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Scientific Report 2007-2009
Particle physics
P28. Investigations on particle Dark Matter
with DAMA/LIBRA at Gran Sasso
DAMA/LIBRA (Large sodium Iodide Bulk for RAre
processes) is part of the DAMA project, which is mainly
based on the development and use of low background
scintillators. In particular, the former DAMA/NaI and
the present DAMA/LIBRA set-ups, at the Gran Sasso
National Laboratory, have the main aim to perform a
direct detection of Dark Matter (DM) particles in the
galactic halo using the model independent annual modulation
signature. This signature exploits the effect of the
Earth revolution around the Sun on the number of events
induced by DM particles in a suitable low background
set-up placed deep underground. In particular, as a consequence
of its annual revolution, the Earth should be
crossed by a larger flux of DM particles around roughly
June 2 nd (when its rotational velocity is summed to the
one of the solar system with respect to the Galaxy) and
by a smaller one around roughly December 2 nd (when
the two velocities are subtracted). The annual modulation
signature is very distinctive since the effect induced
by DM particles must simultaneously satisfy many peculiar
requirements (see Ref. [1]).
The former DAMA/NaI experiment has pointed out
a model independent evidence for the presence of DM
particles in the galactic halo with high C.L. The
DAMA/LIBRA set-up, now in data taking, is the result
of a second generation R&D project to develop new
highly radiopure NaI(Tl) detectors (250 kg total mass) to
further investigate Dark Matter particles and other rare
processes. The set-up, its main features and radiopurity
have been discussed in Ref. [2]. After 4 annual cycles
Residuals (cpd/kg/keV)
DAMA/NaI ≈ 100 kg
(0.29 ton×yr)
2-6 keV
DAMA/LIBRA ≈ 250 kg
(0.53 ton×yr)
Time (day)
Figure 1: Model-independent residual rate of the single-hit
scintillation events, measured by DAMA/NaI and the new
DAMA/LIBRA experiments (exposure of 0.82 ton×yr) in
the (2-6) keV energy interval as a function of the time (here
keV is keV electron equivalent). The superimposed curves
represent the cosinusoidal functions behaviors A cos ω(t − t 0)
with a period T = 2π = 1 yr, with a phase t0 = 152.5
ω
day (June 2 nd ) and with modulation amplitudes, A, equal to
the central values obtained by best fit over the whole data:
(0.0129 ± 0.0016) cpd/kg/keV. The dashed vertical lines correspond
to the maximum of the signal (June 2 nd ), while the
dotted vertical lines to the minimum. The χ 2 test disfavors
the hypothesis of unmodulated behavior (A = 0) giving a
probability of 1.8 × 10 −4 (χ 2 /d.o.f. = 116.4/67).
(exposure of 0.53 ton×yr) the model independent result
achieved by the second generation DAMA/LIBRA setup
confirms the evidence of the presence of Dark Matter
particles in the galactic halo with high confidence level;
a cumulative C.L. of 8.2 σ is reached when considering
the data of the former DAMA/NaI experiment and the
ones of DAMA/LIBRA all together (exposure of 0.82
ton×yr) [1]. The collected DAMA/LIBRA data satisfy
Normalized Power
20
18
16
14
12
10
8
6
4
2
0
0 0.002 0.004 0.006 0.008
Frequency (d -1 )
Figure 2: Power spectrum of the measured single-hit residuals
for the (2-6) keV (solid line) and (6-14) keV (dotted line)
energy intervals (exposure of 0.82 ton×yr). The principal
mode in the (2-6) keV energy interval corresponds to a frequency
of 2.737 × 10 −3 d −1 (vertical line); that is to a period
of ≃ 1 year. A similar peak is not present in the (6-14) keV
energy interval just above.
all the many peculiarities of the DM annual modulation
signature. Neither systematic effects nor side reactions
able to account for the observed modulation amplitude
and to contemporaneously satisfy all the several requirements
of this DM signature are available [1].
The collection of a larger exposure with
DAMA/LIBRA will allow the improvement of the
corollary information about the nature of the candidate
particle(s) and about various related astrophysical,
nuclear and particle Physics scenarios as well as the
investigation of the other DM features and of second
order effects with very high sensitivity. Several rare
processes other than DM will also be investigated (see
e.g. Ref. [3]).
References
1. R. Bernabei et al., Eur. Phys. J. C 56, 333 (2008).
2. R. Bernabei et al., Nucl. Instr. Meth. A 592, 297 (2008).
3. R. Bernabei et al., Eur. Phys. J. C 62, 327 (2009).
Authors
D. Prosperi, A. Incicchitti 1 , F. Cappella
http://people.roma2.infn.it/dama
Sapienza Università di Roma 135 Dipartimento di Fisica