The Heterotic String

Abstract

In this chapter we construct and study a new class of string theories: the heteroric ones which on the left side are the bosonic string and on the right one the superstring. Two of these theories are supersymmetric (16 supercharges) with spacetime gauge group Spin(32 ) and E8× E8, respectively. We construct these theories in two different (but equivalent) ways: using either chiral fermions or chiral bosons to represent their ‘internal’ left-moving 2d d.o.f. We study the toroidal compactifications of these theories, their dualities, and equivalences. We also discuss the non-SUSY heterotic strings, and in particular the tachyon-free one with gauge group SO(16 ) × SO(16 ). Finally we describe the BPS states of the SUSY heterotic strings. In the Appendix the N= 2 superstring is briefly reviewed.