Search for Light Weakly-Interacting-Massive-Particle Dark Matter by Annual Modulation Analysis with a Point-Contact Germanium Detector at the China Jinping Underground Laboratory

We present results on light weakly interacting massive particle (WIMP) searches with annual modulation (AM) analysis on data from a 1-kg mass $p$-type point-contact germanium detector of the CDEX-1B experiment at the China Jinping Underground Laboratory. Datasets with a total live time of 3.2 yr within a 4.2-yr span are analyzed with analysis threshold of 250 eVee. Limits on WIMP-nucleus ($\chi \text{−}N$) spin-independent cross sections as function of WIMP mass ($m_{\chi }$) at 90% confidence level (C.L.) are derived using the dark matter halo model. Within the context of the standard halo model, the 90% C.L. allowed regions implied by the DAMA/LIBRA and CoGeNT AM-based analysis are excluded at $>99.99\%$ and 98% C.L., respectively. These results correspond to the best sensitivity at $m_{\chi }<6\mathrm{GeV}/c^2$ among WIMP AM measurements to date.

Figure 1

Count rates of CDEX-1B as function of time, where the shaded area denotes the period of gamma source calibration. (a) Time variation of the background count rates at 20–40 and 2.0–4.0 keVee. Stability in the signal regions is demonstrated with good ${\chi }^2/\mathrm{d}.\mathrm{o}.\mathrm{f}.$ and $p$ values under stable-background hypothesis—17.30/20(0.63) for ${\mathrm{AC}}^{-}$ in Run 1, $16.44/13(0.23)$ for Run 2 at 20–40 keVee, and $28.15/20(0.11)$ for ${\mathrm{AC}}^{-}$ in Run 1, $18.66/20(0.54)$ for ${\mathrm{AC}}^{+}$ in Run 1, $9.24/13(0.75)$ for Run 2 at 2.0–4.0 keVee. (b) The B/S corrected bulk event counts versus time at three energy ranges which are most relevant to the sensitivities at $m_{\chi }\sim 8\mathrm{GeV}/c^2$, with the overall ${\chi }^2/\mathrm{d}.\mathrm{o}.\mathrm{f}.$ of $35.5/60$ for Run 1 and $24.6/39$ for Run 2 in these energy ranges. The uncertainties in these energies are dominated by the systematic uncertainties from $\mathrm{B}/\mathrm{S}$ correction, in which overestimated upper bound was used [18]. A bin size of 200 eVee is used in this plot, while different bin size is adopted in the analysis.

Figure 2

Best-fit solutions of modulation amplitude $A_i$ at $\text{phase}=152.5\mathrm{d}$. The distributions show consistency with null results, i.e., no significant modulation signatures. Derived modulation signals (based on standard halo model) from 90% C.L. allowed regions of CoGeNT (3 yr) [8] and best-fit modulation amplitudes of CDMS-II distributed results [29] are superimposed. A bin size of 200 eVee was used for better illustration.

Figure 3

Limits at 90% C.L. from CDEX-1B AM-analysis (red) on spin-independent WIMP-nucleon cross section. Also shown are other AM-based results: 90% C.L. upper limits of XMASS-1 (dark gray) [21], allowed regions of DAMA/LIBRA phase1 (Na-recoil, pale blue: $5\text{−}\sigma$, blue: 90% C.L.) [3, 4, 37], and CoGeNT (green: 90% C.L.) [8]. Constraints from the CDEX-1B time-integrated spectral analysis [18] are displayed (black dotted line) as comparison. We note that the DAMA/LIBRA regions shown in this plot stem from the previous DAMA/LIBRA-phase1 data [3, 4], under the assumption of canonical isospin-conserving spin-independent WIMP-nucleus scattering in the standard halo model. New DAMA/LIBRA-phase2 data [5] have considerable impact on the best-fit regions and disfavor the canonical assumption [38].

Figure 4

Best-fit ${\sigma }_{\mathrm{SI}}$ values versus $\varphi$ at $m_{\chi }=7.9\mathrm{GeV}/c^2$, taking $\varphi$ as a free parameter in $\sum {\chi }_i^2$. The results show that the null hypothesis is within $1\text{−}\sigma$ to the best-fit ${\sigma }_{\mathrm{SI}}$ values (similar conclusions also apply for $m_{\chi }$ in the range $3.2–17\mathrm{GeV}/c^2$).