XENON1T Anomaly and its Implication for Decaying Warm Dark Matter
  An excess in the electronic recoil data was observed in the XENON1T detector.
One of plausible explanations for the excess is absorption of a vector bosonic
particle with the mass of $2-3\,{\rm keV}$. For this, the kinetic mixing
$\kappa\!\sim\!10^{-15}$ of the dark photon with the photon is required if the
dark photon explains the current DM abundance. We recently proposed a model
where the main component DM today is a decaying warm dark matter (WDM) with the
life time comparable to the age of the current universe. The WDM decays to a
massless fermion and a massive dark photon. This model was originally designed
for addressing both the small scale problems which $\Lambda$CDM suffers from
and the $H_{0}$ tension. In this letter, we show that the massive dark photon
produced by the WDM decay can be identified with the vector boson inducing the
anomalous excess in the XENON1T experiment. Depending on a lifetime of the
parent decaying WDM, the dark photon could be either the main component or the
sub-component of DM population today.
