Spin-independent elastic WIMP scattering and the DAMA annual modulation signal

TL;DR

This work tests whether DAMA's annual modulation can be explained by spin-independent elastic WIMP scattering with light masses, incorporating channeling and the spectral shape of the modulation. Using the standard isothermal halo, the DAMA-preferred region near $m_χ\approx12$ GeV and $\sigma_p\approx1.3\times10^{-41}$ cm$^2$ is strongly incompatible with constraints from CDMS and XENON, with a global fit yielding $p$-values around $10^{-5}$. The authors further explore non-standard halos (e.g., Via Lactea) and find only modest improvements, while extreme halo properties (e.g., very low velocity dispersion and strong radial anisotropy) can somewhat alleviate tensions but rely on arguably unrealistic astrophysical configurations. They also show that updated XENON calibrations (notably $L_{\text{eff}}$) modestly reduce the discrepancy. Overall, the standard spin-independent light-WIMP interpretation of the DAMA signal is disfavored, highlighting the critical role of halo modeling, channeling, and spectral information in direct-detection analyses.

Abstract

We discuss the interpretation of the annual modulation signal seen in the DAMA experiment in terms of spin-independent elastic WIMP scattering. Taking into account channeling in the crystal as well as the spectral signature of the modulation signal we find that the low-mass WIMP region consistent with DAMA data is confined to WIMP masses close to $m_χ\simeq 12$ GeV, in disagreement with the constraints from CDMS and XENON. We conclude that even if channeling is taken into account this interpretation of the DAMA modulation signal is disfavoured. There are no overlap regions in the parameter space at 90% CL and a consistency test gives the probability of $1.2\times 10^{-5}$. We study the robustness of this result with respect to variations of the WIMP velocity distribution in our galaxy, by changing various parameters of the distribution function, and by using the results of a realistic N-body dark matter simulation. We find that only by making rather extreme assumptions regarding halo properties can we obtain agreement between DAMA and CDMS/XENON.

Figures (6)

• Figure 1: Allowed regions at 90% and 99.73% CL for WIMP mass and scattering cross section on nucleon for DAMA, and exclusion contours for CDMS-Si, CDMS-Ge and XENON at 90% CL. We also display the limit from CoGeNT extracted from figure 2 of Aalseth:2008rx. The global best fit for DAMA is marked with a star, the allowed region around $m_\chi \simeq 50$ GeV is defined with respect to the local minimum, which is marked with a dot. For DAMA we show the regions obtained from using only the modulation amplitude for 2--6 keVee (gray curves) and from using the spectral shape of the modulation signal (shaded regions). For parameters above the dashed curve the predicted number of events in DAMA/LIBRA is larger than the observed number of events.
• Figure 2: Left: Energy distribution of the annual modulation amplitude from DAMA/NaI and DAMA/LIBRA data extracted from figure 9 of Bernabei:2008yi (points with error bars), together with the prediction for three examples of WIMP masses and scattering cross sections (curves). Right: Energy distribution of the time averaged rate observed in DAMA/LIBRA extracted from figure 1 of Bernabei:2008yi (points), together with the prediction for two examples of WIMP masses and scattering cross sections (thick curves) as well as the corresponding un-identified background (thin curves). The data are corrected for the energy dependent efficiency.
• Figure 3: The parameters $\alpha_i$ and $f_i$ explained in the text fitted to the radial and tangential velocity dispersions of dark matter at different radii from the centre of the Via Lactea simulation. The vertical lines indicate the position of the Sun at $r=8.5$ kpc. The velocity dispersions are clearly non-Gaussian as one approaches the centre of the galaxy.
• Figure 4: Allowed regions at 90% and 99.73% CL for DAMA, and exclusion contours for CDMS-Si, CDMS-Ge and XENON at 90% CL for the DM halo obtained in the Via Lactea simulation (a), an isotropic Maxwellian halo with dispersion $\bar{v} = 110$ km/s (b), an asymmetric Maxwellian halo with dispersion $\bar{v}_R = 142$ km/s in the radial direction and $\bar{v}_T = 63$ km/s in the tangential direction (c), and an isotropic Maxwellian halo with dispersion $\bar{v} = 220$ km/s and escape velocity $v_\mathrm{esc} = 450$ km/s (d). The best fit for DAMA is marked with a star. In the panels (b) and (c) we show also the 90% and 99.73% CL regions for the global data combining all experiments, as well as the global best fit point (marked with a dot).
• Figure 5: Here we plot the radial velocity dispersion $\bar{v}_R$ at the solar radius $r=8.5$ kpc as a function of the concentration of the dark matter halo $r_{vir}/a$ for two different dark matter profiles. We have assumed that the velocity dispersion anisotropy parameter $\beta_{vel}=0.8$ and is a constant with respect to radius. The horizontal line corresponds to the value of $\bar{v}_R$ which helps explain the discrepancy. It appears that only for sets of halo parameters such as $(\alpha,\beta,\gamma)=(1,4,1.5)$ can one reconcile such a high value of $\beta$ with a low enough radial velocity dispersion to help explain the discrepancy between DAMA and XENON/CDMS.