Astroparticle physics experiments aim at the study of the cosmos and rare phenomena that can be investigated in underground laboratories or in deep marine sites.

KM3NeT

KM3NeT is one of the pan-European research infrastructures included in the ESFRI (European Strategy Forum on Research Infrastructures) roadmap 2018. The infrastructure comprises two neutrino detectors in the Mediterranean Sea for the study of high energy neutrinos of astrophysical origin and neutrino oscillations. The ARCA detector, which is under construction offshore the Sicilian coast of Capo Passero, aims at observing high-energy neutrinos from galactic and extra-galactic sources.
High-energy cosmic neutrinos, first observed in 2013 by the IceCube detector at the South Pole, can provide crucial information about the mechanisms that take place in the most powerful cosmic accelerators. In fact, the emission mechanisms of extreme energy particles are not yet understood. In particular, huge detectors, designed to study the high-energy cosmic radiation that irradiates the Earth have measured particles up to extremely high energies far exceeding those achievable with any man-made accelerator. However, the origin of these particles is still unknown.
Cosmic rays are charged particles (mainly protons). Being deflected by galactic and intergalactic magnetic fields while travelling in space, they lose information about their original direction. On the other way neutrinos, which are chargeless, are not deflected during their journey through the cosmos, and can therefore provide direct information about their sources. Furthermore, being weakly interacting particles, neutrinos can travel enormous distances, allowing to study the internal regions of astrophysical sources and phenomena that occur in remote regions of the Universe. To detect neutrinos, instruments with size of the order of a cubic kilometre and installed in places where background signals are reduced by a million times are needed. The only viable solution is to install these telescopes in the abysses of the sea or in the depths of the Antarctic ice. Water or ice have a threefold function: shield for the background of atmospheric muons; target for converting neutrinos into charged particles; radiator for the Cherenkov light produced by the secondary particles produced in neutrino interactions. By measuring with a three-dimensional array of optical sensors the space-time correlation of Cherenkov photons it is possible to infer the direction of the incoming neutrino. This direction can be reconstructed with an accuracy of less than a tenth of a degree making possible to point back at the emitting source.
The LNS are strongly involved in the detector design and construction activities. In the past the design aspects have been addressed and the submarine site has been identified and characterized. The site selected for the installation of the ARCA detector is located at a depth of 3500 m, 80 km offshore Capo Passero. The technological challenges related to the hostile environment (pressure, corrosion, difficult accessibility) are many and have required the development of new technologies that have been validated with the implementation of some prototypes. Currently the KM3NeT group at the LNS, consisting of about 20 physicists and engineers, is heavily involved in the construction phase of the seafloor infrastructure and the integration of the detection units.
The LNS group is also taking part in the ANTARES experiment, a 0.01 km³ prototype forerunner of KM3NeT, installed off Toulon at a depth of 2500 m.

DARKSIDE

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 being completed data at the Gran Sasso Laboratory (LNGS), deploying 50 kg of active liquid Argon. The next phases of the DarkSide project, that are currently under design and preparation, envisage the scaling to 50 tons of active liquid Argon (DS20k), hosted at LNGS, and then to 300 tons (ARGO). The main committment of the LNS group within the DarkSide Collaboration is the ReD (Recoil Directionality) project: a miniaturized dual-phase TPC, which is the downscaled version of the TPC in preparation for DS20k, is being characterized and calibrated by means of an artificial neutron beam. The neutron beam is produced by impinging a primary ⁷Li beam accelerated by the TANDEM onto a polyethylene target.

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.
After a first phase which ended in 2013, the data daking restarted with improved performance in December 2015 and was recently completed in December 2019. In fact GERDA achieved all its design goals in terms of background and of accumulated data. No indication was found in GERDA data for neutrinoless double beta decay of ⁷⁶Ge and a lower limit on its half life was set at 1.8 10²⁶ yr at 90% confidence level.
The GERDA infrastructure at the LNGS is being handled to the project LEGEND, which will search for the neutrinoless double beta decay of ⁷⁶Ge with further increased sensitivity.

NU AT FNAL

The LNS participate to the NU_AT_FNAL research program funded by the CSN2. The program includes two different neutrino physics experiments at the Fermi National Accelerator Laboratory (FNAL): SBN and DUNE.
The Short Baseline Neutrino (SBN) experiment is designed to prove or disprove the existence of sterile neutrinos with a mass of the order of 1 eV. The search for light sterile neutrinos is motivated by a series of anomalies observed on neutrino oscillations, in the results of some experiments such as LSND and MiniBooNE. Sterile neutrinos, if they exist, are not directly observable since they do not interact with ordinary matter through weak interaction but modify the patterns of oscillations between standard neutrino flavors. Their discovery would therefore open the door to new physics beyond the Standard Model of elementary particles.
The SBN experiment involves the use of a neutrino beam and three liquid argon detectors: SBND, MicroBoone and ICARUS at different distances. In particular, the ICARUS detector, which will act as far detector, has been refurbished after the data taking at the Laboratori Nazionali del Gran Sasso, where it operated underground from 2010 to 2014, using the neutrino beam from CERN. SBN is under installation at the Fermilab in Chicago and data collection is expected to start in 2020. The Laboratori Nazionali del Sud participated in the construction of the ICARUS Cosmic Ray Tagger and will take part in the Monte Carlo data analysis and simulation activities. SBN will also provide important information on the detection of neutrinos in liquid argon, used with much larger volumes also in the Deep Underground Neutrino Experiment (DUNE), at the forefront of a complete investigation of neutrino oscillations.
DUNE is a long-baseline experiment in which an intense neutrino beam produced at the FNAL will be detected by a near detector at 375 m and by a far detector installed in the Sanford Underground Research Laboratory, in South Dakota, 1,300 kilometers downstream from the source of beam. One of the main goals of DUNE is the search of CP symmetry violation in the leptonic sector. This violation could explain why in the Universe matter prevails strongly on antimatter although the big bang theory provides an equal number of particles and anti-particles. The CP violation has already been observed in meson decays, composed of quark-antiquark pairs, but is too small to explain the matter-antimatter asymmetry. The observation of the violation of CP in the leptonic sector could therefore make an important contribution to the knowledge of the nature of matter and the evolution of the Universe.
The LNS are involved in the design of the DUNE near detector. This will consist of a complex of detection systems including the System for on-Axis Neutrino Detection (SAND) apparatus. The SAND system, proposed by the Italian component of the collaboration, provides for the re-use of the KLOE detector, in operation from 1999 to 2018 with the collector DAΦNE of the Laboratori Nazionali di Frascati. SAND will allow you to continuously monitor the spectrum of the neutrino beam by measuring the pulse and energy of the particles produced in the neutrino interactions. The Laboratori Nazionali del Sud also participate in the characterization and qualification of the SiPMs used for the light detection in the far detector.

GRAVITATIONAL WAVES

LNS researchers contribute as well to Gravitation Wave physics experiments that have experienced an overwhelming interest in recent years, thanks to the first detection of GW emitted in the coalescence of two black holes and to the recent discovery of GW emitted by the coalescence of two neutron stars, which has opened the new field of multi-messenger astronomy. Gravitational waves (GWs) are a consequence of the theory of General Relativity, published by Albert Einstein in 1916. They are ripples in the fabric of the spacetime that propagate at the speed of light and are produced when huge masses are accelerated or deformed. This happens in many astrophysical scenarios, including supernova explosions or the gravitational interactions between black holes or neutron stars. GWs travel through spacetime, deforming it and producing mini "spacetime quakes".

VIRGO

Virgo is a giant laser interferometer designed to detect gravitational waves. It has been designed and built by a collaboration between the French Centre National de la Recherche Scientifique (CNRS) and the INFN. It is now operated and improved in Cascina, a small town near Pisa on the site of the European Gravitational Observatory (EGO), by an international collaboration of scientists from France, Italy, the Netherlands, Poland, and Hungary. The Virgo detector consists mainly in a Michelson laser interferometer made of two orthogonal arms being each 3 kilometers long. Multiple reflections between mirrors located at the extremities of each arm extend the effective optical length of each arm up to 120 kilometers. Virgo detector is currently taking date (3^rd Observation Run, O3) jointly with the two Advanced LIGO detectors in the US and KAGRA in Japan.
The LNS group is mainly involved in the data analysis of GW detectors, in particular in the search of generic GW transients, or bursts, especially in the search of GW expected to have an electromagnetic counterpart. The analysis is focused on two types of bursts: short fast radio burst (FRB), and gamma-ray burst (GRB).

ET

Present GW detectors are called "Advanced Detectors" because they are the evolution of the "First Generation Detectors" that ran in the past decade. They are based on an infrastructure that is about 20 years old, both for LIGO and Virgo, and that were conceived with the knowledge of gravitational sources and technologies of those years. GW Advanced detectors are typically limited in duty-cycle by adverse weather conditions and, more importantly, the final sensitivity at low frequency is expected to be limited by the Newtonian noise.
Currently, GW community is preparing the third generation of GW detectors and in Europe the community has grown around the Einstein Telescope (ET) project. Many technological developments will characterize the ET detector: it will benefit from novel optical techniques both on laser and optical components, on novel seismic attenuation and control systems, on cryogenic temperature of critical optics and suspensions, faster data analysis systems, new detection schemes and techniques. But, above all, it will require a new infrastructure. The ET detector will be underground, cryogenic and with at least 10 km arm length. Several case studies have been conducted in Europe to identify an underground site with suitable low noise. These preliminary studies have indicated the Sos Enattos mine as one of the best sites in Europe.
LNS group is strongly involved in supporting the Sos Enattos candidature, by means of geological investigations, the measurement of seismic and environmental noise and the computation of the ET sensitivity curve, particularly at low frequencies (1-10 Hz), attainable at the Sos Enattos site.