Effect of the galactic halo modeling on the DAMA/NaI annual modulation result: an extended analysis of the data for WIMPs with a purely spin-independent coupling

TL;DR

The paper tackles how Galactic halo modeling uncertainties affect the DAMA/NaI annual modulation result for WIMPs with purely spin-independent couplings. It systematically replaces the standard isothermal halo with a broad set of self-consistent halo models (spherical isotropic/non-isotropic, axisymmetric, and triaxial) and computes the corresponding WIMP velocity distributions using Eddington inversion and analytic axisymmetric solutions, analyzing the data via a maximum-likelihood approach in the (m_W, ξ σ^{(nucleon)}_{scalar}) plane. The study finds that the modulation signature remains favored across all halo classes, with the allowed WIMP mass and cross-section region shifting depending on v0,ρ0, and halo rotation; non-rotating models typically permit up to m_W ≈ 270 GeV, while co-rotating halos extend this to approximately 500–900 GeV for cross-sections of order few ×10^{-9}–2×10^{-8} nbarn. This work provides a quantitative framework to propagate halo-DF uncertainties into direct-detection interpretations and highlights halo rotation as a key factor in expanding the DAMA-compatible parameter space.

Abstract

The DAMA/NaI Collaboration has observed a 4-sigma C.L. model independent effect investigating the annual modulation signature in the counting rate of an NaI(Tl) set-up (total exposure of 57986 kg day) and the implications of this effect have been studied under different model-dependent assumptions. In this paper we extend one of the previous analyses, the case of a WIMP with a purely spin-independent coupling, by discussing in detail the implications on the results of the uncertainties on the dark matter galactic velocity distribution. We study in a systematic way possible departures from the isothermal sphere model, which is the parameterization usually adopted to describe the halo. We specifically consider modifications arising from various matter density profiles, effects due to anisotropies of the velocity dispersion tensor and rotation of the galactic halo. The hypothesis of WIMP annual modulation, already favoured in the previous analysis using an isothermal sphere, is confirmed in all the investigated scenarios, and the effects of the different halo models on the determination of the allowed maximum-likelihood region in the WIMP mass and WIMP-nucleon cross-section are derived and discussed.

Figures (37)

• Figure 1: The quantities $M_{vis}$ (upper panel) and $v_{rot}^{100}\equiv v_{rot}(R = 100 ~{\rm kpc})$ (lower panel) calculated as a function of $\rho_0$ for the halo models summarized in Table \ref{['tab:models']} and for $v_0$=220 km sec$^{-1}$. The different curves correspond, from left to right, to the following halo models: A4, A0, D3, A3, A6, A1, A5, D1, A7, C1, C4, C3, A2, C2 (upper panel); A3, A0, D3, A5, A1, A6, A2, A4, C3, A7, C2, C1 (lower panel: here C4 and D1 are not plotted because indistinguishable from A7). The horizontal lines indicate the constraints discussed in Section \ref{['sec:constraining']}. The B1-7 models have the same density distribution of the corresponding A1-7 models. D2 and D4 have the same distribution as D1 and D3, respectively.
• Figure 2: The same as in Figure \ref{['fig:check_densities_220']} for $v_0=170$ km sec$^{-1}$ (the ordering of all the different curves is maintained).
• Figure 3: The same as in Figure \ref{['fig:check_densities_220']} with $v_0=270$ km sec$^{-1}$ (the ordering of all the different curves is maintained).
• Figure 4: Plot of the 3$\sigma$ annual--modulation region in the plane $\xi \sigma_{\rm scalar}^{\rm (nucleon)}$ versus $m_{W}$ using for the velocity distribution of WIMPs the isothermal sphere model (Model A0, see Table \ref{['tab:models']}). The three panels of the figure correspond to $v_0$=170, 220, 270 km sec$^{-1}$ from left to right. Upper (lower) regions correspond to $\rho_0$=$\rho_0^{min}$ ($\rho_0^{max}$) where $\rho_0^{min}$ and $\rho_0^{max}$ are given in Table \ref{['tab:intervals']}.
• Figure 5: The same as in Fig. \ref{['fig:a0']} for the velocity distribution of model A1 (see table \ref{['tab:models']}).