Dark Matter particles in the galactic halo: results and implications from DAMA/NaI

TL;DR

The DAMA/NaI collaboration reports a model-independent annual modulation signal in the 2–6 keV single-hit events, with amplitude $(0.0200 \pm 0.0032)$ cpd/kg/keV, period $T \approx 1$ year, and phase $t_0 \approx 152.5$ days, observed over seven annual cycles and $107731$ kg·days. The modulation persists only in single-hit, low-energy events and is inconsistent with a flat or higher-energy background; extensive tests show no known systematic can mimic all signature requirements, supporting a Dark Matter interpretation with a WIMP component in the galactic halo. The paper also surveys corollary model-dependent quests across SI/SD and inelastic scattering under various halo models, highlighting large astrophysical and nuclear uncertainties in translating the signal into particle properties. Looking forward, DAMA/LIBRA, with ~250 kg of radiopure NaI(Tl), aims to increase sensitivity, control systematics, and probe halo substructure and alternative DM scenarios, reinforcing the ongoing effort to characterize the nature of Dark Matter.

Abstract

The DAMA/NaI experiment (about 100 kg highly radiopure NaI(Tl)) was proposed, designed and realised to effectively investigate in a model independent way the presence of a Dark Matter particle component in the galactic halo by exploiting the annual modulation signature. With a total exposure of 107731 kg day, collected over seven annual cycles deep underground at the Gran Sasso National Laboratory of the I.N.F.N., it has pointed out -- at 6.3 sigma C.L. -- an effect which satisfies all the peculiarities of the signature and neither systematic effects nor side reactions able to mimic the signature were found. Moreover, several (but still few with respect to the possibilities) corollary model dependent quests for the candidate particle have been carried out. In this paper the obtained results are summarized and some perspectives are discussed at some extent.

Figures (14)

• Figure 1: Schematic view of the Earth motion around the Sun.
• Figure 2: Experimental residual rate for single-hit events, in the (2--4), (2--5) and (2--6) keV energy intervals as a function of the time elapsed since January 1-st of the first year of data taking. The experimental points present the errors as vertical bars and the associated time bin width as horizontal bars. The superimposed curves represent the cosinusoidal functions behaviours expected for a WIMP signal with a period equal to 1 year and phase at $2^{nd}$ June; the modulation amplitudes have been obtained by best fit. See text. The total exposure is 107731 kg $\cdot$ day.
• Figure 3: Experimental single-hit residual rate in a single annual cycle from the total exposure of 107731 kg $\cdot$ day for the (2--6) keV and (6--14) keV energy intervals. The experimental points present the errors as vertical bars and the associated time bin width as horizontal bars. The initial time is taken at August 7$^{th}$. Fitting the data with a cosinusoidal function with period of 1 year and phase at 152.5 days, the following amplitudes are obtained: $(0.0195 \pm 0.0031)$ cpd/kg/keV and $-(0.0009 \pm 0.0019)$ cpd/kg/keV, respectively. Thus, a clear modulation is present in the lowest energy region, while it is absent just above.
• Figure 4: Power spectrum of the measured single-hit residuals for the (2--6) keV (continuous line) and (6--14) keV (dotted line) energy intervals calculated according to ref. Lomb, including also the treatment of the experimental errors and of the time binning. As it can be seen, the principal mode present in the (2--6) keV energy interval corresponds to a frequency of $2.737 \cdot 10^{-3}$ d$^{-1}$, that is to a period of $\simeq$ 1 year. A similar peak is not present in the (6--14) keV energy interval.
• Figure 5: Distributions of the variable $\frac{S_m-<S_m>}{\sigma}$ (where $\sigma$ is the error associated to the $S_m$) evaluated for each detector, for each annual cycle and each considered energy bin: i) in the region of interest for the observed modulation, (2--6) keV (left panel); ii) including also the energy region just above, (2--14) keV (right panel). See text.