The DAMA/LIBRA experiment is a particle detector experiment designed to detect dark matter using the direct detection approach, by using a scintillation detector to search for Weakly Interacting Massive Particles (WIMPs) in the galactic halo. The experiment aims to find an annual variation 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 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).
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. In particular, the upgrade in 2010 allows an increase of the set-up’s sensitivity, the lowering of the energy threshold, and several other kinds of investigations.
Operation and results
Data collection started in Sept 2003. The DAMA/LIBRA data released so far correspond to 4 and 7 annual cycles. Considering these data together with those by DAMA/NaI, a total exposure (1.17 ton x yr) has been collected over 13 annual cycles. This experiment has further confirmed the presence of model-independent evidence with high statistical significance on the basis of the dark matter signature. DAMA/LIBRA is sensitive to 2 keV to 20 keV and annual modulation is seen in the 2-6 keV range.
Final phase 1 results were published in 2013, confirming an annual modulation.
Interpretation and comparisons
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 minimized 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 literatures. 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.
An improved version of DAMA/LIBRA, named SABRE (Sodium-iodide with Active Background REjection) is under construction in two copies. One will be installed at LNGS, and the other will be installed in Australia at the Stawell Underground Physics Laboratory, a laboratory being constructed 1025 m below the surface in a gold mine in Stawell, Victoria. First results are expected in 2017.
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