Classification of left–right symmetric heterotic string vacua

The classification method of the free fermionic heterotic string vacua is extended to models where the SO(10) GUT symmetry is broken directly at the string scale to the Left–Right Symmetric subgroup. Our method involves using a fixed set of basis vectors which are defined by the boundary conditions assigned to the free fermions before enumerating the string vacua by varying the Generalised GSO (GGSO) projection coefficients. It allows the derivation of algebraic expressions for the GGSO projections for each sector that generates massless states in the models. This enables a computerised analysis of the entire massless spectrum of a given choice of GGSO projection coefficients. The total number of vacua in the class of models chosen is 266≈7.38×1019. A statistical sampling is performed and a sample size of 1011 vacua with the Left–Right Symmetric gauge group is extracted. We present the results of the classification, noting that contrary to the previous classification of Pati–Salam models, no three generation exophobic models were found. The results obtained demonstrate the existence of three generation models with the necessary Higgs representations needed for viable spontaneous symmetry breaking, and with a leading top quark Yukawa coupling.