First model independent results from DAMA/LIBRA-phase2

Abstract

The first model independent results obtained by the DAMA/LIBRA-phase2 experiment are presented. The data have been collected over 6 annual cycles corresponding to a total exposure of 1.13 ton × yr, deep underground at the Gran Sasso National Laboratory (LNGS) of the I.N.F.N. The DAMA/LIBRA-phase2 apparatus, ≃ 250 kg highly radio-pure NaI(Tl), profits from a second generation high quantum efficiency photomultipliers and of new electronics with respect to DAMA/LIBRA-phase1. The improved experimental configuration has also allowed to lower the software energy threshold. New data analysis strategies are presented. The DAMA/LIBRA-phase2 data confirm the evidence of a signal that meets all the requirements of the model independent Dark Matter (DM) annual modulation signature, at 9.5 σ 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 (exposure 1.33 ton × yr, collected over 14 annual cycles), the achieved C.L. for the full exposure (2.46 ton × yr) is 12.9 σ; the modulation amplitude of the single-hit scintillation events is: (0.0103 ± 0.0008) cpd/kg/keV, the measured phase is (145 ± 5) days and the measured period is (0.999 ± 0.001) 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. 1

Annual modulations from secular variations: not relaxing DAMA?

“…The results of the DAMA/NaI and DAMA/LIBRA experiments are interpreted by the DAMA collaboration as a strong evidence of the presence of Dark Matter (DM) particles in the galactic halo [1][2][3][4][5][6]. This conclusions derive from the compatibility of the annually modulated rate with a sinusoidal signal, characterized by a phase and a period in agreement with those expected from the interaction of DM particles.…”

“…The single-hit scintillation events rate is used to look for a possible signal of DM interactions with matter over the large backgrounds due to natural radioactivity of the detector and surrounding environment. The event rate, expressed in in cpd/kg/keVee (where 'ee' stands for electron equivalent), has only been published [2,4,6] after the subtraction of its time-average over each cycle (of roughly one-year duration) following the procedure outlined in the previous section. Each cycle starts every year roughly around the beginning of September.…”

“…In a recent paper [7] it has been discussed the possibility that, due to the analysis procedure followed by the DAMA collaboration, the observed annual modulation could be reproduced by a slowly varying time-dependent background. This possibility has been used to interpret the modulation of residuals of the single-hit scintillation rate as a function of time published by the DAMA collaboration [2,4,6].…”

Annual modulation in direct dark matter searches

“…Physics data was taken with DAMA/NaI from 1995 -2002 and with DAMA/LIBRAphase I from 2003 -2010 [84], continuing with DAMA/LIBRA-phase II from 2011 onward [85]. A model independent analysis [85] of twenty years of data confirms a modulation of an event rate in the (2 − 6) keV ee region over 14 annual cycles from DAMA/LIBRA phase I and II, as shown in Figure 2. A corollary analysis of this data [86] reduces the software energy threshold from 2 to 1 keV ee for the first six annual cycles and confirms the annual modulation at a 12.9σ detection level.…”

“…‡ Beware that the total event rate includes the constant WIMP contribution as well as the background in this energy region. Figure 2: Variations in the event rate of the two DAMA/LIBRA phases from [85]. In both cases, only single interactions in the detectors are taken into account.…”