UCLA

We reexamine the current direct dark matter (DM) detection data for several types of DM candidates, both assuming the Standard Halo Model (SHM) and in a halo-independent manner. We consider the potential signals for light WIMPs that have appeared in three direct detection searches: DAMA, CDMS-II-Si, and CoGeNT, and we analyze their compatibility with the null results of other direct detection experiments.

We first consider light WIMPs with exothermic scattering with nuclei (exoDM). Exothermic interactions favor light targets, thus reducing the importance of upper limits derived from Xe targets, the most restrictive of which is at present the LUX limit. In our SHM analysis the CDMS-II-Si and CoGeNT regions become allowed by these bounds, however the SuperCDMS limit rejects both regions for exoDM with isospin-conserving couplings. An isospin-violating coupling of the exoDM, in particular one with a neutron to proton coupling ratio of -0.8 (which we call "Ge-phobic"), maximally reduces the DM coupling to Ge and allows the CDMS-II-Si region to become compatible with all upper bounds. This is also clearly shown in our halo-independent analysis.

Next, we extend and correct a recently proposed maximum-likelihood halo-independent method to analyze unbinned direct DM detection data. Instead of the recoil energy as an independent variable, we use the minimum speed a DM particle must have to impart a given recoil energy to a nucleus. This has the advantage of allowing us to apply the method to any type of target composition and interaction, e.g. with general momentum and velocity dependence, and with elastic or inelastic scattering. We prove the method and provide a rigorous statistical interpretation of the results. As first applications, we find that for dark matter particles with elastic spin-independent interactions and neutron to proton coupling ratio $f_n/f_p=-0.7$ ("Xe-phobic", which reduces maximally the coupling to Xe), the WIMP interpretation of the signal observed by CDMS-II-Si is compatible with the constraints imposed by all other experiments with null results. We also find a similar compatibility for exothermic inelastic spin-independent interactions with $f_n/f_p=-0.8$.

Finally, we reexamine the interpretation of the annual modulation signal observed by the DAMA experiment as due to WIMPs with a spin-dependent coupling mostly to protons. We consider both axial-vector and pseudo-scalar couplings, and elastic as well as endothermic and exothermic inelastic scattering. We conclude that the DAMA signal is in strong tension with null results of other direct detection experiments, particularly PICASSO and KIMS.