Rare ghostly particles produced inside the sun just detected under a mountain in Italy

For the first time ever, physicists have spotted rare, ghostly particles produced by a weird kind of fusion inside the sun. 

The particles, called CNO-produced neutrinos, traveled from the sun to a detector buried deep beneath a mountain in Italy. This discovery brings humans one step closer to understanding the fiery nuclear reactions fueling our home star.

"With this outcome," physicist Gioacchino Ranucci, a physicist at Italy's National Institute for Nuclear Physics in Milan, told Live Science. "Borexino has completely unraveled the two processes powering the sun."

Two types of nuclear fusion reactions occur in the sun's core. The first, and most common, is proton-proton fusion, where protons fuse to transform hydrogen into helium. Scientists predict such reactions generate 99% of the sun's energy. Rarely, nuclear fusion occurs via a six-step process, called the CNO cycle, where hydrogen is fused to helium using carbon (C), nitrogen (N), and oxygen (O). Proton-proton fusion and the CNO-cycle create different types of neutrinos, subatomic particles that are nearly massless and can pass through ordinary matter without a hint of their presence, at least most of the time. Physicists routinely detect neutrinos created during the proton-proton process. However, on June 23, at the Neutrino 2020 Virtual Meeting, researchers from Italy's Borexino detector announced that they had detected CNO-produced solar neutrinos for the very first time.

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The underground Borexino Experiment, at the Laboratori Nazionali del Gran Sasso, near the town of L'Aquila, Italy, was designed to study these extremely rare neutrino interactions. The Borexino detector consists of a tank approximately 60 feet (18 meters) tall that contains 280 tons (254 metric tons) of scintillating liquid — which flashes light when electrons in the liquid interact with a neutrino. A bright flash, which indicates higher energy, is more likely to be from CNO-produced neutrinos. 

Buried deep underground and cocooned in a water tank, Borexino's internal tank is lined with sensitive detectors that are extremely isolated from background radiation from cosmic rays present at Earth's surface. Without this shielding, other signals would drown out the rare signals coming from CNO neutrinos. 

Ranucci also credits the "unprecedented purity" of the scintillating liquid with much of the experiment's success. 

Comparing the observed CNO neutrino observation with the number of observed proton-proton neutrinos will help reveal how much of the sun is made up of elements heavier than hydrogen such as carbon, nitrogen and oxygen. The current results, although not yet peer-reviewed and published in a scientific journal, showed a significance greater than 5 sigma with a greater than 99% confidence level, meaning there is just a 1 in 3.5 million chance that the signal was produced by random fluctuations, rather than the CNO process.

The Borexino international collaboration is made up of researchers from Italy, France, Germany, Poland, Russia, and three universities from the United States, Princeton, Virginia Tech and the University of Massachusetts at Amherst.

Originally published on Live Science.