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

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.