Implications of CoGeNT and DAMA for Light WIMP Dark Matter

TL;DR

The paper investigates whether the CoGeNT excess and DAMA modulation can be explained by a light WIMP with mass in the $5-10$ GeV range and a spin-independent cross section of order $10^{-41}$ cm$^2$, examining compatibility with XENON10 and CDMS under detector and halo-systematic uncertainties. It develops a model-independent operator framework and surveys Asymmetric Dark Matter and NMSSM realizations that can produce the observed direct-detection signals while yielding the correct relic density. It then analyzes indirect-detection implications, showing that neutrino and gamma-ray searches with Super-Kamiokande and FGST are near current sensitivity for testing these models, with charged cosmic rays and white-dwarf heating offering complementary probes. Overall, the CoGeNT/DAMA-compatible region remains viable within reasonable astrophysical and experimental uncertainties and motivates targeted indirect-detection tests.

Abstract

In this paper, we study the recent excess of low energy events observed by the CoGeNT collaboration, and discuss the possibility that these events originate from the elastic scattering of a light (m_DM ~ 5-10 GeV) dark matter particle. We find that such a dark matter candidate may also be capable of generating the annual modulation reported by DAMA, as well as the small excess recently reported by CDMS, without conflicting with the null results from other experiments, such as XENON10. A dark matter interpretation of the CoGeNT and DAMA observations favors a region of parameter space that is especially attractive within the context of Asymmetric Dark Matter models. In such models, the cosmological dark matter density arises from the baryon asymmetry of the universe, naturally leading to the expectation that m_DM ~ 1-10 GeV. We also discuss neutralino dark matter from extended supersymmetric frameworks, such as the NMSSM. Lastly, we explore the implications of such a dark matter candidate for indirect searches, and find that the prospects for detecting the neutrino and gamma ray annihilation products of such a particle to be very encouraging.

Figures (6)

• Figure 1: The regions in the elastic scattering cross section (per nucleon), mass plane in which dark matter provides a good fit to the CoGeNT excess, compared to the region that can generate the annual modulation reported by DAMA at 90% confidence (darker grey regions). In this figure, we have adopted $v_0=270$ km/s and use two values of the galactic escape velocity: $v_{\rm esc}=490$ km/s (left) and $v_{\rm esc}=650$ km/s (right). In calculating the DAMA region, we have neglected the lowest energy bin (the effect of this is shown in later figures) and treated channeling as described in Ref. Bernabei:2007hw. If a smaller fraction of events are channeled in DAMA than is estimated in Ref. Bernabei:2007hw, the DAMA region will move upward, toward the yellow regions (near $\sigma_N\approx 10^{-39.5}$ cm$^2$, which include no effects of channeling), improving its agreement with CoGeNT. Also shown is the 90% C.L. region in which the 2 events observed by CDMS can be produced. If the escape velocity of the galaxy is taken to be relatively large, this region can also approach those implied by CoGeNT and DAMA. Constraints from the null results of XENON10 and the CDMS silicon analysis are also shown. For the XENON10 constraint, we have used the lower estimate of the scintillation efficiency (at $1\sigma$) as described in Ref. Leff.
• Figure 2: Left panel: To illustrate the effect of uncertainties in the XENON10 scintillation factor, $L_{\rm eff}$, we show the CoGeNT and DAMA allowed regions alongside constraints from the new XENON10 analysis using $L_{\rm eff}=0.19$ (light gray dashed), and $L_{eff}$ at the central and lower $1\sigma$ values from the new measurement Leff (the two thick black dashed curves). In between the central and lower $1\sigma$$L_{eff}$ curves we have taken $-\frac{1}{2} \sigma$$L_{eff}$ values in only the three energy bins near XENON10's threshold (black dotted). We also show constraints from the CDMS silicon analysis (red dot-dashed), and the region in which the two events observed by CDMS can be produced (blue long dashed). Here, we have used $v_0=220$km$/$s and $v_{\rm esc}=500$km$/$s. With the lower values of $L_{\rm eff}$ values, the tension between XENON10 and CoGeNT is alleviated. Right panel: To illustrate the effect from changing the halo model, CoGeNT, CDMS, XENON10, and DAMA results are shown, but with $v_0 = 270$km$/$s and $v_{\rm esc}$=490km$/$s.
• Figure 3: Left panel: The DAMA region and XENON10 constraint are the same as in the right panel of Fig. \ref{['fig:noLeff']} but, for illustration, with the effects of removing the lowest bin of data from the DAMA region (gray), or with the effects of increasing the errors in those bins by a factor of two (purple and blue for 90% and 99%). We see that the DAMA region is shifted to smaller masses and becomes more consistent with the XENON10 constraint. Right panel: The same in in the left frame, but with dark matter couplings only to neutrons, $f_n = 1$ and $f_p = 0$. The constraints from CDMS silicon relative to the DAMA region are weaker than for $f_n=f_p$. Only the 90% confidence limit for DAMA removing the lowest bin is shown.
• Figure 4: The effective dark matter-fermion couplings required to generate a given spin-independent elastic scattering cross section with nucleons (dashed lines), and to produce a thermal abundance consistent with the observed dark matter density (solid lines). In each frame, the dotted region represents the approximate range of couplings and masses that leads to a signal consistent with that reported by CoGeNT. The various frames show results for a fermionic dark matter particle with scalar interactions (upper left), pseudoscalar interactions (upper right), vector interactions (center), or a scalar dark matter particle with scalar interactions (lower left), or vector interactions (lower right). In the case of a scalar dark matter particle with approximately universal scalar couplings to Standard Model fermions, the coupling size required to generate the excesses observed by CoGeNT and DAMA is also approximately the size required to thermally generate the observed density of dark matter in the early universe.
• Figure 5: The upper limit on a light WIMP's spin-independent elastic scattering cross section with nuclei from the Super Kamiokande experiment, for various choices of the dominant annihilation channel. Also shown is the region of the plane which provides a good fit to the excess observed by CoGeNT. From this figure, we conclude that Super Kamiokande excludes a dark matter particle which annihilates entirely to neutrinos or taus as a source of the CoGeNT excess. This figure was adapted from Ref. Hooper:2008cf.