A Simple Dark Matter Model to Explain the LZ Event and Galactic Center Excess

We propose that the dark matter (DM) is a pseudo-Dirac fermion with a mass of approximately 20-100 GeV that couples inelastically to the Standard Model through a hadrophilic spin-1 mediator, V, the gauge boson of spontaneously broken baryon number. The two DM eigenstates, χ 1 and χ 2, acquire approximately equal abundances after thermal freeze-out and are depleted through the processes χ 1 χ 1 to VV and χ 2 χ 2 to VV. Although the heavier state is unstable, for a mass splitting below the electron-positron threshold and sufficiently small mass mixing with the Z boson, its lifetime can exceed the age of the universe, allowing both states to remain approximately equally abundant in the Galactic halo. For a thermal-relic annihilation cross section, this model can produce a signal that is consistent with the observed features of the longstanding Galactic Center Gamma-Ray Excess (GCE). If δ about 1000 keV, the same parameter space that explains the GCE can also explain the LZ event through exothermic χ 2 to χ 1 downscattering off xenon nuclei.