Low-energy neutrinos provide direct evidence of solar proton–proton reactions

Nature : Proton–proton reactions are the first step in the hydrogen fusion process that powers the Sun and 90% of the other stars in the Milky Way. In the process, two protons are fused together by the pressure in the star’s core. One of the protons then transforms into a neutron by releasing a positron and a neutrino, and the proton–neutron pair forms the nucleus of a deuterium atom. The reaction produces 90% of the neutrinos that escape the Sun, but they are hard to detect on Earth where radioactive decay processes produce much more energetic neutrinos. Andrea Pocar of the University of Massachusetts Amherst and his colleagues have now used the Borexino detector at Gran Sasso National Laboratory in Italy to obtain the first direct detection of these low-energy neutrinos. The detector measures the flux of the neutrinos with a precision of 10%. If that precision can be increased to just 1%, it could provide a way of looking for a decrease in the neutrinos’ energy as they pass through dark matter.