On-shell mediator dark matter models and the Xenon1T anomaly

We present a dark matter model to explain the excess events in the electron recoil data recently reported by the Xenon1T experiment. In our model, dark matter $chi$ annihilates into a pair of on-shell particles $phi$ which subsequently decay into $psi psi$ final state; $psi$ interacts with electron to generate the observed excess events. Due to the mass hierarchy, the velocity of $psi$ can be rather large and can have an extended distribution, which provides a good fit to the electron recoil energy spectrum. We estimated the flux of $psi$ from dark matter annihilations in the galaxy and further determined the interaction cross section which is sizable but small enough to allow $psi$ to penetrate the rocks to reach the underground labs.