Laboratori Nazionali del Sud

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Research

Astroparticle Physics

Astroparticle experiments aim at the study of the cosmos and of very rare phenomena that can be investigate only in underground laboratories

The LNS researchers have an intense activity in the field of astroparticle physics.

KM3NET

The main effort concerns high-energy neutrino astronomy. Neutrinos, that are neutral particles very weakly interacting with matter, can provide information on the denser regions of astrophysical sources and on the most violent phenomena occurring at the far edge of the Universe. As well as the detection of solar neutrinos provided detailed information on how the Sun works, high-energy cosmic neutrinos, detected for the first time in 2013 by IceCube at the South Pole, are expected to shed light on the mechanisms which take place in the most powerful cosmic accelerators. Cosmic rays are detected in huge arrays, tens of thousands of square kilometer in size, up to energies of order of magnitudes larger than energies achievable with any accelerator that could be made on Earth. Still 100 years after their discovery their origin is still unknown. Being cosmic rays mostly charged particle (proton and nuclei), they are deflected by galactic and extra-galactic magnetic fields that scramble their directions. Neutrinos do not suffer these deflections and can therefore provide unperturbed information on the cosmic ray production and acceleration mechanisms. However, their  detection requires instrumented volumes of about one cubic kilometer that can only be installed in deep sea o glaciers. The water or ice acts the same time as shield for atmospheric muons, as a target for neutrino interaction and as a radiator through the Cherenkov effect. A three-dimensional array of optical sensors allows to estimate the energy and to reconstruct the arrival direction of the neutrino with an accuracy of about 0.1°.

The construction and operation of the cubic kilometer telescope KM3NeT required a long and complex R&D activity where the LNS were deeply involved. The selection and characterization of the site and the development of technology has taken  many years. The site selected for the KM3NeT installation is at 3500 m depth 80 km off-shore Capo Passero. The design of an underwater detector contains technological challenges due to the huge pressure, corrosion and accessibility. An intense activity of prototype design, construction and in-situ validation has been carried out. The first phase of the project is under construction. The LNS team also takes part in ANTARES, the underwater neutrino telescope precursor of KM3NeT with an instrumented volume of 0,01 km3 installed off-shore Toulon at 2500 m depth.

 

DARKSIDE

The DarkSide experiment, which aims at the detection of WIMP-like dark matter and the GERDA experiment, which looks for a rare nuclear decay, the “neutrinoless double beta decay”. [cross-link alla pagina NUMEN@LNS].

It is known that dark matter constitutes up to about 30% of our Universe and that it is “non-baryonic”, i.e. not made up by protons, neutrons, neither by electrons, as the ordinary matter. Some theories predict that the dark matter is made by new kinds of particles, named WIMPs (Weakly Interacting Massive Particles), which are streaming within the Galaxy as a sort of wind. It can very rarely happen that a WIMP hits on a “normal” nucleus and set it in motion, just like a billiard ball would do. The DarkSide experiment is indeed looking for the interactions of WIMP-like dark matter particles on Argon nuclei, by using the dual-phase Time Projection Chamber (TPC) technology. Argon, which is normally a gas at room temperature, is kept as a liquid target, at -187 C. The WIMP interactions on Argon nuclei are so rare that the experiment must be performed in an underground laboratory and must be equipped with shielding and appropriate tools in order to suppress the background interactions by “normal” particles (e.g. neutrons or gamma-rays), which could mimic the WIMP signal. The DarkSide 50 (DS50) experiment is currently taking data at the Gran Sasso Laboratory, deploying 50 kg of active liquid Argon. The next phase of the DarkSide project, which is under preparation, envisages the scaling to 20 tons of active liquid argon (DS20k).

GERDA

Neutrinoless double beta decay is a rare nuclear transition, which has never been observed so far but which is predicted by many theoretical models. In the neutrinoless double beta decay, a nucleus is transformed into another nucleus, which sits two places away in the periodic table, with the emission of two electrons and no neutrinos. Such a transition violates the lepton number conservation: should the existence of neutrinoless double beta decay be confirmed experimentally, it would imply that the neutrino is a Majorana particle (i.e. identical to its own anti-particle) instead of a Dirac particle. The Dirac vs. Majorana nature of the neutrino is closely related to the matter/anti-matter asymmetry in the Universe. A nucleus that could potentially undergo neutrinoless double beta decay is 76Ge. The experiment GERmanium Detector Array (GERDA) at the Gran Sasso underground laboratory is searching for the neutrinoless double beta decay of 76Ge by using ultra high-purity Germanium detectors, which are enriched in 76Ge. Also in this case, it is critical that the experiment is performed in a underground site and that appropriate tools and shieldings are set up to prevent that the very rare genuine signal from double beta decay is overwhelmed by the much more abundant background events. Germanium detectors are therefore operated immersed in a bath of ultra-pure liquid Argon, which is acting as a cooling medium and as a shielding. The experiment completed the first phase in 2013 and started the second phase of data taking, with improved performance, on December 2015.

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