Historic neutrino detection shines new light on the sun

James Marshall
November 27, 2020

Buried deep beneath the Apennine Mountains, the INFN lab is the largest underground research center in the world.

The rare emissions - which travelled 90 million miles to reach us - are produced in certain nuclear reactions that account for less than a per cent of the Sun's energy.

An worldwide team of about 100 scientists of the Borexino Collaboration, including particle physicist Andrea Pocar at the University of Massachusetts Amherst, report in Nature this week detection of neutrinos from the sun, directly revealing for the first time that the carbon-nitrogen-oxygen (CNO) fusion-cycle is at work in our sun. At the same time, although the CNO cycle plays a minor role in our Sun, it is most likely the predominant way of producing energy in other more massive and hotter stars.

Nearly 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. Their lack of interaction also makes these subatomic particles hard to detect. This not just affirms that CNO is a main thrust behind greater stars, however the universe on the loose.

Neutrinos are neutral, subatomic, "ghostly" particles with a mass close to zero.

These are almost massless - and are capable of passing through ordinary matter without giving up any indication of their presence.

Physicists have wanted to study these emissions from the Sun, however, as better understanding how the CNO cycle works in our star will offer insights into how larger stars - where this process is dominant - burn their nuclear fuel.

But after they enlarged the Borexino detector (which was already huge) and improved its sensitivity, physicists were able to detect seven neutrinos with the signature energy of the CNO cycle.

More than 100 scientists came together in the Borexino detector on the Italian border to measure nuclear fusion occurring in the Sun's core.

Most stars in existence are much larger than our modest yellow sun: Betelgeuse, a red giant star, is roughly 20 times more massive and roughly 700 times the diameter of the sun.

These are picked up by camera-like sensors and analysed by powerful hardware.

While the Borexino Collaboration has been able to detect neutrinos originating from several reactions along the pp chain in recent years, their current achievement has been to explicitly identify neutrinos released in the CNO cycle, which are significantly less abundant in comparison.

According to physicist Gioacchino Ranucci, also of Milan, the success of the experiment should be credited to the "unprecedented purity" of the solution.

Furthermore, the scientists suggest in their paper in the journal Nature, it may even be possible to refine the neutrino measurements enough to be able to calculate the amount of carbon, nitrogen and oxygen in our Sun's core - a direct experimental measurement of what astrophysicists call its metallicity (its content of elements heavier than hydrogen and helium).

The study showed how our star carries out a process called the carbon-nitrogen-oxygen (CNO) fusion cycle - which uses heavier elements than scientists thought a star of the sun's size would.

Confirming this from Earth, however, requires looking at the neutrinos each produces as a byproduct and distinguishing those from the CNO cycle from those from the pp process - a decades-long challenge. Crucially, this confirms that the CNO cycle exists on an empirical basis - a task left undone since the process was first hypothesized in the 1930s, Futurism reports.

" It is the culmination of a tireless, years-long effort that has led us to push technology further than anything previously reached, " said Marco Pallavicini, a spokesperson for the Burexino experiment, a physicist from the University of Genoa.

Other reports by Click Lancashire

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