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Register a new userInvestigating Earth shadowing effect with DAMA/LIBRA-phase1
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R. Bernabei
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In the present paper the results obtained in the investigation of possible diurnal effects for low-energy single-hit scintillation events of DAMA/LIBRA-phase1 (1.04 ton $times$ yr exposure) have been analysed in terms of an effect expected in case of Dark Matter (DM) candidates inducing nuclear recoils and having high cross-section with ordinary matter, which implies low DM local density in order to fulfill the DAMA/LIBRA DM annual modulation results. This effect is due to the different Earth depths crossed by those DM candidates during the sidereal day.
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The results obtained in the search for possible diurnal effect in the single-hit low energy data collected by DAMA/LIBRA-phase1 (total exposure: 1.04 ton x yr) deep underground at the Gran Sasso National Laboratory (LNGS) of the I.N.F.N. are presented. At the present level of sensitivity the presence of any significant diurnal variation and of diurnal time structures in the data can be excluded for both the cases of solar and sidereal time. In particular, the diurnal modulation amplitude expected, because of the Earth diurnal motion, on the basis of the DAMA Dark Matter annual modulation results is below the present sensitivity.
The results obtained with the total exposure of 1.04 ton x yr collected by DAMA/LIBRA-phase1 deep underground at the Gran Sasso National Laboratory (LNGS) of the I.N.F.N. during 7 annual cycles (i.e. adding a further 0.17 ton x yr exposure) are presented. The DAMA/LIBRA-phase1 data give evidence for the presence of Dark Matter (DM) particles in the galactic halo, on the basis of the exploited model independent DM annual modulation signature by using highly radio-pure NaI(Tl) target, at 7.5 sigma C.L.. Including also the first generation DAMA/NaI experiment (cumulative exposure 1.33 ton x yr, corresponding to 14 annual cycles), the C.L. is 9.3 sigma and the modulation amplitude of the single-hit events in the (2-6) keV energy interval is: (0.0112 pm 0.0012) cpd/kg/keV; the measured phase is (144 pm 7) days and the measured period is (0.998 pm 0.002) yr, values well in agreement with those expected for DM particles. No systematic or side reaction able to mimic the exploited DM signature has been found or suggested by anyone over more than a decade.
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.
Several of the many proposed Dark Matter candidate particles, already investigated with lower exposure and a higher software energy threshold, are further analyzed including the first DAMA/LIBRA--phase2 data release, with an exposure of 1.13 ton $times$ yr and a lower software energy threshold (1 keV). The cumulative exposure above 2 keV considering also DAMA/NaI and DAMA/LIBRA--phase1 results is now 2.46 ton $times$ yr. The analysis permits to constraint the parameters space of the considered candidates restricting their values -- with respect to previous analyses -- thanks to the increase of the exposure and to the lower energy threshold.
The DAMA/LIBRA experiment ($sim$ 250 kg of highly radio-pure NaI(Tl)) is running deep underground at the Gran Sasso National Laboratory (LNGS) of the I.N.F.N. Here we briefly recall the results obtained in its first phase of measurements (DAMA/LIBRA--phase1, total exposure: 1.04 ton $times$ yr). DAMA/LIBRA--phase1 and the former DAMA/NaI (cumulative exposure: $1.33$ ton $times$ yr) give evidence at 9.3 $sigma$ C.L. for the presence of DM particles in the galactic halo by exploiting the model-independent DM annual modulation signature. No systematic or side reaction able to mimic the exploited DM signature has been found or suggested by anyone over more than a decade. At present DAMA/LIBRA--phase2 is running with increased sensitivity.
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