A Z2xZ2 standard model

TL;DR

The paper constructs a heterotic string model based on a Z2xZ2 orbifold with a freely acting Z2 involution that non-locally breaks SU(5) to the Standard Model, enabling a complete blow-up of orbifold singularities without hypercharge flux. This yields an exact chiral MSSM spectrum with a gauged matter parity and supports precision gauge coupling unification, addressing long-standing issues of threshold corrections and GUT-scale matching. The authors also discuss a semi-realistic vacuum that decouples exotics, preserves the MSSM gauge group, and yields a large top Yukawa, while acknowledging challenges such as the mu-term and the need for an explicit smooth Calabi-Yau realization with consistent Bianchi identities. Overall, the work demonstrates a concrete string construction where orbifold and smooth compactifications coherently describe a viable MSSM framework with non-local GUT breaking, laying groundwork for more complete phenomenology and geometric embedding.

Abstract

We present a Z_2 x Z_2 orbifold compactification of the E_8 x E_8 heterotic string which gives rise to the exact chiral MSSM spectrum. The GUT breaking SU(5) to SU(3)_C x SU(2)_L x U(1)_Y is realized by modding out a freely acting symmetry. This ensures precision gauge coupling unification. Further, it allows us to break the GUT group without switching on flux in hypercharge direction, such that the standard model gauge bosons can remain massless when the orbifold singularities are blown up. The model has vacuum configurations with matter parity, a large top Yukawa coupling and other phenomenologically appealing features.