The newfound particles are produced when two protons and an electron interact to make deuterium, a heavy form of hydrogen that helps feed the sun’s fusion. About 1 in 400 deuterium atoms in the sun are made in this proton-electron-proton, or pep, reaction.
Scientists can probe the sun’s inner workings by studying the particles produced in its thermonuclear reactions — in particular, the neutrinos that flood through Earth in great numbers but hardly interact with matter here. Researchers must build detectors underground to screen out these elusive solar neutrinos from other particle chatter.
In 2007, an Italian-led collaboration known as Borexino started trying to do just that in the Gran Sasso National Laboratory, buried in a mountain in central Italy. Borexino consists of a giant vat of liquid, which sets off tiny sparkles when neutrinos interact with it.
Team scientists knew they could spot neutrinos from the more common and higher-energy proton-proton, or pp, reaction. “We didn’t expect to be able to see the pep neutrino when we started,” says Frank Calaprice, a team member also at Princeton. “We knew it might be possible, but there were huge barriers.”