Constraints on Spin-Independent Nucleus Scattering with sub-GeV Weakly Interacting Massive Particle Dark Matter from the CDEX-1B Experiment at the China Jinping Underground 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 day exposure and 160 eVee threshold for TI analysis, and 1107.5 kg 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(SI)(chi N) at 90% confidence level are derived as 2 x 10(-32) similar to 7 x 10 (-35) cm(2) for TI analysis at m(chi) similar to 50-180 MeV/c(2), and 3 x 10(-32) similar to 9 x 10 (-38) cm(2) for AM analysis at m(chi) similar to 75 MeV/c(2)-3.0 GeV/c(2).