Neutrinos are often called ghost particles: trillions stream through every human body each second without leaving a trace, which makes detecting them one of experimental physics' great challenges. Produced in the nuclear reactions at the Sun's core, solar neutrinos have been observed interacting with only a handful of target materials. A study led by University of Oxford physicists, published in Physical Review Letters, adds carbon to that short list for the first time.
The observation was made with the SNO+ detector at SNOLAB, an international facility housed two kilometres underground in a working mine in Sudbury, Canada, where the rock overhead shields the experiment from cosmic rays. The team searched for a distinctive two-flash signature: an initial signal as a high-energy solar neutrino strikes a carbon-13 nucleus and transforms it into radioactive nitrogen-13, followed roughly ten minutes later by the flash of that nitrogen decaying.
This delayed coincidence method allowed genuine interactions to be separated from background noise, yielding 5.6 observed events across a 231-day exposure, statistically consistent with the 4.7 predicted by theory. Though the numbers are small, the detection technique lays the groundwork for future studies of low-energy neutrino interactions, particles that are essential to understanding stellar processes, nuclear fusion and the evolution of the universe.