Neutrino detector finds new type of "ghost particle" from the Sun

Borexino, a huge underground particle detector in Italy, has picked up a never-before-seen type of neutrino coming from the Sun. These neutrinos confirm a 90-year-old hypothesis and complete our picture of the fusion cycle of the Sun and other stars.

Neutrinos are ultra-light particles produced in nuclear reactions, and the majority of those detected on Earth are produced by the Sun as it fuses hydrogen into helium. But in the 1930s it was predicted that the Sun should also make another type of neutrino through reactions involving carbon, nitrogen and oxygen – so-called CNO neutrinos. And now Borexino has detected these neutrinos for the very first time.

This kind of CNO reaction only accounts for a tiny fraction of the Sun’s energy, but in more massive stars it’s thought to be the primary driver of fusion. Experimentally detecting CNO neutrinos means that scientists have now slotted together the last long-missing puzzle pieces in the solar fusion cycle.

“Confirmation of CNO burning in our sun, where it operates at only a one percent level, reinforces our confidence that we understand how stars work,” says Frank Calaprice, a principal investigator for Borexino.

Detecting CNO neutrinos was no easy task. Although around 65 billion solar neutrinos strike every square centimeter of the Earth’s surface every second, they very rarely interact with matter, passing straight through the entire planet like it was air.

Neutrino detectors are designed to keep watch for the rare times that these “ghost particles” happen to bump into another atom. They usually involve huge volumes of a detector fluid or gas that will give off a flash of light on impact by a neutrino, and these experiments are usually run inside a chamber deep underground, away from interference from other cosmic rays.

Frustratingly, CNO neutrino signals are even harder to spot than more common solar neutrinos. That’s because their signatures resemble those of particles produced by the huge nylon balloon that encases the liquid hydrocarbons Borexino uses as a detector.

To get around that problem, the team spent years adjusting the temperature of the instrument to slow down the fluid movement inside the detector, and focused on signals coming from the center, away from the edges of the balloon. And sure enough, in February 2020 the team finally picked up the signal they were looking for.

Since then, the central part of the detector has become even more sensitive, which could allow for further detections into next year. This data can not only improve our understanding of the fusion cycle of stars, but help scientists figure out how “metallic” the Sun and other stars are.

The research was published in the journal Nature.

Source: Princeton University