Neutrinos point to rare stellar fusion

Neutrinos captured under a mountain in central Italy have provided the first direct evidence for a nuclear reaction involved in the conversion of hydrogen to helium inside the Sun. The observation was made by the Borexino collaboration, which next hopes to ensnare as-yet-unseen neutrinos from fusion reactions taking place in stars heavier than our own.

Most of the Sun’s heat is generated in fusion reactions that form what is known as the “proton–proton cycle”. This involves the fusion of two hydrogen nuclei (protons) to form heavy hydrogen, the fusion with a third hydrogen nucleus to form helium-3 and then, via various pathways, the creation of extremely stable helium-4.

Physicists can learn about this cycle by intercepting the chargeless, nearly massless particles known as neutrinos that are produced in many of the constituent reactions. In fact, by measuring the fluxes of these particles, they can learn not just about the structure and dynamics of the Sun, but also about the properties of neutrinos themselves. To date, however, most neutrino detectors have been sensitive to the highest energy solar neutrinos – those having energies of between about 5–18 MeV. However, the vast majority of solar neutrinos have energies below 5 MeV, and Borexino was built specifically to study these particles.

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