The DAMA/LIBRA experiment[1] is an experiment designed to detect dark matter using the direct detection technique. It is located at the Laboratori Nazionali del Gran Sasso in Italy. The successor to the DAMA/NaI experiment, it uses very similar detector technology but has a larger target mass of 250 kg. DAMA/LIBRA has been collecting data since 2003 and first results were reported in 2008.[2]
The DAMA/LIBRA setup consists of scintillation thallium-doped sodium iodide (NaI) radioactivity pure crystals. Nuclei recoiling after a collision cause emissions of photons that are detected using photomultiplier tubes. A detected nuclear recoil can be caused by dark matter particles or by the background (thermal neutrons, radioactivity or cosmic radiation). The revolution of the Earth around the Sun can cause an annual modulation of the dark matter flux. This should give rise to an annual modulation in the detected recoils and thus provides a simple way to extract a dark matter signal from the background.
The DAMA/LIBRA and the DAMA/NaI experiments have used this technique and are the only ones to have reported a positive signal. The results of this experiment are controversial because other searches[3] have not detected nuclear recoils due to dark matter interactions. All these other searches use sophisticated background elimination techniques instead of the annual modulation technique. An independent check[4] of the DAMA/LIBRA signal using the Super-Kamiokande neutrino detector is proposed.
The signal itself[2] comprise annual modulation of recoil events frequency in the 2 keV – 6 keV energy bin with modulation depth 2% and maximum around 2nd of June. Signal to noise level is high enough to exclude zero modulation hypothesis, although it is unclear if it enough to rule out Solar origin of modulation, because in case of the Solar origin the modulation maximum will be close to the 22nd of June. Independent experiment in the south hemisphere may prove or disprove an extrasolar origin of the modulation.
Early explanations for the DAMA/LIBRA results includes mirror matter[5] because of the unexpected low energy of anomalous recoil events. Recently some papers[6] suggest that results from DAMA/LIBRA are incompatible with the Weakly interacting massive particle (WIMP) models, so introduction of the daemon concept may be necessary. More conventional explanations using the dark matter concept include an exotic selection of dark matter clustering parameters.[7]