Experimental Constraints on a Dark Matter Origin for the DAMA Annual Modulation Effect

A claim for evidence of dark matter interactions in the DAMA experiment has been recently reinforced. We employ a new type of germanium detector to conclusively rule out a standard isothermal galactic halo of weakly interacting massive particles as the explanation for the annual modulation effect leading to the claim. Bounds are similarly imposed on a suggestion that dark pseudoscalars might lead to the effect. We describe the sensitivity to light dark matter particles achievable with our device, in particular, to next-to-minimal supersymmetric model candidates.

Figure 1

Improvements in threshold and resolution in a PPC design (bottom), compared to a typical coaxial HPGe [34] (top). Cosmogenic peaks are clearly resolved in the PPC spectrum. BE stands for binding energy, EC for electron capture.

Figure 2

Parameter space region (crosshatched) able to explain the DAMA modulation via spin-independent couplings from an isothermal light-WIMP halo [5]. Lines delimit the coupling (${\sigma }_{\mathrm{SI}}$) vs WIMP mass ($m_{\chi }$) regions excluded by relevant experiments [5]. All regions are defined at the 90% confidence level. Inset: PPC spectral region (0.47–3.94 keV) used for the extraction of present limits. The solid line is the best fit (null hypothesis) to the data. Other lines display the signals expected from some reference WIMP candidates (Dotted line: $m_{\chi }=8\mathrm{GeV}/c^2$, ${\sigma }_{\mathrm{SI}}={10}^{-4}\mathrm{pb}$. Dashed line: $m_{\chi }=6\mathrm{GeV}/c^2$, ${\sigma }_{\mathrm{SI}}=0.002\mathrm{pb}$. Dash-dotted line: $m_{\chi }=4\mathrm{GeV}/c^2$, ${\sigma }_{\mathrm{SI}}={10}^{-2}\mathrm{pb}$). The modest maximum recoil energy imparted by these light WIMPs imposes a requirement for the lowest possible detector threshold.

Figure 3

Hatched region: viable parameter space in an interpretation of the DAMA modulation involving an axio-electric coupling $g_{a\overline{e}e}$ from pseudoscalars composing a dark isothermal halo, according to [20]. The validity of this interpretation is now challenged [22] (see text). The solid line indicates present limits, dotted lines recent astrophysical bounds [35]. Inset: expected pseudoscalar interaction rates in Ge and NaI, for a fixed value of $g_{a\overline{e}e}$, as a function of pseudoscalar mass $m_a$.

Figure 4

Solid lines: spin-independent sensitivity from leading experiments in the WIMP low-mass region. A theoretically favored NMSSM phase space is denoted by crosses. Dashed lines: predicted sensitivity for PPC HPGe in a number of scenarios [7]: (a) expected reduction in background from cryostat upgrade, (b) background reduction to best achieved in HPGe [36] plus an improvement to 100 eV threshold, (c) very conservative limiting sensitivity imposed by $\sim 15\mathrm{d}$ of cosmogenic ${}^3\mathrm{H}$ production at sea level for the Majorana demonstrator array (a best estimate represents a $\times 10$ further improvement).