New breakthrough in detecting “ghost particles” from the Sun

Neutrinos are one of the most mysterious particles in the universe, often called “ghost particles” because they rarely interact with anything else. Trillions stream through our bodies every second, yet leave no trace. They are produced during nuclear reactions, including those that take place in the core of our Sun. Their tendency to not interact often makes detecting neutrinos notoriously difficult.

Capturing this interaction is an extraordinary achievement. Despite the rarity of the carbon isotope, we were able to observe its interaction with neutrinos, which were born in the Sun’s core and travelled vast distances to reach our detector.

Lead author Gulliver Milton, Department of Physics, University of Oxford

Neutrinos from the Sun have only been seen to interact on a handful of different targets. Now, for the first time, scientists have succeeded in also observing them transform carbon atoms into nitrogen inside a vast underground detector.

The breakthrough, led by researchers at Oxford, was made using the SNO+ detector located two kilometres underground in SNOLAB, an international world-class facility housed in a working mine in Sudbury, Canada. The deep location was crucial to shield the lab from cosmic rays and background radiation that would mask the faint neutrino signals.

The team searched for events where a carbon-13 nuclei is struck by a high-energy neutrino and transformed into radioactive nitrogen-13, which decays about ten minutes later. They used a “delayed coincidence” method, which looks for two linked signals: an initial flash from a neutrino striking a carbon-13 nucleus, followed several minutes later by a second flash from the resulting radioactive decay. This distinctive pattern allows researchers to confidently separate real neutrino interactions from background noise.

The analysis found 5.6 observed events over a 231-day period, from 4 May 2022 to 29 June 2023. This is statistically consistent with the 4.7 expected to be generated by neutrinos during this time.

Neutrinos are bizarre particles that are essential for understanding stellar processes, nuclear fusion, and the evolution of the universe. According to the researchers, this discovery lays the groundwork for future studies of similar low-energy neutrino interactions.

The Sudbury Neutrino Observatory cavity and detector under construction two kilometres underground in Sudbury, Ontario, Canada. Image credit: SNOLAB.

Lead author Gulliver Milton, a DPhil student at the University of Oxford’s Department of Physics, said: ‘Capturing this interaction is an extraordinary achievement. Despite the rarity of the carbon isotope, we were able to observe its interaction with neutrinos, which were born in the Sun’s core and travelled vast distances to reach our detector.’

Co-author Professor Steven Biller (Department of Physics, University of Oxford) added: ‘Solar neutrinos themselves have been an intriguing subject of study for many years, and the measurements of these by our predecessor experiment, SNO, led to the 2015 Nobel Prize in physics. It is remarkable that our understanding of neutrinos from the Sun has advanced so much that we can now use them for the first time as a “test beam” to study other kinds of rare atomic reactions!’

Event display console in the SNO+ control room at SNOLAB. Image credit: SNOLAB.

‘This discovery uses the natural abundance of carbon-13 within the experiment's liquid scintillator to measure a specific, rare interaction,” Kraus said. “To our knowledge, these results represent the lowest energy observation of neutrino interactions on carbon-13 nuclei to date and provides the first direct cross-section measurement for this specific nuclear reaction to the ground state of the resulting nitrogen-13 nucleus.’

The study ‘First Evidence of Solar Neutrino Interactions on ¹³C’ has been published in Physical Review Letters.

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