An international collaboration of particle physicists has announced the detection of solar neutrinos originating from the secondary fusion cycle powering the Sun. This is a world-first that could shine a light on the otherwise unseen solar core and further our understanding of stellar evolution. The Borexino Collaboration, using the eponymous Borexino detector at the Gran Sasso National Laboratory in Italy, reported its discovery in the journal Nature.
The Sun, like other stars, is powered by a series of reactions in its core, fusing hydrogen nuclei to form helium while converting some of their mass into energy as per Einstein’s famous equation E = mc2. According to the solar standard model (SSM), the current mathematical theory of solar structure and evolution, 99% of the Sun’s energy comes from the so-called proton-proton chain (p-p chain) of fusion reactions. This type of reaction is thought to dominate energy production in lighter, cooler stars. The remaining 1% is produced in the largely temperature-dependent carbon-nitrogen-oxygen cycle (CNO cycle) which only becomes prevalent in more massive stars with core temperatures of over 17 million kelvins. While falling slightly short of the mark, the core of our Sun still chugs along at the no less formidable temperature of 15 million kelvins.
Both reaction cycles release an abundance of neutrinos, the lightest known subatomic particles, which only interact with matter weakly. Thanks to this unsociable attitude, most neutrinos glide through the solar sphere undisturbed, carrying insights straight into the Sun’s core. They then flood the Earth with around 60 billion such solar neutrinos flowing through the area of an average shirt button (about 1 cm2) each second. However, this indifference to obstacles also makes them immensely difficult to detect on Earth, requiring background noise levels only found deep underground and earning neutrinos the nickname ‘ghost particle’.
“Hidden under 1400 metres of solid rock beneath Gran Sasso, the highest mountain in the Italian Apennines, The Borexino experiment sees the flux of corruptive cosmic muons reduced to a millionth of its value on the surface.”
Hidden under 1400 metres of solid rock beneath Gran Sasso, the highest mountain in the Italian Apennines, the Borexino experiment sees the flux of corruptive cosmic muons (higher energy particles originating in the upper atmosphere) reduced to a millionth of its value on the surface. The detector itself, centred around an eight-metre-wide spherical vessel filled with 278 tonnes of luminescent liquid, makes use of a phenomenon called elastic scattering in which incoming neutrinos essentially bounce off electrons in the liquid, producing a flash of light which is then detected by over 2000 photomultiplier tubes around the chamber.
Despite the experiment having been in constant operation since 2007, it was only a substantial upgrade to its thermal cladding in 2015 that finally made the chamber sufficiently free of radioactive contaminants for neutrinos produced in the CNO cycle to be detected. Over a data collection period of almost three years, the experiment registered a daily average of 7.2 interactions per 100 tonnes of scintillator liquid. This is equivalent to about 700 million CNO neutrinos passing through the aforementioned shirt button each second.
“Over a data collection period of almost three years, the experiment registered a daily average of 7.2 interactions per 100 tonnes of scintillator liquid. This is equivalent to about 700 million CNO neutrinos passing through the aforementioned shirt button each second.”
The CNO cycle was originally proposed by the physicists Hans Bethe and Carl Friedrich von Weizsäcker as early as the 1930s, over a decade before the proton-proton chain was discovered. But while solar neutrinos from the p-p chain have been studied directly for years, including a full spectral analysis conducted by the Borexino Collaboration in 2018, the new detection marks the first experimental confirmation of the secondary fusion reaction cycle in the Sun.
Since the rate of the CNO cycle largely depends on the metallicity of the solar core, the researchers believe a precise measurement of the flux of CNO neutrinos could solve the long-standing solar metallicity problem and help differentiate between two competing solar models. As the metallicity, defined as the abundance of elements heavier than hydrogen and helium, is related to the chemical composition of the Sun at its birth, such a measurement could also shed light on the early history of our Sun and stellar formation in general.
Originally set for decommissioning before the new year due to environmental concerns, the Borexino experiment is now in a race against time to improve on its groundbreaking measurement. Background noise levels have been pushed down to unprecedented levels and data collection may still continue into 2021. Should Borexino fall short of its goal, however, the next generation of neutrino detectors is sure to take up the torch. Among these, the SNO+ experiment currently in the early stages of its observational run in Ontario, Canada, has the precise measurement of solar metallicity in its sights. Expect more cosmic ghost sightings in the future.