Constraints on cosmic-ray boosted DM in CDEX-10

Dark matter (DM) direct detection experiments have been setting strong limits on the DM-nucleon scattering cross section at the DM mass above a few GeV, but leave large parameter space unexplored in the low mass region. DM is likely to be scattered and boosted by relativistic cosmic rays in the expanding universe if it can generate nuclear recoils in direct detection experiments to offer observable signals. Since low energy threshold detectors using Germanium have provided good constraints on ordinary halo GeV-scale DM, it is necessary to re-analyze 102.8 kg$\times$day data in the CDEX-10 experiment assuming that DM is boosted by cosmic rays. For the DM mass range 1 keV $<m_\chi <$ 1 MeV and the effective distance within 1 kpc, we reach an almost flat floor limit at $8.32\times10^{-30}$ cm$^2$ on spin-independent DM-nucleon scattering cross section at a 90\% confidence level. The CDEX-10 result is able to close the gap unambiguously in the parameter space between MiniBooNE and XENON1T constraints which was partially hindered by the Earth attenuation effect. We also quantitatively calculate expected neutrino floor on searching for CRBDM in future direct detection experiments using Germanium.