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Dark Matter

 For many years there have been many evidences of matter in the Universe not visible in light and other electro-magnetic radiations. The earliest and probably most convincing evidence came from observation of rotational velocities of the visible objects such as clouds of gas and stars in the galaxies, which suggest the presence of invisible matter inside the galaxies, called Dark Matter (DM).

The dark matter is observed in similar way, i.e. through rotation of the visible objects, also in other large scale structures of the universe such as e.g. clusters of galaxies, where mass of DM is much larger than baryonic mass. The most precise determination of the fraction of DM in the universe is performed at present by measurements of the cosmological parameters performed by WMAP satellite experiment and it comprises about 21 - 23 % of the total energy of the universe.

Among many models that have been proposed to explain the nature of DM, one of the most attractive postulates the existence of a yet undiscovered, massive particles. These particles must interact very weakly with nucleons and nuclei otherwise they would have been already discovered. These hypothetical particles fill the whole universe and all universe structures, including clusters of galaxies, galaxies and the solar system. They cross the Earth practically without any interaction. Such kind of a particle appears in a theory called supersymmetry and is named neutralino. It is stable, it does not have an electric charge, and its mass can be typically in the range from several tens of GeV up to 1000 GeV. 

How to search for these elusive DM particles? Suppose you have a sensitive detector deep underground, where particles from outside, such as high energy protons or photons cannot penetrate. For very weakly interacting DM particles the thickness of the Earth is not an obstacle. They easily penetrate through the depth of the Earth and one of them can scatter (with a very small probability) on a nucleon or a nucleus in the detector. The recoil nucleon/nucleus can be observed signaling that a DM particle passed.

This is the principle of search for DM particles in Gran Sasso Laboratory, in which CFA is also involved. Protected from the outside cosmic radiation by the great depth of the Gran Sasso mountain, detectors are hunting for very rare events, the recoil of nucleons/nuclei. CFA is involved into the Dark Matter search through the activity of the participating institutions, particularly the Gran Sasso Laboratory, promoting networking and collaborations between scientists. In the following we will mention some of these experiments.

The WARP experiment uses the liquid Argon at very low temperature (-186 C) to separate the possible interactions of dark matter particles from ordinary background coming from the radioactivity of surrounding materials and from the residual cosmic radiation. Several institutions participate in WARP, including LNGS and L'Aquila University. A somewhat similar experiment is called XENON, it makes use of liquid Xenon instead of Argon. XENON project is lead by a US group based at Columbia University with a participation of LNGS scientist.

Another experimental approach is used by the CRESST experiment, a German-lead experiment with a participation of LNGS. In this case crystals of CaWO4 are used, kept at a temperature very near to the absolute zero. The combined measurement of the crystal vibrations (phonons) induced by the interactions of particles and of the scintillation light enable a powerful selection of DM particle interactions from background.

A different technique to detect Dark Matter particles is employed by the DAMA/LIBRA experiment, a collaboration of Rome Tor Vergata University, Rome La Sapienza University, Gran Sasso and IHEP (China). The detector based on highly radio-pure NaI crystals is designed to measure the expected annual modulation of the DM particle flux. The principle is based on the variation of the velocity of movement of the detector relative to the DM halo, which occurs because of the annual revolution of the  Earth around the Sun and the Sun velocity in the galactic halo.  

The positive modulation effect is seen for 13 annular cycles. The latest results of DAMA/LIBRA presented at TAUP conference in Rome in July 2009 (and also at BEYOND 2010 conference) are summarised as ``a model-independent evidence for presence of Dark Matter particles in the galactic halo ... at 8.9 sigma CL''.  It is worth to note, that there is at present no other experiment in the world which has the necessary  sensitivity to confirm or contradict the  DAMA/LIBRA result.  



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