Heterotic string theory (I). The free heterotic string

Abstract

A new theory of closed orientable superstrings is constructed as a chiral combination of the closed D = 26 bosonic and D = 10 fermionic strings. The resulting ten-dimensional theory is N = 1 supersymmetric, Lorentz invariant, and free of tachyons. An alternate formulation where sixteen of the bosonic coordinates are replaced by fermionic coordinates is presented and used to construct a manifestly Lorentz covariant and reparametrization invariant action for the theory. The theory is consistent only for gauge group Spin(32)/Z 2 or E 8×E 8.