Semi-annihilating Z3 dark matter for XENON1T excess

The recently reported result from XENON1T experiment indicates that there is an excess with $3.5\sigma$ significance in the electronic recoil events. Interpreted as new physics, sources are needed to account for scattering. We show a dark fermion $\psi$ mass about $ \mathcal{O}\left(10\right)$ MeV semi-annihilation of $Z_3$ matter $X$ and subsequent decay gauge boson $V_\mu$ may be responsible excess. relevant process $X+X\rightarrow \overline{X}+V_\mu (\rightarrow \psi + \overline{\psi})$, which final has box-shape energy spectrum. fast-moving can scatter electron through additional mixes photon kinetically. shape signals this model consistent observed interaction proton makes testable future searches nucleus well. Because lightness invisible particle introduced, scenario requires non-standard cosmology accommodate radiation component.