Constraints on Spin-Independent Nucleus Scattering with sub-GeV Weakly Interacting Massive Particle Dark Matter from the CDEX-1B Experiment at the China Jin-Ping Laboratory

We report results on the searches of weakly interacting massive particles (WIMPs) with sub-GeV masses ($m_{chi}$) via WIMP-nucleus spin-independent scattering with Migdal effect incorporated. Analysis on time-integrated (TI) and annual modulation (AM) effects on CDEX-1B data are performed, with 737.1 kg$cdot$day exposure and 160 eVee threshold for TI analysis, and 1107.5 kg$cdot$day exposure and 250 eVee threshold for AM analysis. The sensitive windows in $m_{chi}$ are expanded by an order of magnitude to lower DM masses with Migdal effect incorporated. New limits on $sigma_{chi N}^{rm SI}$ at 90% confidence level are derived as $2times$10$^{-32}sim7times$10$^{-35}$ $rm cm^2$ for TI analysis at $m_{chi}sim$ 50$-$180 MeV/$c^2$, and $3times$10$^{-32}sim9times$10$^{-38}$ $rm cm^2$ for AM analysis at $m_{chi}sim$75 MeV/$c^2-$3.0 GeV/$c^2$.