Italian scientists have detected solar neutrinos with an energy of 1.4 MeV (i.e. mega-electron-volt) which had been predicted to arise from proton-electron-proton interactions that give rise to deuterium in the sun.
About 1 in 400 solar deuterium atoms are made this way, rather than through the usual proton-proton fusion process that produces much higher energy neutrinos. Neutrinos arising from interactions at particle colliders are likewise much more energetic (on the order of 140 MeV). About three such neutrinos are detected each day at their lab placed deep underground to filter out interference from sources other than neutrinos which interact very weakly with ordinary matter, confirming our understanding the nuclear physics that are going on in the sun.
High energy neutrinos move so close to the speed of light that it is virtually impossible to make meaningful determinations about neutrino rest mass, since their relativistic kinetic energy so profoundly dwarfs their very small mass. In principle, measurements of the speed of low energy solar neutrinos traveling at a speed distinguishably less than that of the Lorentz transform speed limit of special relativity could make it possible to directly infer the rest mass of a neutrino with much greater accuracy.
The energetic equivalent of the mass of an electron is about 0.5 MeV/c^2, and the mass of the up and down quarks respectively are on the order of single digit MeV equivalents. The various varieties of ordinary neutrinos are believed to have rest masses in the KeV to eV range, closer in mass-energy to photons than other fermions.