Dark Matter as the Z_2 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 Z_2 symmetry is exactly equivalent to an interchange symmetry between two Higgs doublets, making dark matter the Z_2-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 248 keV nuclear-recoil event in the LUX-ZEPLIN experiment. For dark-matter masses of 440--600 GeV, a technically natural, mass-dependent neutral-state splitting of order 350 keV defines a narrow strip near the kinematic boundary where the fixed off-diagonal Z interaction can produce the observed recoil. Because the scattering probes the extreme high-velocity tail of the Galactic distribution, this interpretation predicts a large annual modulation, with future events preferentially occurring around early summer. Neither observation is yet conclusive, but their common interpretation is sharply testable.