FIRST INDICATION OF SOLAR NEUTRINOS FROM THE CNO CYCLE REACTIONS WITH THE BOREXINO EXPERIMENT

Solar neutrinos play a unique and irreplaceable role for the comprehension of the mechanisms powering our star. The study of solar neutrino fluxes allows us to definitely prove that the Sun is powered by thermonuclear reactions occurring in its core. Several questions concerning solar neutrinos are still unsolved. In particular, neutrinos emitted in the CNO cycle of reactions are the only undetected piece of the solar fusion puzzle. This cycle is believed to be the main engine of very massive stars, while it is expected to contribute only ~1% to the solar luminosity. Observing neutrinos from the CNO reactions would have a striking importance in astrophysics, providing the first direct confirmation of the existence of this important energy source in the core of the stars. The Borexino detector is an ultrapure liquid scintillator-based detector located at the Laboratori Nazionali del Gran Sasso. Throughout a more than ten-year long data taking, Borexino has achieved outstanding results about solar neutrino physics, measuring all the neutrino fluxes emitted from the pp-chain nuclear reactions. This thesis is devoted to the CNO solar neutrino search with Borexino: analyzing the latest three years of data-taking (Phase-III) I provide the first direct experimental indication of a signal of CNO neutrinos. I describe the details of the analysis, the CNO neutrino results and their implications in terms of solar physics.