Physicists Directly Detect Neutrinos from Our Sun for the Very First Time

For the first time, physicists have directly detected neutrinos created by the "keystone" proton-proton (pp) fusion process occurring at the sun's core. The findings could shed some light on our closest star.

The pp reaction is actually the first step of a reaction sequence that's responsible for about 99 percent of the sun's power. Solar neutrinos in particular are produced in nuclear processes and radioactive decays of different elements during fusion reactions at the sun's core. In fact, these particles stream out of the star at nearly the speed of light, and as many as 420 billion of these particles hit every square inch of Earth's surface per second. Yet because they only interact through the nuclear weak force, they pass through matter and can be very difficult to detect.

In order to detect the neutrinos, the researchers employed one of the most sensitive neutrino detectors on the planet-the Borexino instrument, which is located deep beneath Italy's Apennine Mountains. This instrument detects neutrinos as they interact with the electrons of an ultra-pure organic liquid scintillator at the center of a large sphere surrounded by 1,000 tons of water.

"As far as we know, neutrinos are the only way we have of looking into the sun's interior," said Andrea Pocar, one of the researchers, in a news release. "These pp neutrinos, emitted when two protons fuse forming a deuteron, are particularly hard to study. This is because they are low energy, in the range where natural radioactivity is very abundant and masks the signal from their interaction."

By examining these neutrinos, the researchers can learn a bit more about our sun. This, in turn, can tell them a bit more about how the particles that the sun emits will interact with our own planet.

"By comparing the two different types of solar energy radiated, as neutrinos and as surface light, we obtain experimental information about the sun's thermodynamic equilibrium over about a 100,000-year timescale," said Pocar. "If the eyes are the mirror of the soul, with these neutrinos, we are looking not just at its face, but directly into its core. We have glimpsed the sun's soul."

The findings are published in the journal Nature.