Type (0)over-bar heterotic string orbifolds

The Z(2) x Z(2) heterotic string orbifold gives rise to a large space of phenomenological three generation models that serves as a testing ground to explore how the Standard Model of particle physics may be incorporated in a theory of quantum gravity. Recently, we demonstrated the existence of type 0 Z(2) x Z(2) heterotic string orbifolds in which there are no massless fermionic states. In this paper we demonstrate the existence of non-supersymmetric tachyon-free Z(2) x Z(2) heterotic string orbifolds that do not contain any massless bosonic states from the twisted sectors. We dub these configurations type (0) over bar models. They necessarily contain untwisted bosonic states, producing the gravitational, gauge and scalar moduli degrees of freedom, but possess an excess of massless fermionic states over bosonic ones, hence producing a positive cosmological constant. Such configurations may be instrumental when trying to understand the string dynamics in the early universe. (C) 2021 The Author(s). Published by Elsevier B.V.

Automated summary

The Z(2) x Z(2) heterotic string orbifold provides a large space of phenomenological three-generation models for exploring the incorporation of the Standard Model into a theory of quantum gravity. Recent findings show the existence of type 0 Z(2) x Z(2) heterotic string orbifolds lacking massless fermionic states. The non-supersymmetric tachyon-free Z(2) x Z(2) heterotic string orbifolds dubbed as type (0) over bar models contain untwisted bosonic states and an excess of massless fermionic states over bosonic ones, potentially aiding in understanding string dynamics in the early universe.