Doublet-triplet splitting in fertile left-right symmetric heterotic string vacua

Classification of Left-Right Symmetric (LRS) heterotic-string vacua in the free fermionic formulation, using random generation of Generalised GSO (GGSO) projection coefficients, produced phenomenologically viable models with probability 4 x 10(-11). Extracting substantial number of phenomenologically viable models requires modification of the classification method. This is achieved by identifying phenomenologically amenable conditions on the Generalised GSO projection coefficients that are randomly generated at the SO(10) level. Around each of these fertile cores we perform a complete LRS classification, generating viable models with probability 1.4 x10(-2), hence increasing the probability of generating phenomenologically viable models by nine orders of magnitude, and producing some 1.4 x10(5) such models. In the process we identify a doublet-triplet selection mechanism that operates in twisted sectors of the string models that break the SO(10) symmetry to the Pati-Salam subgroup. This mechanism therefore operates as well in free fermionic models with Pati-Salam and Standard-like Model SO(10) subgroups. (C) 2020 The Author(s). Published by Elsevier B.V.