Neutrino detection reaches the heart of the Sun

Write in Nature, the Borexino collaboration¹ reports results that go beyond a milestone in neutrino physics. They 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 nucleus and offer crucial information on the formation of heavy stars.

Neutrinos are tiny subatomic particles. They were first postulated by Wolfgang Pauli in 1930, to explain the energy that was apparently lacking during β decay, a process in which energetic electrons are emitted from an atomic nucleus. The presence of a massless particle that could carry any fraction of the decay energy would explain why the energy spectrum of the emitted electrons is continuous. Pauli’s explanation for why neutrinos had never been observed was that they interact incredibly weakly with matter. The decades of research that followed have provided a wealth of information on Pauli’s ‘ghost particle’, including the Nobel Prize-winning discovery that neutrinos actually have mass^2–4, although so small that it is beyond the range of current measurements.

Fusion reactions in the Sun produce an astonishing number of neutrinos: approximately 100 billion solar neutrinos pass through each of your vignettes every second. Due to the weakness of their interactions, they are hardly deterred from their path even when they have to traverse the whole body of the Earth: cutting-edge experiences⁵ (see also go.nature.com/36sktyj) found it difficult to observe a difference in the neutrino flux measured between day and night, due to the extremely small scale of this effect.

Neutrinos are therefore both difficult to observe and able to offer information about otherwise inaccessible regions of the Universe, such as distant supernovae or the interiors of stars. Energy produced in the center of the Sun in the form of photons takes tens of thousands of years to escape, but a solar neutrino can escape from the Sun and reach Earth in just eight minutes. This gives us a unique window into the heart of this flaming star.

The Sun is powered by fusion reactions that occur in its nucleus: in the intense heat of this highly pressurized environment, protons merge to form helium. This occurs in two separate 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 about 1% of solar energy, but dominates energy production in heavier stars.⁶.

The first solar neutrino detection experiment was performed using a detector at Homestake Mine in 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 of this experiment is that only about a third of the number of neutrinos of the expected type (flavor) were detected.^seven.

A decades-long campaign of experiments followed, seeking to solve this “solar neutrino problem”. Nobel Prize-winning results from Sudbury Neutrino Observatory in Ontario, Canada, ultimately explained the deficit: Neutrinos changed flavor between their production and detection³. The Borexino experiment at the Gran Sasso National Laboratory in Italy followed this result with a comprehensive spectral analysis of neutrinos from many stages of the pp chain⁸. This analysis ultimately allowed the field to come full circle, reopening the possibility of using solar neutrinos as a means of probing the Sun’s interior.

The Borexino collaboration is now reporting another groundbreaking achievement of its experiment: the first detection of neutrinos in the CNO cycle. This result is a huge leap forward, offering the possibility of solving the mystery of the elemental composition of the Sun’s core. In astrophysics, anything heavier than helium is called a metal. The exact metal content (metallicity) of a star’s core affects the speed of the CNO cycle. This, in turn, influences the temperature and density profile – and therefore 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, helioseismological measurements have been in agreement with SSM predictions for the speed of sound in the Sun, giving confidence in this model. However, more recent spectroscopic measurements of solar opacity have produced results significantly lower than previously thought, leading to discrepancies with the helioseismological data.⁹. Accurate measurements of CNO cycle neutrinos provide the only independent handle to study this difference. Such measurements would also shed additional light on stellar evolution.

The main obstacles to performing these measurements are the low energy and flux of CNO neutrinos, and the difficulty in separating the neutrino signal from background signal sources, such as radioactive decay processes. The Borexino experiment detects the light produced when solar neutrinos scatter electrons in a large vessel of liquid scintillator – a medium that produces light in response to the passage of charged particles. Accurate measurement of the energy and temporal profile of the detected light helps differentiate scintillation caused by solar neutrinos from light from other sources, such as radioactive contamination in the scintillator itself and in surrounding detector components.

The Borexino collaboration conducted a multi-year purification campaign to ensure unprecedented levels of radioactive contaminants in the scintillator. Even so, minor convection currents caused by temperature changes allowed radioactive contaminants to diffuse from the outer edges of the detector. The researchers mitigated this effect by establishing extremely precise control of thermal variations in the detector (Fig. 1), allowing them to achieve the extremely difficult feat of detecting CNO neutrinos. The resulting measurements are not yet precise enough to resolve the question of solar metallicity, but they offer a path towards this goal.

Future experiments will seek to improve the accuracy obtained by Borexino, by developing innovative methods to identify and reject background noise caused by radioactive contamination. In the meantime, the formidable achievement of the Borexino Collaboration brings us closer to a full understanding of our Sun and massive star formation, and is likely to set the goal in this area for years to come.