In this chapter we construct the superstring theories in flat Minkowski spacetime. Their main advantage with respect to bosonic string theory is that they lead to spacetime fermions. In addition, they naturally lead to spacetime supersymmetry, which guarantees the absence of spacetime tachyons, and thus provide stable vacua. There are five such superstring theories, called the type IIA and IIB theories, the E8 × E8 and SO(32) heterotic theories, and type I theory, which we study in that order. These superstrings are the best studied string theories, and have been efficiently applied to the construction of realistic particle physics models, to be developed in later chapters.

Fermions on the worldsheet

We would like to construct new string theories which avoid the shortcomings of bosonic string theory, and in particular can lead to spacetime fermions. This requires modifying the worldsheet theory. However, the modification cannot amount to just changing the worldsheet action keeping the same 2d fields, since as discussed in Section 3.1.5 this just leads to the same bosonic string theory in a different spacetime background. Rather, we need to modify the worldsheet field content.

The most elegant modifications of the field content of a theory are those based on a symmetry principle; this is exploited by superstrings, whose worldsheet theory contains additional 2d fermionic fields as required by a 2d supersymmetry on the worldsheet (hence the name).