Search for an Annual Modulation in a P-type Point Contact Germanium Dark Matter Detector

TL;DR

Fifteens months of CoGeNT data from a PPC germanium detector are analyzed to search for an annual modulation signature expected from light-mass WIMPs. After correcting for cosmogenic L-shell electron capture backgrounds and verifying detector stability, a modulated component is observed with ~2.8 sigma significance, maximized in the 0.5–3 keVee bulk-energy region. The best-fit modulation parameters (amplitude ~16.6%, period ~347 days, phase near mid-October) are broadly consistent with a light-WIMP interpretation within reasonable astrophysical uncertainties, and overlap the DAMA/LIBRA-favored region. The paper discusses constraints from CDMS and XENON and notes that larger exposure and alternative analyses are needed to confirm a dark-matter origin.

Abstract

Fifteen months of cumulative CoGeNT data are examined for indications of an annual modulation, a predicted signature of Weakly Interacting Massive Particle (WIMP) interactions. Presently available data support the presence of a modulated component of unknown origin, with parameters prima facie compatible with a galactic halo composed of light-mass WIMPs. Unoptimized estimators yield a statistical significance for a modulation of ~2.8 sigma, limited by the short exposure.

Figures (4)

• Figure 1: Top: Uncorrected (i.e., prior to threshold efficiency correction) spectrum displaying all expected K-shell EC cosmogenic peak positions. The dotted histogram shows the spectrum before rejection of surface background events. Bottom: Uncorrected low-energy spectrum following removal of surface events. Dotted Gaussian peaks show the predicted L-shell EC contribution, devoid of any free parameters (see text). A dashed line traces their envelope. A second dashed line indicates the combined threshold efficiency (trigger + software cuts) ourprl2, an arrow pointing from it to the right scale. Inset: Spectra corrected by this efficiency and stripped of L-shell contribution and flat background component. Examples of light WIMP signals are overlapped on it (see text).
• Figure 2: ROI extracted from the irreducible spectra in Fig. 1 (inset) under consideration of a light-WIMP hypothesis. A small dotted line bisects it, approximately separating the domains favored by the black dot (left) or unfilled circle (right) spectra in Fig. 1. ROI definition and uncertainties able to shift it are described in the text. The DAMA/LIBRA ROI includes present uncertainties in its position prddan, with the exception of ion channeling channeling, conservatively assumed to be absent. Solid and dotted lines are CDMS limits from CDMSSUF and CDMSsoudan, respectively. A dashed line corresponds to recent XENON100 claims lastXENON. Uncertainties in these constraints and those by XENON10 peter are examined in cdmscritcomp.
• Figure 3: Assessment of the stability of the CoGeNT PPC at Soudan (see text). First panel: daily average in detector electronic noise (shaping time 10 $\mu$s). Second panel: stability of the trigger threshold level. Third panel: negligible correction to the counting rate in the region 0.5-0.9 keV$_{ee}$ induced by it. Fourth panel: expected counting rate in this same region originating in L-shell EC. The observed stability augurs well for WIMP modulation searches using next-generation PPCs like those planned for the upcoming expansion of CoGeNT (C-4), MAJORANA, GERDA, and CDEX.
• Figure 4: Rate vs. time in several energy regions (the last bin spans 8 days). A dotted line denotes the best-fit modulation. A solid line indicates a prediction for a 7 GeV/c$^{2}$ WIMP in a galactic halo with Maxwellian velocity distribution. Background contamination and/or a non-Maxwellian halo can shift the amplitude of this nominal modulation (see text). Dotted and solid lines overlap for the bottom panels.