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, ${\chi }_1$ and ${\chi }_2$, acquire approximately equal abundances after thermal freeze-out and are depleted through the processes ${\chi }_1{\chi }_1\to VV$ and ${\chi }_2{\chi }_2\to VV$. 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 $\delta \sim 1000$ keV, the same parameter space that explains the GCE can also explain the LZ event through exothermic ${\chi }_2\to {\chi }_1$ downscattering off xenon nuclei.