Dark Matter as the Z2 Partner of the Standard Model Higgs Boson

Abstract

A possible gamma-ray excess from the Galactic halo favors dark matter with a mass of several hundred GeV. In previous work, we found that an electroweak scalar doublet is a particularly economical—and nearly uniquely selected—interpretation among simple renormalizable thermal dark-matter models. We point out that its unbroken Z2 symmetry is exactly equivalent to an interchange symmetry between two Higgs doublets, making dark matter the Z2-odd partner of the Standard Model Higgs boson and suggesting that the two doublets may share a common weak-scale origin. We connect this model to the recently reported 248keV nuclear-recoil event in the LUX-ZEPLIN experiment. For dark-matter masses of several hundred GeV, the fixed off-diagonal Z interaction can produce the observed recoil within a narrow, halo-dependent strip near the kinematic boundary, predicting a large annual modulation for a smooth Standard Halo Model. Dedicated solar-capture calculations, however, exclude this Standard-Halo interpretation in the thermal-relic mass range. An enhanced high-velocity tail, potentially associated with the Large Magellanic Cloud, shifts the LZ-preferred splitting upward and may leave marginal compatibility with the solar-neutrino constraint. The solar-neutrino constraint does not affect the identification of inert-doublet dark matter as the Z2 partner of the Standard Model Higgs boson or its possible connection to the Galactic halo excess.

The authors' abstract, as published at the source. Symmetry, 2026 · DOI ↗