The dark matter: DAMA/LIBRA and its perspectives
  Experimental observations and theoretical arguments point out that Dark
Matter (DM) particles are one of the most prominent component of the Universe.
This motivated the pioneer DAMA experiment to investigate the presence of these
particles in the galactic halo, by exploiting the model independent signature
of the DM annual modulation of the rate and very highly radio-pure apparatus in
underground site. In this paper the results obtained by other two annual cycles
of DAMA/LIBRA-phase2 are presented and the long-standing model-independent
annual modulation effect measured by DAMA deep underground at the Gran Sasso
National Laboratory (LNGS) of the I.N.F.N. with different experimental
configurations is summarized. The improved experimental configuration of
DAMA/LIBRA-phase2, $\simeq$ 250 kg highly radio-pure NaI(Tl), allowed to lower
the software energy threshold. The total exposure of DAMA/LIBRA-phase2 over 8
annual cycles is 1.53 ton $\times$ yr. DAMA/LIBRA-phase2 confirms the evidence
of a signal that meets all the requirements of the model independent Dark
Matter annual modulation signature, at 11.8 $\sigma$ C.L. in the energy region
(1-6) keV. In the energy region between 2 and 6 keV, where data are also
available from DAMA/NaI and DAMA/LIBRA-phase1 (2.86 ton $\times$ yr), the
achieved C.L. is 13.7 $\sigma$; the modulation amplitude of the single-hit
scintillation events is: $(0.01014 \pm 0.00074)$ cpd/kg/keV, the measured phase
is $(142.4 \pm 4.2)$ days and the measured period is $(0.99834 \pm 0.00067)$
yr, all these values are well in agreement with those expected for DM
particles. No systematics or side reaction able to mimic the exploited DM
signature (i.e. to account for the whole measured modulation amplitude and to
simultaneously satisfy all the requirements of the signature), has been found
or suggested by anyone throughout some decades thus far.
