Progresses of theory on underground light dark matter direct detection—Cosmic-ray boosted dark matter and the Migdal effect

Currently, the dominant underground dark matter (DM) direct detection experiments in the world aim to research the property of DM interaction by probing the possible scattering signals between DM particles near the Earth and nuclei in the detectors. Since the DM particles in the galactic DM halo are mainly non-relativistic, and currently the thresholds of the detectors of DM direct detection experiments are mainly around keV, the nuclear recoil energy induced by DM particles with mass below a few GeV is too small to be observed, which becomes a bottleneck problem for underground direct detection of light DM. In recent years, a few proposals based on well-known physical processes to detect light DM with mass below GeV in traditional DM direct detection experiments have been accorded significant importance. Among them, detecting electron signals induced by the Migdal effect in the DM-nucleus scattering processes can significantly lower the detection threshold of experiments. High-energy astrophysical processes, such as cosmic-ray boosted DM (CRDM), can boost a small fraction of non-relativistic DM particles into relativistic ones. These boosted DM particles can generate nuclear recoil signals above experimental detection thresholds, which are possible to detect. By considering these physical processes with the existing experimental data, we can set limits on the scattering cross section for DM particles with mass around MeV. However, the allowed DM-nucleus scattering cross sections are large in general. For the large scattering cross section case, it is necessary to consider the Earth shielding effect for the analysis of existing experimental data from deeply underground DM direct detection experiments. This article reviews the theoretical progresses and experimental applications in the underground direct detection of light DM in recent years, focusing on the Migdal effect and CRDM, as well as the impact of the Earth shielding effect on the underground direct detection of light DM.