First results from DAMA/LIBRA and the combined results with DAMA/NaI

TL;DR

The paper tests the model-independent annual modulation signature as direct evidence for Dark Matter in the galactic halo using the DAMA/LIBRA NaI(Tl) detector array, with support from previous DAMA/NaI data. The analysis finds a cosine-like modulation in the 2–6 keV single-hit events with a ~1-year period and ~June 2 phase, attaining 8.2 sigma significance when combined with DAMA/NaI. Extensive systematic studies—including temperature stability, background sources (neutrons, Radon), noise, calibration, and efficiencies—find no plausible alternatives that could mimic the signal. The results provide robust, high-confidence evidence for Dark Matter within the Galactic halo, while planning for larger exposures and dedicated model-dependent interpretations in future work.

Abstract

The highly radiopure $\simeq$ 250 kg NaI(Tl) DAMA/LIBRA set-up is running at the Gran Sasso National Laboratory of the I.N.F.N.. In this paper the first result obtained by exploiting the model independent annual modulation signature for Dark Matter (DM) particles is presented. It refers to an exposure of 0.53 ton$\times$yr. The collected DAMA/LIBRA data satisfy all the many peculiarities of the DM annual modulation signature. Neither systematic effects nor side reactions can account for the observed modulation amplitude and contemporaneously satisfy all the several requirements of this DM signature. Thus, the presence of Dark Matter particles in the galactic halo is supported also by DAMA/LIBRA and, considering the former DAMA/NaI and the present DAMA/LIBRA data all together (total exposure 0.82 ton$\times$yr), the presence of Dark Matter particles in the galactic halo is supported at 8.2 $σ$ C.L..

Figures (22)

• Figure 1: Cumulative low-energy distribution of the single-hit scintillation events (that is each detector has all the others as veto), as measured by the DAMA/LIBRA detectors in an exposure of 0.53 ton $\times$ yr. The energy threshold of the experiment is 2 keV and corrections for efficiencies are already applied.
• Figure 2: Model-independent residual rate of the single-hit scintillation events, measured by the new DAMA/LIBRA experiment in the (2 -- 4), (2 -- 5) and (2 -- 6) keV energy intervals as a function of the time. The residuals measured by DAMA/NaI and already published in ref. RNCijmd are also shown. The zero of the time scale is January 1$^{st}$ of the first year of data taking of the former DAMA/NaI experiment. 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 $A \cos \omega(t-t_0)$ with a period $T = \frac{2\pi}{\omega} = 1$ yr, with a phase $t_0 = 152.5$ day (June 2$^{nd}$) and with modulation amplitudes, $A$, equal to the central values obtained by best fit over the whole data, that is: $(0.0215 \pm 0.0026)$ cpd/kg/keV, $(0.0176 \pm 0.0020)$ cpd/kg/keV and $(0.0129 \pm 0.0016)$ cpd/kg/keV for the (2 -- 4) keV, for the (2 -- 5) keV and for the (2 -- 6) keV energy intervals, respectively. See text. The dashed vertical lines correspond to the maximum of the signal (June 2$^{nd}$), while the dotted vertical lines correspond to the minimum. The total exposure is 0.82 ton$\times$yr.
• Figure 3: Modulation amplitudes of each single year of DAMA/NaI and DAMA/LIBRA experiments in the (2 -- 4), (2 -- 5) and (2 -- 6) keV energy intervals. The same time scale as in Fig. \ref{['fig1']} is adopted. The solid horizontal lines shows the central values obtained by best fit over the whole data set (see Fig. \ref{['fig1']}). The $\chi^2$ test and the run test accept the hypothesis at 90% C.L. that the modulation amplitudes are normally fluctuating around the best fit values. See text.
• Figure 4: Power spectrum of the measured single-hit residuals for the (2--6) keV (solid lines) and (6--14) keV (dotted lines) energy intervals calculated according to ref. Lomb, including also the treatment of the experimental errors and of the time binning. The data refer to: left - just to the DAMA/LIBRA data; right - to the cumulative DAMA/NaI and DAMA/LIBRA data; the case of DAMA/NaI has been given in ref. RNCijmd. As it can be seen, the principal mode present in the (2--6) keV energy interval corresponds to a frequency of $2.705 \times 10^{-3}$ d$^{-1}$ and $2.737 \times 10^{-3}$ d$^{-1}$, respectively (vertical lines); that is, they correspond to a period of $\simeq$ 1 year. A similar peak is not present in the (6--14) keV energy interval just above.
• Figure 5: Experimental residuals in the (2 -- 6) keV region and those in the (6 -- 14) keV region just above for the DAMA/LIBRA data considered as collected in a single annual cycle. 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$^{th}$. The clear modulation is present in the lowest energy interval, while it is absent just above. See text.