Science and Technology in very low energy neutrinos physics with Borexino
G.Bellini, A.Ianni, G. Ranucci, Phys. Report , 906 (2021) 1-6
The study of very low energy neutrinos requires a suppression of the natural radioactivity up to ultra-traces level. This is what the Borexino experiment succeeded to achieve with the development of technologies for liquid purification and non-standard approaches in constructing and assembling the detector, allowing a threshold down to 50 keV. Thanks to these measures, the entire solar neutrino spectrum has been detected, neglecting the hep neutrinos which account for less than 10 of the total flux, together with a small, but clean and robust geoneutrino signal; they represent new achievements in the physics of the Sun, the stars and neutrinos, and they open new horizons in the study of the Earth’s interior too. This report describes the technology employed to achieve these goals and the results obtained throughout more than 10 years of data collection. Among the results, actual breakthroughs can be counted: the identification of the nuclear reactions which produce 99% of solar energy and consequently solar luminosity, combined with the demonstration of the solar stability on a 105 years scale; the confirmation of the Standard Solar Model predictions through the measured neutrino fluxes, leading the way for a significant indication in favor of the high metallicity composition of the Sun; the independent demonstration of the validity of the MSW neutrino oscillation solution, through the determination in a single experiment of the distinctive transition of the electron neutrino survival probability from the matter to the vacuum regime; and finally the achievement of the first experimental evidence of the CNO cycle, which dominates in the stars. To this date BOREXINO is a unique experiment with its unprecedented radiopurity level, achieved by means of innovative techniques, which have now become a standard reference for the whole physics field of rare events search.