Probing ultra-light dark photon from inverse Compton-like scattering

Dark photon not only provides a portal linking dark sector particles and ordinary matter but also is a well-motivated dark matter candidate. We propose to detect the dark photon dark matter through the inverse Compton-like scattering process $p+\gamma^\prime \to p+\gamma$. Thanks to the ultra-high energy primary cosmic rays, we find that such a method is able to probe the dark photon mass from $10^{-2}$ eV down to $10^{-19}$ eV with the expected sensitivity of eROSITA $X$-ray telescope, which can extend the current lower limit of dark photon mass from Jupiter's magnetic fields experiment by about three orders of magnitude.