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 $\times$ yr, deep underground at the Gran Sasso National Laboratory (LNGS) of the I.N.F.N. The DAMA/LIBRA-phase2 apparatus, $\simeq$ 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 $\sigma$ 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 (exposure $1.33$ ton $\times$ yr, collected over 14 annual cycles), the achieved C.L. for the full exposure (2.46 ton $\times$ yr) is 12.9 $\sigma$; the modulation amplitude of the single-hit scintillation events is: $(0.0103 \pm 0.0008)$ cpd/kg/keV, the measured phase is $(145 \pm 5)$ days and the measured period is $(0.999 \pm 0.001)$ 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.

Examination of DAMA/LIBRA–Phase2 Results and Dark Matter Annual Modulation

The paper "First model independent results from DAMA/LIBRA–phase2" presents the results from the DAMA/LIBRA-phase2 experiment, conducted at the Laboratori Nazionali del Gran Sasso (LNGS), which aims to detect dark matter (DM) particles through the annual modulation signature inherent to their interaction with terrestrial targets. The DAMA/LIBRA experimental initiative, including its predecessor DAMA/NaI, seeks to investigate dark matter by observing the predicted yearly variations in detection rates resulting from the Earth's motion relative to the galactic DM halo.

Key Experimental Setup

DAMA/LIBRA–phase2 employs an array of 25 NaI(Tl) scintillation detectors, enhanced with higher efficiency photomultipliers and advanced electronics compared to the prior phase. This configuration enabled reducing the energy threshold to 1 keV, crucial for observing low-energy DM scattering events. The accumulated exposure is 1.13 ton×yr, with the overall cumulative exposure, including DAMA/NaI and DAMA/LIBRA-phase1, reaching 2.46 ton×yr.

Data Analysis and Results

The experiment aims for a model-independent verification of DM-induced annual modulation by analyzing single-hit scintillation events. The data reveal a modulation amplitude of (0.0103±0.0008) cpd/kg/keV in the 2–6 keV energy interval, with a statistical significance of 12.9σ when combined with previous phases. This modulation, exhibited by single-hit events, conforms to the features expected from dark matter interaction:

• An annual modulation following the Earth's orbital dynamics, with a period of approximately one year.
• A phase peaking around June 2, consistent with models predicting maximum DM flux during this time.
• The absence of modulation in multi-hit events, which serve as control data to differentiate background noise.

The reported period and phase of the modulation, verified through various statistical methods such as χ² tests and frequency analysis, strongly substantiate the DM signal hypothesis. Furthermore, extensive checks for systematic effects or background processes capable of mimicking the observed signature have not identified any plausible alternatives.

Implications and Future Directions

These findings reinforce DAMA's longstanding claims of dark matter detection based on annual modulation. This persistent signal, consistent across multiple experimental phases and corroborated by rigorous statistics, underscores the necessity for theoretical frameworks to incorporate scenarios accommodating such results. The analyses suggest compatibility with a multitude of DM models, necessitating further theoretical and experimental explorations to narrow down viable candidates and interaction mechanisms.

With the expansion of detection sensitivity to include lower energy thresholds, future investigation will either bolster or challenge current understandings of dark matter characteristics. Moreover, corroborative endeavors like cross-experimental analyses or upgrades in detection technology will be paramount in establishing a more definitive narrative regarding dark matter's presence and nature.

Conclusion

The DAMA/LIBRA–phase2 findings provide compelling support for the existence of dark matter particles within the galactic halo, utilizing a signature largely unconstrained by specific DM properties. While theoretical and experimental corroboration from other initiatives remains indispensable, DAMA’s results serve as a cornerstone in the pursuit of unraveling dark matter's elusive nature. Continued enhancement of experimental protocols and inter-disciplinary collaborations are crucial steps forward in this complex scientific inquiry.