Do current WIMP direct measurements constrain light relic neutralinos?

TL;DR

This work addresses whether current WIMP direct-detection measurements constrain light neutralinos in SUSY models without gaugino-mass unification. It highlights the crucial role of the Galactic halo velocity distribution by evaluating a broad set of halo models (A–D) and their associated velocity distribution functions to compute the key rate factor $\mathcal{I}(v_{\min})$ that modulates $dR/dE_R$. Using CDMS data, the authors derive upper limits on $\xi \sigma_{\rm scalar}^{(\rm nucleon)}$ across models and demonstrate that limits for light WIMPs are highly sensitive to halo assumptions, with substantial variability at $m_\chi \lesssim 50$ GeV. They conclude that current direct-detection results do not exclude light neutralinos in SUSY without gaugino-universality, though certain extreme halo scenarios could constrain the 7–25 GeV range, underscoring the importance of astrophysical uncertainties and the potential complementarities with indirect searches and DAMA’s modulation region.

Abstract

New upper bounds on direct detection rates have recently been presented by a number of experimental collaborations working on searches for WIMPs. In this paper we analyze how the constraints on relic neutralinos which can be derived from these results is affected by the uncertainties in the distribution function of WIMPs in the halo. Various different categories of velocity distribution functions are considered, and the ensuing implications for supersymmetric configurations derived. We conservatively conclude that current experimental data do not constrain neutralinos of small mass (below 50 GeV).

Figures (7)

• Figure 1: Function ${\cal I}(v_{\rm min})$ for an isothermal sphere (model A0 of Table \ref{['tab:models']}) and $v_{\rm esc}=650$ km sec$^{-1}$. The solid curves (from top to bottom, as seen on the extreme left of the plot) refer to the three values of $v_0$ (and ensuing $\rho_0$) given in Table \ref{['tab:intervals']}: $v_0=170$ km sec$^{-1}$ (top), $v_0=220$ km sec$^{-1}$ (medium), $v_0=270$ km sec$^{-1}$ (bottom). The dashed line shows the modification of the median isothermal case when $v_{\rm esc}=450$ km sec$^{-1}$.
• Figure 2: Values of $v_{\rm min}$ as a function of the nuclear recoil energy $E_R$, for a Ge detector. The different curves refer to WIMP masses of: 10, 20, 50, 100, 200 GeV and 1 TeV, from top to bottom.
• Figure 3: Function ${\cal I}(v_{\rm min})$ for all the galactic model of Table \ref{['tab:models']}, other than the isothermal sphere, for $v_{\rm esc}=650$ km sec$^{-1}$. The label which identifies the model is written in the bottom--left corner of each panel. Notations are as in Fig. \ref{['fig:ivmin_isothermal']}. In panels (g) and (h), which correspond to axisymmetric models, the dotted and dashed lines refer to maximal galactic co--rotation and counter--rotation, respectively. In panel (h), the long--dashed line shows the modification of the $v_0=270$ km sec$^{-1}$ case when $v_{\rm esc}=450$ km sec$^{-1}$.
• Figure 4: The solid lines show the upper limit on the quantity $\xi \sigma_{\rm scalar}^{(\rm nucleon)}$ as a function of the WIMP mass $m_\chi$ for the CDMS detector and for an isothermal sphere (model A0 of Table \ref{['tab:models']}). The curves refer to the three values of $v_0$ (and corresponding $\rho_0$) given in Table \ref{['tab:intervals']}: $v_0=170$ km sec$^{-1}$ (top), $v_0=220$ km sec$^{-1}$ (medium), $v_0=270$ km sec$^{-1}$ (bottom). The colored region shows the values of $\xi \sigma_{\rm scalar}^{(\rm nucleon)}$ for neutralino dark matter, obtained in a scan of the minimal supersymmetric model defined in Refs. lowneulowmassdirlowindlowpbar. The funnel for low neutralino masses (below 50 GeV) corresponds to supersymmetric models without gaugino--mass unification.
• Figure 5: Ratio of the upper limits of Fig. \ref{['fig:limits_isothermal']}, obtained for an isothermal sphere (model A0). The upper curve is the ratio between the upper and the central curves in Fig. \ref{['fig:limits_isothermal']}. The lower curve is the ratio between the lower and central curves