Writing in Nature, the Borexino Collaboration1 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.
Neutrinos are tiny, subatomic particles. They were first postulated to exist by Wolfgang Pauli in 1930, to account for the energy that was apparently missing 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 energy from the decay would explain why the spectrum of emitted electron energies is continuous. Pauli’s explanation for why neutrinos had never been observed was that they interact incredibly weakly with matter. Subsequent decades of research have yielded a wealth of information about Pauli’s ‘ghost particle’, including the Nobel-prizewinning discovery that neutrinos do, in fact, have a mass2–4, albeit one so small as to be beyond the reach of current measurements.
Fusion reactions in the Sun produce an astonishing number of neutrinos: roughly 100 billion solar neutrinos pass through each of your thumbnails every second. Because of the weakness of their interactions, they are barely deterred from their path even when they have to pass through the entire body of the Earth: cutting-edge experiments5 (see also go.nature.com/36sktyj) have struggled to observe a difference in the measured neutrino flux between daytime and night-time, owing to the vanishingly small scale of this effect.
Neutrinos are therefore both challenging to observe and yet able to offer insights into other-wise unreachable regions of the Universe, such as distant supernovae or the interiors of stars. Energy produced in the centre of the Sun in the form of photons takes tens of thousands of years to escape, but a solar neutrino can escape the Sun and reach Earth in just eight minutes. This gives us a unique window into the core of this blazing star.
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 detected7.
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 detection3. 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 chain8. 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 data9. 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.
The chief obstacles to making these measurements are the low energy and flux of CNO neutrinos, and the difficulty of separating the neutrino signal from sources of background signals, such as radioactive-decay processes. The Borexino experiment detects light produced when solar neutrinos scatter off electrons in a large vat of liquid scintillator — a medium that produces light in response to the passage of charged particles. A precise measurement of the energy and time profile of the detected light allows the scintillation caused by solar neutrinos to be differentiated from light resulting from other sources, such as radio-active contamination in the scintillator itself and in surrounding detector components.
The Borexino Collaboration carried out a multi-year purification campaign to ensure unprecedentedly low levels of radioactive contaminants in the scintillator. Even so, minor convection currents caused by temperature variations allowed radioactive contaminants to diffuse from the outer edges of the detector. The researchers mitigated this effect by establishing exquisitely fine control of thermal variations in the detector (Fig. 1), thus allowing them to achieve the extremely challenging 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 objective.
Figure 1 | The Borexino neutrino detector. The Borexino experiment detects light produced when solar neutrinos scatter off electrons in a large vat of liquid scintillator — a medium that produces light in response to the passage of charged particles. The Borexino Collaboration wrapped the detector in thermal insulation to control temperature variations in the detector. This helped the team to take the highly precise measurements needed to detect solar neutrinos produced by the Sun’s secondary solar-fusion cycle1.Source: Ref. 1
Future experiments will seek to improve on the precision achieved by Borexino, by developing innovative methods to identify and reject background noise caused by radioactive contamination. In the meantime, the Borexino Collaboration’s tremendous accomplishment moves us closer to a complete understanding of our Sun, and of the formation of massive stars, and is likely to define the goal in this field for years to come.
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November 25, 2020 at 11:04PM
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