Xenophobic dark matter

We consider models of xenophobic dark matter, in which isospin-violating dark matter–nucleon interactions significantly degrade the response of xenon direct detection experiments. For models of near-maximal xenophobia, with neutron-to-proton coupling ratio $f_n/f_p\approx -0.64$, and dark matter mass near 8 GeV, the regions of interest for CoGeNT and CDMS-Si and the region of interest identified by Collar and Fields in CDMS-Ge data can be brought into agreement. This model may be tested in future direct, indirect, and collider searches. Interestingly, because the natural isotope abundance of xenon implies that xenophobia has its limits, we find that this xenophobic model may be probed in the near future by xenon experiments. Near-future data from the LHC and Fermi-LAT may also provide interesting alternative probes of xenophobic dark matter.

Figure 1

Regions of interest and exclusion contours in the $(m_X,{\sigma }_p)$ for neutron-to-proton coupling ratios $f_n/f_p=1$ (left), $f_n/f_p=-0.70$ (center) and $f_n/f_p=-0.64$ (right). Plotted are the 90% CL ROIs for CDMS-Si [4], CoGeNT [2], and CDMS-Ge (Collar/Fields) [20], the 90% and $3\sigma$ ROIs for DAMA [1] as determined in Refs. 45, 46, and exclusion contours from XENON100 [5, 6], Edelweiss [47], and CDMS [17], and projected exclusion bounds from LUX [21]. Recent TEXONO [48] and CDEX-1 [49] bounds (not shown) are similar but moderately weaker than CDMS bounds over the mass interval shown. Also plotted are 90% CL exclusion contours from CMS and from the Fermi-LAT, assuming dark matter is either a complex scalar or Dirac fermion coupling only to first generation quarks through an effective contact interaction permitting unsuppressed spin-independent scattering and $S$-wave annihilation. The thin dot-dashed violet and dashed teal lines correspond to the systematic uncertainty in the Fermi-LAT bounds from astrophysical uncertainties for complex scalar and Dirac fermion candidates, respectively. In the center and right panels the CMS complex scalar exclusion bounds exceed the plotted range by between 1 and 2 orders of magnitude, and thus place no constraints on the disputed region.

Figure 2

Ratio of the normalized-to-nucleon cross section reported by direct detection experiments to the true nucleon cross section. Results are shown for ${\sigma }_N^Z/{\sigma }_p=D_p^Z$ (left) and ${\sigma }_Z^N/{\sigma }_n=D_n^Z$ (right) as a function of $f_n/f_p$ for various elements. The entire range of $f_n/f_p$ is shown (top) as well as the xenophobic region (bottom). All plots assume $m_X=8\mathrm{GeV}$, but are highly insensitive to this choice. (a) ${\sigma }_Z^N/{\sigma }_p$, entire $f_n/f_p$ range. (b) ${\sigma }_Z^N/{\sigma }_n$, entire $f_n/f_p$ range. (c) ${\sigma }_Z^N/{\sigma }_p$, xenophobic region. (d) ${\sigma }_Z^N/{\sigma }_n$, xenophobic region.

Figure 3

Proton cross section for various experiments as a function of $f_n/f_p$ for $m_X=8\mathrm{GeV}$. Plotted are slices of the 90% CL ROIs for CDMS-Si [4], CoGeNT [19], and CDMS-Ge (Collar/Fields) [20], the $3\sigma$ ROI for DAMA [1], and exclusion contours for XENON100 [6]. Also plotted are 90% CL exclusion contours for CMS [39] and for the Fermi-LAT [31], assuming dark matter is either a complex scalar or Dirac fermion coupling only to first generation quarks through an effective contact interaction permitting unsuppressed spin-independent scattering and $S$-wave annihilation. The thin dot-dashed violet and dashed teal lines correspond to the systematic uncertainty in the Fermi-LAT bounds from astrophysical uncertainties for complex scalar and Dirac fermion candidates, respectively. (a) Entire $f_n/f_p$ range. (b) Xenophobic region.

Figure 4

Ratio of ${\sigma }_N^Z$ in various experiments to ${\sigma }_N^{\mathrm{Xe}}$. Results are shown as a function of $f_n/f_p$ for scattering off various elements, as well as for LHC and Fermi determinations. In the xenophobic region the behavior of LHC and Fermi bounds for a given operator are visually identical. (a) Entire $f_n/f_p$ range. (b) Xenophobic region.