Borexino has a new preprint where they try to measure neutrinos from proton-proton fusion in the sun. For a star the size of the sun, this is the main fusion reaction creating neutrinos and also releasing the energy that heats the sun. For a heavier star with a hotter core, the CNO cycle becomes the dominant fusion source. Proton-proton fusion generates most of the solar neutrino flux but is difficult to measure because the neutrinos don’t have much energy (less than an MeV). This energy range means that not only are other neutrino-generating processes important, but radioactive contamination in the detector, which has an active volume made of liquid scintillator, are important as well. Because there are significant backgrounds, the analysis needs to find the signal within a complicated spectrum of irreducible but well understood backgrounds.
The final result is consistent with theoretical predictions, so nothing surprising seems to be in the data. Borexino says that a measurement of pp neutrinos can help constrain the possibility of alternative models of solar power production.