Are Direct Search Experiments Sensitive to All Spin-Independent Weakly Interacting Massive Particles?

The common analysis of direct searches for spin-independent weakly interacting massive particles (WIMPs) assumes that a spin-independent WIMP couples with the same strength with both nucleons, i.e., that the spin-independent interaction is also fully isospin independent. Though in a fully isospin-dependent interaction scenario the spin-independent WIMP-nucleus cross section is strongly quenched, the leading experiments are still sensitive enough to set limits 1–2 orders of magnitude less stringent than those traditionally presented. In the isospin-dependent scenario the difference between the limits of CDMS-II and ZEPLIN-I is significantly reduced. Here, a model-independent framework is discussed and applied to obtain the current general model-independent limits.

Figure 1

Comparison between the exclusion limits normally presented by the experiments (left) and those for a fully isospin-dependent interaction (right).

Figure 2

Cross section representation survey for $\frac{g_p}{g_n}>0$ (left) and $\frac{g_p}{g_n}<0$ (right) at $M_W=50\mathrm{GeV}/c^2$. The overall allowed regions are shaded. The inset in the right-hand figure shows the shape of the area allowed by CDMS-II alone.

Figure 3

Spin-independent survey in the coupling strength representation. The intersection of all experiments but DAMA/NaI is shaded. The DAMA/NaI allowed region is the outside of the visible contour.

Figure 4

Overall spin-independent limits on $|g_p|$ (left) and $|g_n|$ (right) from the intersection of CDMS-II and either ZEPLIN-I or CRESST-II. In both cases the intersection points have $\frac{g_p}{g_n}<0$.