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Nearly a century of compelling astronomical observations has indicated that the visible and ordinary matter constitute only a small fraction of the Universe. A large part of the Universe is indeed expected to be in form of non-visible matter, Dark Matter, and of a mysterious energy component, Dark Energy.
The measurements on the anisotropies of the Cosmic Microwave Background strongly suggest that the Universe is flat, having a density equal to the critical one (ΩTOT = ρTOT/ρcrit =1, with ρcrit =3H2/8πG about 6 hydrogen atoms per m3). Cosmological models through the CMB measurements, the observational data of the Supernovae IA, the Baryonic Acoustic Oscillation and the large-scale structures, allow one to estimate the relative fraction of all the matter and energy components in the Universe. They can be expressed in terms of their density values, ρx, in unit of critical density (Ωx = ρx/ρcrit). The obtained values are: Ωrad ≈ 5 × 10-5 for the radiation density, Ωb ≈ 0.05 for the baryonic matter, ΩDM ≈ 0.27 for the non-baryonic Dark Matter, Ωλ ≈ 0.68 for the Dark Energy. Regarding the neutrino content of the Universe, the neutrino density is strongly constrained by the large-scale structure of the Universe and a value below Ων ≈ 0.01 is obtained. The estimated baryonic matter contribution, Ωb ≈ 0.05, is also supported by the Big-Bang nucleosynthesis (BBN) that predicts the abundances of the light elements in the Universe. This value is much larger than the cosmic density of the luminous matter that is only, Ωlum ≈ 0.004, suggesting that most baryons are dark and, probably, in the form of a diffuse intergalactic medium. Thus the values of the cosmological parameters support that most of the matter in the Universe is Dark, i.e. do not emit or adsorb light, in form of relic particles from Big Bang with non-baryonic nature.
From the astronomical point of view, many observations gave evidence of the existence of a non-luminous matter component in the Universe at very different scales.
The first evidence dates back to 1933 when F. Zwicky, measuring the velocity dispersion of the galaxies in the COMA cluster, realized that these galaxies move too fast to be bound to the system if the only visible matter components contribute to the gravitational field of the cluster. A large part of the non-visible matter needs to be present in the cluster to explain the gravitational attraction felt by the galaxies.
A crucial step for the existence of Dark Matter in the Universe was then performed in 1970’s when its presence inside spiral galaxies was pointed out. It was observed that the rotational velocity in the galactic plane doesn’t decrease outside the halo of visible matter — rather it stays roughly constant. This implies the existence of a Dark halo that contributes to the mass of the galaxy proportionally to its radius. The amount of non-luminous matter is estimated to be about four times larger than luminous one.
Other evidence of the existence of the Dark Matter has been obtained from the observation of X-ray emitting gases surrounding elliptical galaxies, the velocity distribution of hot intergalactic plasma in clusters, and weak gravitational lensing. All these observations are based on gravitational effects and lead to the same conclusion on the prominent presence of Dark Matter in the universe. Most recently a persuasive result was obtained in 2004 by the observation of the “Bullet Cluster” (1E0657-558): a collision between two clusters of galaxies in which the hot baryonic matter component of the two clusters - observed by the X-ray radiation - collided and decelerated, remaining in the central part of the system, while the mass distribution was concentrated in the outer part of the system (as revealed by the weak lensing.) This displacement of ordinary matter with respect to the obtained mass profile supports that in the cluster the largest matter component is collisionless and has non-luminous nature. The “Bullet Cluster” can be explained only by considering the existence of non-baryonic Dark Matter in the clusters and, moreover, cannot be interpreted in terms of theories of modified gravity.
The Milky-Way is also embedded in a large dark halo whose density in the solar neighborhood has been estimated to be of order of ≈ 0.3 GeV/cm3, depending on the halo model.
All the evidence supports the presence of Dark Matter in the universe as an important ingredient for its evolution, playing a crucial role in the structure formation. Therefore, a large component of the Universe is expected in form of non-baryonic relic particles from the Big Bang. These particles, to survive with a sizeable abundance up to the present epoch, have to be neutral, stable or with a lifetime comparable with the age of the Universe and have very low interaction rate with ordinary matter. It is remarkable that no particle belonging to the Standard Model of particle physics can be a good Dark Matter candidate; this can be considered as a motivation for theories extending the Standard Model in which, indeed, many candidates having different nature and interaction types have been proposed. Among the many Dark Matter candidates are stable particles proposed in SUSY theories (as e.g. neutralino or sneutrino in various scenarios), inelastic Dark Matter, electron interacting Dark Matter, sterile neutrino, Kaluza-Klein particles, self-interacting Dark Matter, axions or axion-like (light pseudoscalar and scalar candidate), mirror Dark Matter, etc. Moreover it is important to note that, considering the richness of the ordinary particles in the visible Universe, one could also expect that the Dark Matter particles may be multicomponent.
A widely considered interaction process of Dark Matter particle with ordinary matter is the elastic coherent scattering; in this case an experiment - with the aim of detecting Dark Matter interactions with target nuclei - would measure the nucleus recoil energy; anyhow, other detection processes can be considered for various Dark Matter candidates in which the interaction signal is in total or in part of electromagnetic nature. Since the velocity of the particle is of the order of around 300 km/s, and considering the detector-quenching factor for nuclear recoils, the energy released in the interaction is expected in the keV energy range. Moreover the interactions are also predicted to be rare because of the very low cross section and thus the experiments investigating Dark Matter direct detection - to be sensitive to the expected signal with respect to background - exploit low background techniques. Firstly, they are carried out in underground laboratories where the radiation produced by the cosmic ray is highly suppressed. To further reduce the measured background radiation, the detectors are placed inside massive shield able to adsorb the external environmental radiation; in addition all the employed materials, as well as the detector itself, are characterized by a very low content of residual radioactive isotopes. Various experimental solutions and detector configurations have been developed specifically by each experiment to further reduce residual background. The experiments carried out in the Gran Sasso laboratory use different target materials and exploit different approaches. A model independent approach, based on the study of the time dependence of the Dark Matter signal, the so called Dark Matter annual modulation signature is exploited; this approach has the merit to be model independent (i.e. no hypothesis on the Dark Matter nature and interaction is needed) and to require the satisfaction of many specific peculiarities offering an unambiguous signature for Dark Matter. On the other hand other approaches are followed by various experiments in which hypothesis on the nature of the Dark Matter particle interaction are required. This is the case of experiments that apply rejection procedures of the electromagnetic rate, assuming the Dark Matter signal to be present only as nuclear recoils.
Neutrinos are perhaps the most elusive particles in our universe and yet we are surrounded and perpetually bombarded by them. Since from their start, to study the properties of neutrinos was one of the major research interests within the Gran Sasso National Laboratories. The measurement of the particular characteristics of neutrino propagation (neutrino oscillations), the study of the information that neutrinos bring to us from celestial objects near and far (neutrino astronomy) and the determination of the intrinsic characteristics of this particle (Double Beta Decay and Majorana neutrino) are since many years fundamental pieces of the prolific scientific activity in the underground laboratories.
Among all fields of astronomy, the one that studies neutrinos has unique and amazing characteristics. In fact, their enormously penetrating power allows exploring the core of the Sun, where nuclear reactions provide the solar energy that we observe in the form of light.
At the Gran Sasso National Laboratories some of the pioneering experiments in this field have been done. The experiment GALLEX (1991-1997) was one of the first in the world in the field of neutrino astronomy measuring the total flux of low energy solar neutrinos.
In recent years the Gran Sasso National Laboratories maintained their leadership in this field. The experiment Borexino published in 2014 the first direct measurement of the low-energy neutrinos generated by proton fusion reactions in the nucleus of the Sun. These neutrinos make up for the biggest part of neutrinos produced in the Sun and are directly linked to its radiation energy. The Sun is of special importance in astronomy, being the closest star and providing the ideal test bench on which to check the validity of our theories. Neutrino telescopes (as for instance the Large Volume Detector (LVD) and Borexino), are able to observe the final moments in the life of the biggest stars of our galaxy, when they collapse under their own weight, producing compact stellar objects (e.g. neutron stars, black holes or perhaps even a quark star), giving rise to supernovae by gravitational collapse.
In fact, it is the neutrinos to allow the star releasing the huge amount of excess energy, equal to 10-20% of its total mass. A unique feature of the telescopes hosted at the Gran Sasso National Laboratories is their ability to measure all neutrino flavors. This is due to the fact that the different flavors of emitted neutrinos undergo characteristic reactions with the nuclei composing these telescopes.
The quantitative observation of the neutrino has provided for now the only experimental evidence of the existence of physics beyond the standard model of elementary particles. The discovery of neutrino oscillations, which was observed among others by the MACRO experiment, GALLEX, GNO, OPERA, Icarus and Borexino, all placed in the Gran Sasso National Laboratories, has opened the way towards precision measurements on the nature and the behavior of this elusive particle.
The study of the intrinsic properties of neutrinos is of primary interest for elementary particle physics. One of the most interesting results of the last decade has been the evidence, by means of the detection of the
neutrino oscillation phenomenon, of non zero mass (even if very small). With neutrino oscillation is described the neutrino characteristic of changing, while it travels in space, its capability to interact only with one of the three particles called leptons (electron, muon or tau). In other words, a neutrino produced for example in the sun, that can only interact with electrons, once it reaches the earth may have changed condition (a property called lepton flavor) and can completely ignore the electrons interacting only with muons or taus. In order to understand exactly and completely the oscillation mechanism for what a neutrino type transforms into another, it has been crucial to use different sources of neutrinos such as the sun, the stars, or particle accelerators and reactors (the so called artificial sources).
To complete the neutrino picture the crucial missing information can be provided by the observation of a process called neutrinoless Double Beta Decay (searched by the GERDA, CUORE and Lucifer experiments). This observation will not only confirm the theory originally developed by Ettore Majorna but will completely change our description of elementary particles. The neutrino will be confirmed to be its own antiparticle and some crucial aspects of the universe evolution can be clarified.
Neutrino physics is therefore a benchmark for new theory in elementary particles and for the understanding of the Universe evolution, a window on the knowledge of the infinitely small and on the infinitely large. Due to the elusiveness of the neutrinos, or their unwillingness to interact with matter, the study of their properties requires the use of extremely large detectors placed in environments with very low natural radioactivity, and shielded from the cosmic radiation, therefore in underground laboratories.