Results from a Search for Light-Mass Dark Matter with a $p$-Type Point Contact Germanium Detector

We report on several features in the energy spectrum from an ultralow-noise germanium detector operated deep underground. By implementing a new technique able to reject surface events, a number of cosmogenic peaks can be observed for the first time. We discuss an irreducible excess of bulklike events below 3 keV in ionization energy. These could be caused by unknown backgrounds, but also dark matter interactions consistent with DAMA/LIBRA. It is not yet possible to determine their origin. Improved constraints are placed on a cosmological origin for the DAMA/LIBRA effect.

Figure 1

Decays associated with ${}^{71}\mathrm{Ge}$ ($T_{1/2}=11.4\mathrm{d}$) produced via thermal neutron activation of the PPC in [1] (see text).

Figure 2

Top panel: Rise time in preamplifier traces from ${}^{241}\mathrm{Am}$ gammas in the PPC in [1] and a manual scan of reference electronic pulser signals in the Soudan PPC (see text). Bottom panel: Background events collected by the PPC in Soudan. The dotted line represents the 90% signal acceptance contour for bulk events. Top inset: Vertical cross section of a cylindrical BEGe PPC, showing the two contacts. Bottom inset: Typical preamplifier traces from 1 keVee events, before and after wavelet denoising [19], for $t_{10–90}=0.22\mu \mathrm{s}$ (left) and $t_{10–90}=1.53\mu \mathrm{s}$ (right).

Figure 3

Low-energy spectrum after all cuts, prior to efficiency corrections. Arrows indicate expected energies for all viable cosmogenic peaks (see text). Inset: Expanded threshold region, showing the ${}^{65}\mathrm{Zn}$ and ${}^{68}\mathrm{Ge}$ $L$-shell EC peaks. Overlapped on the spectrum are the sigmoids for triggering efficiency (dotted), trigger $+$ microphonic PSD cuts [1] (dashed) and trigger $+$ PSD $+$ rise time cuts (solid), obtained via high-statistics electronic pulser calibrations. Also shown are uncorrected reference signals from $7\mathrm{GeV}/c^2$ and $10\mathrm{GeV}/c^2$ WIMPs with spin-independent coupling ${\sigma }_{\mathrm{SI}}={10}^{-4}\mathrm{pb}$.

Figure 4

Top panel: 90% C.L. WIMP exclusion limits from CoGeNT overlaid on Fig. 1 from [5]. Green shaded patches denote the phase space favoring the DAMA/LIBRA annual modulation (the dashed contour includes ion channeling). Their exact position has been subject to revisions [20]. The violet band is the region supporting the two CDMS candidate events. The scatter plot and the blue hatched region represent the supersymmetric models in [6] and their uncertainties, respectively. Models including WIMPs with $m_{\chi }\sim 7–11\mathrm{GeV}/{\mathrm{cm}}^3$ provide a good fit to CoGeNT data (red contour, see text). The relevance of XENON10 constraints in this low-mass region has been questioned [21]. Bottom panel: Limits on axioelectric coupling $g_{a\overline{e}e}$ for pseudoscalars of mass $m_a$ composing a dark isothermal galactic halo (see text).