DAMA/LIBRA

The DAMA/LIBRA experiment[1] is a particle detector experiment designed to detect dark matter using the direct detection approach, by using a matrix of NaI(Tl) scintillation detectors to detect dark matter particles in the galactic halo. The experiment aims to find an annual modulation of the number of detection events, caused by the variation of the velocity of the detector relative to the dark matter halo as the Earth orbits the Sun. It is located underground at the Laboratori Nazionali del Gran Sasso in Italy.

It is a follow-on to the DAMA/NaI experiment which observed an annual modulation signature over 7 annual cycles (1995-2002).

Detector

The detector is made of 25 highly radiopure scintillating thallium-doped sodium iodide (NaI(Tl)) crystals placed in a 5 by 5 matrix. Each crystal is coupled to two low background photomultipliers. The detectors are placed inside a sealed copper box flushed with highly pure nitrogen; to reduce the natural environmental background the copper box is surrounded by a low background multi-ton shield. In addition, 1 m of concrete, made from the Gran Sasso rock material, almost fully surrounds this passive shield. The installation has a 3-level sealing system which prevents environmental air reaching the detectors. The whole installation is air-conditioned and several operative parameters are continuously monitored and recorded.

DAMA/LIBRA was upgraded in 2008 and in 2010.[2] In particular, after the upgrade in 2010 the experiment entered in its phase 2, with an increase of the set-up’s sensitivity thanks to the lowering of the energy threshold. The DAMA/LIBRA-phase 2 is in data taking.

Operation and results

DAMA/LIBRA phase 1 data collection started in September 2003. The DAMA/LIBRA released data correspond to 7 annual cycles.[3] Considering these data together with those by DAMA/NaI, a total exposure (1.33 ton × yr) has been collected over 14 annual cycles. This experiment has further confirmed the presence of a model-independent annual modulation effect in the data in 2-6 keV range that satisfy all the features expected for a dark matter signal with high statistical significance.

As previously done for DAMA/NaI, careful investigations on absence of any significant systematics or side reaction in DAMA/LIBRA have been quantitatively carried out.[3][4]

The obtained model independent evidence is compatible with a wide set of scenarios regarding the nature of the dark matter candidate and related astrophysical and particle physics.[5][6]

Interpretation and comparisons

The results can be compared with the CoGeNT signal[7][8][9][10] and other experiment limits to evaluate interpretations as WIMPs,[11] neutralino,[12] and other models. However the CoGeNT-signal has since been shown to have resulted from unaccounted background from surface effects; after accounting for this background, the CoGeNT-signal has been shown to be compatible with null results (that is, no signal at all).

The COSINE-100 collaboration has been working in Korea towards confirming or refuting the DAMA-signal. They are using similar experimental setup to DAMA (NaI(Tl)-crystals). They published their results in December 2018 in the journal Nature; their result rules out spin-independent WIMP–nucleon interactions as the cause of the annual modulation observed by the DAMA collaboration. [13]

A possible explanation of the reported modulation was pointed out as originating from the data analysis procedure. A yearly subtraction of the constant component can give rise to a sawtooth residual in the presence of a slower time dependence.[14]

SABRE

The obvious criticism of the seasonal variation of events recorded in the DAMA/LIBRA experiment is that it is in fact due to some purely seasonal effect unconnected with WIMPs. Although the deep underground location minimizes temperature swings and other direct sunlight effects, there are annual humidity fluctuations and other non-obvious effects. At moment, all these criticisms are taken in account by DAMA collaboration in analysis of the experimental data and they have been excluded, as discussed in published results. A repetition of this experiment in the Southern Hemisphere with the variation in phase with DAMA/LIBRA would discount this objection; if on the other hand variation was detected in the Southern Hemisphere that was 6 months out of phase with DAMA/LIBRA, then the seasonal variation objection would be upheld.

Improved versions of DAMA/LIBRA, named SABRE (Sodium-iodide with Active Background REjection) are under construction in two places. One is at LNGS, and the other is in Australia at the Stawell Underground Physics Laboratory,[15] a laboratory being constructed 1025 m below the surface in a gold mine in Stawell, Victoria. First results are expected in 2017.[16]

The construction of the Stawell Underground Physics Laboratory (SUPL) was halted by the shutdown of its host mine in 2016. Construction restarted around one year later and SUPL is, as of October 2019, well-funded with help from the Australian government.

References
  1. R. Bernabei; et al. (2008). "The DAMA/LIBRA apparatus". Nuclear Instruments and Methods in Physics Research A. 592 (3): 297–315. arXiv:0804.2738. Bibcode:2008NIMPA.592..297B. doi:10.1016/j.nima.2008.04.082.
  2. R. Bernabei; et al. (2012). "Performances of the new high quantum efficiency PMTs in DAMA/LIBRA". European Physical Journal C. 7 (3): 03009. arXiv:1002.1028. Bibcode:2012JInst...7.3009B. doi:10.1088/1748-0221/7/03/P03009.
  3. R. Bernabei; et al. (2013). "Final model independent result of DAMA/LIBRA–phase1". European Physical Journal C. 73 (12): 2648. arXiv:1308.5109. Bibcode:2013EPJC...73.2648B. doi:10.1140/epjc/s10052-013-2648-7.
  4. R. Bernabei; et al. (2012). "No role for muons in the DAMA annual modulation results". European Physical Journal C. 72 (7): 2064. arXiv:1202.4179. Bibcode:2012EPJC...72.2064B. doi:10.1140/epjc/s10052-012-2064-4.
  5. A. Bottino; et al. (2012). "Phenomenology of light neutralinos in view of recent results at the CERN Large Hadron Collider". Physical Review D. 85 (9): 095013. arXiv:1112.5666. Bibcode:2012PhRvD..85i5013B. doi:10.1103/PhysRevD.85.095013.
  6. M. R. Buckley; et al. (2011). "Particle Physics Implications for CoGeNT, DAMA, and Fermi". Physics Letters B. 702 (4): 216–219. arXiv:1011.1499. Bibcode:2011PhLB..702..216B. doi:10.1016/j.physletb.2011.06.090.
  7. C.E. Aalseth; et al. (2011). "Results from a Search for Light-Mass Dark Matter with a P-type Point Contact Germanium Detector". Physical Review Letters. 106 (13): 131301. arXiv:1002.4703. Bibcode:2011PhRvL.106m1301A. doi:10.1103/PhysRevLett.106.131301. PMID 21517370.
  8. C.E. Aalseth; et al. (2011). "Search for an Annual Modulation in a P-type Point Contact Germanium Dark Matter Detector". Physical Review Letters. 107 (14): 141301. arXiv:1106.0650. Bibcode:2011PhRvL.107n1301A. doi:10.1103/PhysRevLett.107.141301. PMID 22107183.
  9. M.T. Frandsen; et al. (2011). "On the DAMA and CoGeNT Modulations". Physical Review D. 84 (4): 041301. arXiv:1105.3734. Bibcode:2011PhRvD..84d1301F. doi:10.1103/PhysRevD.84.041301.
  10. Dan Hooper, Chris Kelso (2011). "Implications of CoGeNT's New Results For Dark Matter". Physical Review D. 84 (8): 083001. arXiv:1106.1066. Bibcode:2011PhRvD..84h3001H. doi:10.1103/PhysRevD.84.083001.
  11. A. Liam Fitzpatrick; et al. (2010). "Implications of CoGeNT and DAMA for Light WIMP Dark Matter". Physical Review D. 81 (11): 115005. arXiv:1003.0014. Bibcode:2010PhRvD..81k5005F. doi:10.1103/PhysRevD.81.115005.
  12. A.V. Belikov; et al. (2011). "CoGeNT, DAMA, and Light Neutralino Dark Matter". Physics Letters B. 705 (1–2): 82–86. arXiv:1009.0549. Bibcode:2011PhLB..705...82B. doi:10.1016/j.physletb.2011.09.081.
  13. The COSINE-100 Collaboration (2018). "An experiment to search for dark-matter interactions using sodium iodide detectors". Nature. 564 (7734): 83–86. arXiv:1906.01791. Bibcode:2018Natur.564...83C. doi:10.1038/s41586-018-0739-1. PMID 30518890.
  14. D. Buttazzo; et al. (2002). "Annual modulations from secular variations: relaxing DAMA?". arXiv:2002.00459 [hep-ex].
  15. Roberts, Glenn Jr. (23 October 2014). "Australia's first dark matter experiment". Symmetry Magazine.
  16. "Dark Matter: A Southern Hemisphere Perspective".
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