Kinetic Mixing of U(1)s in Heterotic Orbifolds

TL;DR

The paper investigates kinetic mixing between massless U(1) gauge factors in bosonic heterotic orbifold compactifications, focusing on non-prime Z_N geometries where N=2 subsectors govern the mixing. It derives a compact, subsector-based expression for the one-loop mixing and applies it to Z_6-II constructions, finding nonzero mixing even without Wilson lines and identifying MSSM-like vacua with a hidden U(1) that can mix with hypercharge. Through detailed analyses of Z_6-II models with and without Wilson lines, the work demonstrates that a broad class of semi-realistic string vacua naturally hosts kinetically mixed hidden sectors with mixing in the 10^{-2} to 10^{-4} range, and it discusses how these hidden sectors may acquire mass and participate in Dark Force phenomenology after SUSY breaking. The results highlight the phenomenological viability of hidden photons in heterotic orbifolds and point to concrete mechanisms by which hidden U(1)s could contribute to observable physics, including potential collider and cosmological signatures. The study thereby bridges explicit string construction with low-energy phenomenology, offering a framework to explore hidden sectors in a globally consistent string context.

Abstract

We study kinetic mixing between massless U(1) gauge symmetries in the bosonic formulation of heterotic orbifold compactifications. For non-prime Z_N factorisable orbifolds, we find a simple expression of the mixing in terms of the properties of the N=2 subsectors, which helps understand under what conditions mixing can occur. With this tool, we analyse Z_6-II heterotic orbifolds and find non-vanishing mixing even without including Wilson lines. We show that some semi-realistic models of the Mini-Landscape admit supersymmetric vacua with mixing between the hypercharge and an additional U(1), which can be broken at low energies. We finally discuss some phenomenologically appealing possibilities that hidden photons in heterotic orbifolds allow.