Neutrinos reveal how the Sun and the stars shine

Mounting of optical fibers in the Borexino detector, equipped as a clean room.

Borexino is an experiment installed at the INFN Gran Sasso laboratories; the concept and the design of the experiment - developed in 1990 - were aimed at measuring the solar neutrino fluxes throughout the entire energy range, which, at that time, had been measured only on the high energy tail (0.1%) because of the natural radioactivity of the environment and the materials.

Borexino's first effort was focused on the development of innovative methods, able to reduce the radioactivity to very low levels, and then to purify the liquid scintillator, the detector active part, and the liquids, as the highly purified water and an aromatic compound, shielding the detector from external radiations. In addition, the construction and installation of the detector required exclusively non-standard components, high radiopurity materials and activities conducted in clean rooms.

These efforts, which lasted more than a dozen years, have been amply rewarded: Borexino has reached an unprecedented radiopurity, not achieved by any other experiment even to this date. Therefore, it is the only experiment capable of measuring neutrinos with a threshold down to 150 keV. Thanks to this radiopurity as well as to the good sensitivity and resolution achieved, Borexino has succeeded beyond the most optimistic expectations.

The first success is the measurement of the entire solar pp cycle, which produces 99% of the solar energy, namely all the nuclear reactions (pp, ⁷Be, pep, ⁸B) emitting neutrinos (electron-neutrinos) except the for the ³He-p nuclear fusion, which is 0.001% of the total flux, the upper limit has been measured. Thanks to the pp cycle determination, the solar luminosity via neutrinos was compared to the photonic luminosity, demonstrating the Sun stability on 10⁵ years scale, the time needed by photons to escape from it, due to various effects undergone in solar matter (neutrinos need few seconds).

The single neutrino flux measurements are aligned with the paradigmatic Standard Solar Model and, from the ⁷Be and ⁸B fluxes, a reasonable indication in favor of the high metallicity composition of the Sun has been obtained. In addition, these fluxes at different energies, provided by a single experiment, have yielded the transition of the electron-neutrino survival probability from the matter to the vacuum regime, thus validating the Mikheyev-Smirnov-Wolfenstein (MSW) model predictions.

The geoneutrinos, antineutrinos from the Earth interior, can be studied only by two experiments: KamLAND and Borexino. Borexino rejected the null hypothesis (no geoneutrino signal) at about 6σ confidence level, the geoneutrinos are providing information on structure and composition of the Earth interior.

The theory of energy generation in the stars theorizes that another cycle, CNO, is the primary channel for hydrogen burning in stars more massive (> 30% more) that the Sun, but an experimental evidence of this cycle has never been achieved. The detection via its 1% contribution to the solar energy is the last, as well as one of the most challenging, of Borexino's goals. The shape of the CNO energy spectrum does not show any particular tagging and falls into an energy region where a ²¹⁰Bi residue is present, with a spectrum shape similar to the CNO one. Nevertheless, the study of Borexino's sensitivity reveals that it is well within its reach. Over the past four years, both hardware and software efforts have been made and the results have been captured in the content of a letter sent to arXiv and to Nature in June 2020.