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.