Neutrinos are elementary particles that are difficult to detect because they very rarely interact with ordinary matter. There are three varieties, or “flavors,” of neutrinos: electron, muon, and tau. Neutrinos forged in the Sun’s core are electron neutrinos, but can change between the other two over time. An international team of over 100 physicists, led in part by Andrea Pocar from the University of Massachusetts Amherst, have been able to directly detect neutrinos from the core of the Sun and describe the particles’ behavior. The paper was published in Nature.
The Sun is fueled by nuclear fusion, the majority of which convert hydrogen atoms into helium. Though there are many ways to do this, the most common way for stars the size of our Sun is called proton-proton (pp) fusion. During this reaction, neutrinos are produced and ejected out of the Sun. Though Earth is constantly getting bombarded with countless neutrinos, the majority pass through the planet without actually interacting with anything, which is why they are hard to detect.
Pocar explained the crux of the experiment in a press release:
"With these latest neutrino data, we are directly looking at the originator of the sun's biggest energy producing process, or chain of reactions, going on in its extremely hot, dense core. While the light we see from the Sun in our daily life reaches us in about eight minutes, it takes tens of thousands of years for energy radiating from the sun's center to be emitted as light.
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. 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.
As far as we know, neutrinos are the only way we have of looking into the Sun's interior. 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."
In order to detect these elusive particles, the researchers used the liquid scintillator that is part of the Borexino experiment in the Apennine Mountains of Italy. The neutrinos were detected by interacting with electrons in a Carbon-14-depleted medium that had been placed in the center of a giant sphere that holds nearly 240,000 gallons of incredibly pure water. These extreme conditions are taken to ensure that there is no interference from unknown sources of radiation and that there was no C14 decay to detect, which can be confused with the signature of neutrino interactions.
"[I]t's a little bit of a coup that we could do it," Pocar admitted. "We pushed the detector sensitivity to a limit that has never been achieved before."