The Borexino Collaboration reports results that blast past a milestone in neutrino physics. They have detected solar neutrinos produced by a cycle of nuclear-fusion reactions known as the carbon–nitrogen–oxygen (CNO) cycle. Measurements of these neutrinos have the potential to resolve uncertainties about the composition of the solar core, and offer crucial insights into the formation of heavy stars.
The Sun is powered by fusion reactions that occur in its core: in the intense heat of this highly pressurized environment, protons fuse together to form helium. This occurs in two distinct cycles of nuclear reactions. The first is called the proton–proton chain (or pp chain), and dominates energy production in stars the size of our Sun. The second is the CNO cycle, which accounts for roughly 1% of solar power, but dominates energy production in heavier stars6.
The first experiment to detect solar neutrinos was carried out using a detector in Homestake Mine, South Dakota. This used measurements of pp-chain solar neutrinos to probe the Standard Solar Model (SSM), which describes nuclear fusion in the Sun. The surprising result from this experiment was that only approximately one-third as many neutrinos of the expected type (flavour) were detected.
A decades-long campaign of experiments followed, seeking to resolve this ‘solar neutrino problem’. Nobel-prizewinning results from the Sudbury Neutrino Observatory in Ontario, Canada, eventually explained the deficit: the neutrinos were changing flavour between their production and detection. The Borexino experiment at the Gran Sasso National Laboratory in Italy followed up this result with a full spectral analysis of neutrinos from many stages of the pp chain. This analysis finally allowed the field to come full circle, re-opening the possibility of using solar neutrinos as a means of probing the Sun’s interior.
The Borexino Collaboration now reports another groundbreaking achievement from its experiment: the first detection of neutrinos from the CNO cycle. This result is a huge leap forward, offering the chance to resolve the mystery of the elemental composition of the Sun’s core. In astrophysics, any element heavier than helium is termed a metal. The exact metal content (the metallicity) of a star’s core affects the rate of the CNO cycle. This, in turn, influences the temperature and density profile — and thus the evolution — of the star, as well as the opacity of its outer layers.
The metallicity and opacity of the Sun affect the speed of sound waves propagating through its volume. For decades, helio-seismological measurements were in agreement with SSM predictions for the speed of sound in the Sun, giving confidence in that model. However, more-recent spectroscopic measurements of solar opacity produced results that were significantly lower than previously thought, leading to discrepancies with the helio-seismological data. Precise measurements of CNO-cycle neutrinos offer the only independent handle by which to investigate this difference. Such measurements would also shed further light on stellar evolution.