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