New results from DAMA/LIBRA

TL;DR

The paper presents model-independent evidence for Dark Matter in the galactic halo through an annual modulation signal observed by DAMA/LIBRA using NaI(Tl) scintillators. An updated analysis of six DAMA/LIBRA cycles, combined with prior DAMA/NaI data for a total exposure of $1.17$ ton×yr, shows a cosine-like modulation in single-hit events within 2–6 keV with period $T \,=\, 1$ year and phase $t_0 \,\approx\, 146$ days, amplitude $A \,=\, 0.0116 \pm 0.0013$ cpd/kg/keV, and a significance of $8.9\sigma$. The modulation is absent in higher energy ranges and in multiple-hit events, and the power spectrum confirms a dominant annual component in the DM-sensitive region. The results are robust across cycles and compatible with a broad set of DM models, though direct cross-experiment comparisons remain complex; future upgrades aim to lower thresholds and refine the signal interpretation.

Abstract

DAMA/LIBRA is running at the Gran Sasso National Laboratory of the I.N.F.N.. Here the results obtained with a further exposure of 0.34 ton x yr are presented. They refer to two further annual cycles collected one before and one after the first DAMA/LIBRA upgrade occurred on September/October 2008. The cumulative exposure with those previously released by the former DAMA/NaI and by DAMA/LIBRA is now 1.17 ton x yr, corresponding to 13 annual cycles. The data further confirm the model independent evidence of the presence of Dark Matter (DM) particles in the galactic halo on the basis of the DM annual modulation signature (8.9 sigma C.L. for the cumulative exposure). In particular, with the cumulative exposure the modulation amplitude of the single-hit events in the (2 -- 6) keV energy interval measured in NaI(Tl) target is (0.0116 +- 0.0013) cpd/kg/keV; the measured phase is (146 +- 7) days and the measured period is (0.999 +- 0.002) yr, values well in agreement with those expected for the DM particles.

Figures (9)

• Figure 1: Experimental model-independent residual rate of the single-hit scintillation events, measured by DAMA/LIBRA,1,2,3,4,5,6 in the (2 -- 4), (2 -- 5) and (2 -- 6) keV energy intervals as a function of the time. The zero of the time scale is January 1$^{st}$ of the first year of data taking of the former DAMA/NaI experiment modlibra. The experimental points present the errors as vertical bars and the associated time bin width as horizontal bars. The superimposed curves are the cosinusoidal functions behaviors $A \cos \omega(t-t_0)$ 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 including also the exposure previously collected by the former DAMA/NaI experiment: cumulative exposure is 1.17 ton $\times$ yr (see also ref. modlibra and refs. therein). The dashed vertical lines correspond to the maximum expected for the DM signal (June 2$^{nd}$), while the dotted vertical lines correspond to the minimum. See text.
• Figure 2: Power spectrum of the measured single-hit residuals in 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: a) DAMA/LIBRA-1,2,3,4,5,6 (exposure of 0.87 ton $\times$ yr); b) the cumulative 1.17 ton $\times$ yr exposure (DAMA/NaI and DAMA/LIBRA-1,2,3,4,5,6). As it can be seen, the principal mode present in the (2--6) keV energy interval corresponds to a frequency of $2.697 \times 10^{-3}$ d$^{-1}$ and $2.735 \times 10^{-3}$ d$^{-1}$ (vertical lines), respectively in the a) and b) case. 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 3: Distribution of the percentage variations of R$_{90}$ with respect to the mean values for all the detectors in the DAMA/LIBRA-5,6 annual cycles (histogram); the superimposed curve is a gaussian fit.
• Figure 4: Experimental residuals in the (2 -- 6) keV region and those in the (6 -- 14) keV energy region just above for the cumulative 1.17 ton $\times$ yr, 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 of the figure is taken at August 7$^{th}$. The clear modulation satisfying all the peculiarities of the DM annual modulation signature is present in the lowest energy interval, while it is absent just above; in fact, in the latter case the best fitted modulation amplitude is: (0.00007 $\pm$ 0.00077) cpd/kg/keV.
• Figure 5: Experimental residual rates over the six DAMA/LIBRA annual cycles for single-hit events (open circles) (class of events to which DM events belong) and for multiple-hit events (filled triangles) (class of events to which DM events do not belong). They have been obtained by considering for each class of events the data as collected in a single annual cycle and by using in both cases the same identical hardware and the same identical software procedures. The initial time of the figure is taken on August 7$^{th}$. The experimental points present the errors as vertical bars and the associated time bin width as horizontal bars. See text and ref. modlibra. Analogous results were obtained for the DAMA/NaI data ijmd.