Can WIMP spin dependent couplings explain DAMA data, in light of null results from other experiments?

We examine whether the annual modulation found by the DAMA dark matter experiment can be explained by Weakly Interacting Massive Particles (WIMPs), in light of new null results from other experiments. CDMS II has already ruled out most WIMP-nucleus spin-independent couplings as an explanation for DAMA data. Hence we here focus on spin-dependent (axial vector; SD) couplings of WIMPs to nuclei. We expand upon previous work by (i) considering the general case of coupling to both protons and neutrons and (ii) incorporating bounds from all existing experiments. We note the surprising fact that CMDS II places one of the strongest bounds on the WIMP-neutron cross section, and show that SD WIMP-neutron scattering alone is excluded. We also show that SD WIMP-proton scattering alone is allowed only for WIMP masses in the 5–13 GeV range. For the general case of coupling to both protons and neutrons, we find that, for WIMP masses above 13 GeV and below 5 GeV, there is no region of parameter space that is compatible with DAMA and all other experiments. In the range (5–13) GeV, we find acceptable regions of parameter space, including ones in which the WIMP-neutron coupling is comparable to the WIMP-proton coupling.

Figure 1

WIMP-proton cross-section limits for the case $a_n=0$. The CDMS Si + Ge limit overlaps that of Ge only; the Si is relatively insensitive in this case. Super-K only analyzed their data for WIMP masses above 18 GeV; their limit is taken from Fig. 14 of 14. Baksan analyzed fluxes down to 13 GeV. Super-K and Baksan rule out the DAMA results over their analysis ranges and CRESST I limits DAMA at low masses, but WIMPs between 6–13 GeV are consistent with all results for this case.

Figure 2

WIMP-neutron cross-section limits for the case $a_p=0$. SIMPLE limits are taken from Fig. 1 of 7. The addition of the Si data clearly benefits CDMS at lower masses, ruling out DAMA for this simple case.

Figure 3

Spin-dependent allowed couplings at several different WIMP masses (in different panels). The legends in the figure indicate regions allowed by different experiments. The thin black horizontal bands (ellipses at higher masses) show the region allowed by DAMA (note that the black bands are indicated as white for regions allowed by all experiments); the allowed couplings for DAMA are those that fall on the bands, not between them. The dark (blue) vertical regions correspond to couplings allowed by the CDMS I Si and CDMS II Ge data sets. The remaining (pink) horizontal region, labeled as other in the legend, represents couplings allowed by the combined null results of Super-K, Baksan, Elegant V, CRESST, DAMA/Xe-2, and Edelweiss (couplings outside of this region are ruled out by at least one of these experiments). The region allowed by all experiments is indicated by white bands. Only inside these white bands is the DAMA signal consistent with the null results from other experiments. For masses above 13 GeV there is no consistent region (no white band).

Figure 4

Spin-dependent couplings allowed by CDMS at a WIMP mass of 10 GeV. The separate limits from Si and Ge each produce extremely long ellipses; the combined limit is significantly smaller.

Figure 5

Spin-dependent allowed couplings at 5 GeV for DAMA (black elliptical band), CDMS (vertical region), and CRESST I (angled region). The CDMS and CRESST I null results only allow for couplings within the intersection of their corresponding allowed regions (hatched region); this intersection does not include any couplings that can reproduce the DAMA observed modulation. Below 5 GeV, the CDMS/CRESST I constraint grows rapidly stronger relative to the DAMA required couplings. Note the axes are not at the same scale.