For the first time, a solar neutrino has transformed the carbon-13 isotope into radioactive nitrogen-13—marking the first direct evidence of neutrino involvement in the weak interaction.

Mysterious Particles of the Universe

Neutrinos are among the most enigmatic particles in modern physics. They interact extremely weakly with matter, carry no electric charge, have a very small mass, and participate only in gravitational and weak interactions. Neutrinos are produced during nuclear reactions, such as those occurring in the core of the Sun. Because they are so elusive, detecting them requires special detectors placed deep underground, in thick layers of water or ice, to shield them from cosmic and background radiation.

A Breakthrough in Neutrino Observation

An international team of scientists has, for the first time, observed solar neutrinos turning carbon atoms into nitrogen inside the giant underground SNO+ detector, located in the SNOLAB laboratory two kilometers below ground in a mine near Sudbury, Canada. This deep placement effectively shields the laboratory from external interference that could drown out the faint neutrino signals. The results of the study have been published in the journal Physical Review Letters.

How the Observation Was Made

Physicists searched for events associated with the weak interaction—specifically, the inverse beta decay. In this process, a neutrino collides with a carbon-13 nucleus, transforms into an electron, and causes one of the neutrons to become a proton, resulting in the formation of nitrogen-13. This isotope is radioactive and exists for about 10 minutes.

To detect such rare events, the researchers used a “delayed coincidence” method: they tracked two related flashes—the first from the neutrino striking the carbon-13 nucleus, and the second, several minutes later, from the radioactive decay of the resulting nitrogen-13. The coincidence of these two signals allows scientists to distinguish genuine neutrino interactions from background noise.

Experimental Results

Analysis of data collected from May 2022 to June 2024 revealed 5.6 observed events over 231 days. Theoretical calculations predicted that neutrinos would cause about 4.7 such events during this period, confirming the theoretical models.

The Significance of the Discovery

“Detecting this interaction is an extraordinary achievement. Despite the rarity of the required carbon isotope, we managed to observe its interaction with neutrinos born in the Sun’s core and traveling vast distances to reach our detector,” noted Gulliver Milton, the lead author of the study.

The History of the SNO+ Experiment

The SNO+ experiment builds on the infrastructure of the original SNO facility, which previously demonstrated that neutrinos can oscillate between three types—electron, muon, and tau neutrinos—on their journey from the Sun to Earth. The head of the SNO experiment and co-author of the article, Arthur B. McDonald, was awarded the 2015 Nobel Prize in Physics for solving the solar neutrino problem.