Searching for ${}^{76}$Ge neutrinoless double beta decay with the CDEX-1B experiment

We operated a p-type point contact high purity germanium (PPCGe) detector (CDEX-1B, 1.008 kg) in the China Jinping Underground Laboratory (CJPL) for 500.3 days to search for neutrinoless double beta ($0\nu \beta \beta$) decay of ${}^{76}$Ge. A total of 504.3 kg$\cdot$day effective exposure data was accumulated. The anti-coincidence and the multi/single-site event (MSE/SSE) discrimination methods were used to suppress the background in the energy region of interest (ROI, 1989$-$2089 keV for this work) with a factor of 23. A background level of 0.33 counts/(keV$\cdot$kg$\cdot$yr) was realized. The lower limit on the half life of ${}^{76}$Ge $0\nu \beta \beta$ decay was constrained as $T_{1\mathrm{/}2}^{0\nu }>1.0\times {10}^{23}\mathrm{y}\mathrm{r}(90\mathrm{\%}\mathrm{C}\mathrm{.}\mathrm{L}\mathrm{.})$, corresponding to the upper limits on the effective Majorana neutrino mass: $⟨m_{\beta \beta }⟩<$3.2$-$7.5$\mathrm{e}\mathrm{V}$.