A wide range of astronomical observations suggest that all the visible stars and galaxies are immersed in a cloud of invisible matter. The existence of a large amount of non luminous matter has been inferred from gravitational effects since 1930. Its nature, however, still remains a deep mistery. To directly detect dark matter and identify its nature is one of the greatest goals in modern physics.

Composite image of the Bullet cluster. The distributions of hot gas (red) and dark matter (blue) are superimposed to the optical image. Credits: NASA.

Elementary particles that arise naturally in theories beyond the Standard Model of Particle Physics are among the proposed solutions to the Dark Matter “puzzle”.

Within this framework, one of the leading Dark Matter candidates is commonly known as WIMP (Weakly Interacting Massive Particles), produced in the early Universe and subsequently clustered with luminous matter through gravitation.

In the standard WIMP Dark Matter scenario, our Galaxy is embedded in a spherical WIMP halo, with isotropic Maxwellian velocity distribution. The thermal motion of WIMPs inside the halo surrounding our Galaxy and the Earth should result in WIMP-nuclear collisions of sufficient energy to be detected in an instrumented target. Direct detection experiments aim to detect the low recoil energy deposited when a WIMP interacts with a nucleus inside the detector sensitive target. Since the collision rates are extremely low, target masses of the order of 0.1 – 10 tons are required and an ultra-low background must be achieved. The detector must be located in a deep underground site in order to reduce background due to cosmic rays and an accurate choice of low radioactivity materials is needed.

In an Earth based experiment the interaction rate between hypothetical DM particles and the sensitive target is expected to modulate yearly due to the change in the Earth’s speed relative to the DM wind. Such modulation is expected due to the change of the Earth’s speed relative to the galactic halo reference frame, with maximum rate around June 2nd.

An annual modulation signal compatible with the presence of  DM particles in our Galaxy has been observed since almost 20 years from the DAMA/LIBRA experiment at Laboratori Nazionali del Gran Sasso  (LNGS).