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Results from a Search for Light-Mass Dark Matter with a P-type Point Contact Germanium Detector

C. E. Aalseth, P. S. Barbeau, N. S. Bowden, B. Cabrera-Palmer, J. Colaresi, J. I. Collar, S. Dazeley, P. de Lurgio, G. Drake, J. E. Fast, N. Fields, C. H. Greenberg, T. W. Hossbach, M. E. Keillor, J. D. Kephart, M. G. Marino, H. S. Miley, M. L. Miller, J. L. Orrell, D. C. Radford, D. Reyna, R. G. H. Robertson, R. L. Talaga, O. Tench, T. D. Van Wechel, J. F. Wilkerson, K. M. Yocum

TL;DR

The paper reports results from an ultra-low-background PPC Ge detector operated at $2,100~\mathrm{m.w.e}$, achieving a sub-keV threshold and enabling detailed low-energy spectroscopy. A new rise-time based surface-event rejection reduces background by a factor of about $2$–$3$, enabling observation of several cosmogenic peaks from activation products such as $^{71}$Ge ($T_{1/2}=11.4~\mathrm{d}$) and L-shell electron captures (e.g., 1.29 keV, 1.1 keV). The analysis focuses on an irreducible excess of bulk-like events below $3~\mathrm{keVee}$ and discusses interpretations in terms of a light WIMP scenario compatible with the DAMA/LIBRA modulation and CDMS hints, as well as axio-electric couplings for pseudoscalars. The study places tighter constraints on cosmological explanations of DAMA/LIBRA and assesses background contributions from muon- and environment-induced neutrons, concluding that conventional backgrounds remain plausible, with no definitive dark matter claim. It also derives WIMP and axio-electric limits and discusses implications for future PPC-based detectors, notably a $60~\mathrm{kg}$ Majorana Demonstrator that could detect annual modulation if such a signal exists.

Abstract

We report on several features present in the energy spectrum from an ultra low-noise germanium detector operated at 2,100 m.w.e. By implementing a new technique able to reject surface events, a number of cosmogenic peaks can be observed for the first time. We discuss several possible causes for an irreducible excess of bulk-like events below 3 keVee, including a dark matter candidate common to the DAMA/LIBRA annual modulation effect, the hint of a signal in CDMS, and phenomenological predictions. Improved constraints are placed on a cosmological origin for the DAMA/LIBRA effect.

Results from a Search for Light-Mass Dark Matter with a P-type Point Contact Germanium Detector

TL;DR

The paper reports results from an ultra-low-background PPC Ge detector operated at , achieving a sub-keV threshold and enabling detailed low-energy spectroscopy. A new rise-time based surface-event rejection reduces background by a factor of about –, enabling observation of several cosmogenic peaks from activation products such as Ge () and L-shell electron captures (e.g., 1.29 keV, 1.1 keV). The analysis focuses on an irreducible excess of bulk-like events below and discusses interpretations in terms of a light WIMP scenario compatible with the DAMA/LIBRA modulation and CDMS hints, as well as axio-electric couplings for pseudoscalars. The study places tighter constraints on cosmological explanations of DAMA/LIBRA and assesses background contributions from muon- and environment-induced neutrons, concluding that conventional backgrounds remain plausible, with no definitive dark matter claim. It also derives WIMP and axio-electric limits and discusses implications for future PPC-based detectors, notably a Majorana Demonstrator that could detect annual modulation if such a signal exists.

Abstract

We report on several features present in the energy spectrum from an ultra low-noise germanium detector operated at 2,100 m.w.e. By implementing a new technique able to reject surface events, a number of cosmogenic peaks can be observed for the first time. We discuss several possible causes for an irreducible excess of bulk-like events below 3 keVee, including a dark matter candidate common to the DAMA/LIBRA annual modulation effect, the hint of a signal in CDMS, and phenomenological predictions. Improved constraints are placed on a cosmological origin for the DAMA/LIBRA effect.

Paper Structure

This paper contains 1 section, 4 figures.

Table of Contents

  1. ACKNOWLEDGMENTS

Figures (4)

  • Figure 1: Decays associated with $^{71}$Ge ($T_{1/2}=11.4$d) produced via thermal neutron activation of a PPC detector (see text).
  • Figure 2: Top panel: Rise time in preamplifier traces from $^{241}$Am gammas and a manual scan of reference electronic pulser signals (see text). A change in digitizer range is noticeable above $\sim4$ keV. Bottom panel:Idem for 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 BEGe PPC detector, showing the two contacts. Bottom inset: Typical preamplifier traces from 1 keVee events, before and after wavelet denoising, for $t_{10-90}=0.22~ \mu s$ (left) and $t_{10-90}=1.53~ \mu s$ (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 $^{65}$Zn and $^{68}$Ge L-shell EC peaks. Overlapped on the spectrum are the sigmoids for triggering efficiency (dotted), trigger + microphonic PSD cuts (dashed) and trigger + PSD + rise time cuts (solid), obtained via high-statistics electronic pulser calibrations. Also shown are reference signals (exponentials) from 7 GeV/c$^{2}$ and 10 GeV/c$^{2}$ WIMPs with spin-independent coupling $\sigma_{SI}=10^{-4}$pb.
  • Figure 4: Top panel: 90% C.L. WIMP exclusion limits from CoGeNT overlaid on Fig. 1 from bottino: 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 dan2. 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 bottino2 and their uncertainties, respectively. Models including WIMPs with $m_{\chi}\sim$7-11 GeV/cm$^{2}$ provide a good fit to CoGeNT data (red contour, see text). The relevance of XENON10 constraints in this low-mass region has been questioned dan. Bottom panel: Limits on axio-electric coupling $g_{a\bar{e} e}$ for pseudoscalars of mass $m_{a}$ composing a dark isothermal galactic halo (see text).