For the first time, scientists detect the ghostly signal that reveals the engine of the universe

In research published Wednesday in the journal Nature, scientists reported that they’ve made the first detection of almost-ethereal particles called neutrinosthat can be traced to carbon-nitrogen-oxygen fusion, known as the CNO cycle, inside the sun.

It’s a landmark finding that confirms theoretical predictions from the 1930s, and it’s being hailed as one of the greatest discoveries in physics of the new millennium.

“It’s really a breakthrough for solar and stellar physics,” said Gioacchino Ranucci of the Italian National Institute for Nuclear Physics (INFN), one of the researchers on the project since it began in 1990.

The scientists used the ultra-sensitive Borexino detector at the INFN’s Gran Sasso particle physics laboratory in central Italy – the largest underground research center in the world, deep beneath the Apennine Mountains about 65 miles northeast of Rome.

The detection caps off decades of study of the sun’s neutrinos by the Borexino project, and reveals for the first time the main nuclear reaction that most stars use to fuse hydrogen into helium.

Almost all stars, including our sun, give off huge amounts of energy by fusing hydrogen into helium – effectively a way of “burning” hydrogen, the simplest and most abundant element and the main fuel source in the universe.

In the case of the sun, 99 percent of its energy comes from proton-proton fusion, which can create beryllium, lithium and boron before breaking them down into helium.

But most stars in the universe are much larger than our sun: the red-giant Betelgeuse, for instance, is about 20 times more massive and about 700 times as wide.

Large stars are also much hotter, which means they are overwhelmingly powered by CNO fusion, which fuses hydrogen into helium by means of atomic nuclei transformed in an endless loop between carbon, nitrogen and oxygen.

The CNO cycle is the dominant source of energy in the universe. But it’s hard to spot inside our relatively cool sun, where it accounts for only one percent of its energy.

The giant Borexino detector looks for neutrinos given off during nuclear fusion at the sun’s core.

Neutrinos barely interact with anything, and so they are ideal for studying distant nuclear reactions — but they are also extremely hard to detect.

Trillions of neutrinos from the sun pass through the Borexino detector every second, but it detects only dozens of them each day by looking for faint flashes of light as they decay in its dark 300-ton water tank.

Ranucci said the Borexino detector has spent decades measuring neutrinos from the sun’s main proton-proton chain reaction, but detecting its CNO neutrinos has been very difficult – only about seven neutrinos with the tell-tale energy of the CNO cycle are spotted in a day.

The discovery required making the detector ever-more sensitive over the last five years, he said, by shielding it from outside sources of radioactivity so that the inner chamber of the detector is the most radiation-free place on Earth.

The result is the only direct sign of CNO fusion ever

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