisfy:
Starting from the same worldsheet CFT, different choices in the GSO projection will lead to string theories with different physical particles and properties in spacetime. For example, the Type II and Type 0 string theories result from different GSO projections on the same worldsheet theory. Furthermore, the two distinct Type II theories, IIA and IIB, differ in their GSO projections. In building models of realistic string vacua, one typically chooses a GSO projection which eliminates the tachyonic ground state of the string and preserves spacetime supersymmetry.