Neutrinos from the sun

Solar neutrinos are simply that: neutrinos emitted by the sun. The sun is the source of the vast majority of neutrinos traveling through you at any given time. Every second, about 100 billion solar neutrinos pass through your fingernail.

Neutrinos are created during the sun’s nuclear fusion process. Protons (the nucleus of the simplest element, hydrogen) fuse together in fusion to generate helium, a heavier element. This emits neutrinos and energy, which eventually reach Earth in the form of light and heat. Electron neutrinos make up all of the neutrinos produced by the sun.

When scientists began seeking for electron neutrinos in the 1960s, something interesting happened. Only around a third to half of the expected quantity of neutrinos were found in detectors. It took nearly four decades to solve the solar neutrino problem, which became known as the solar neutrino problem.

It all started with Ray Davis Jr.’s Homestake experiment. To search for neutrinos, 100,000 litres of dry cleaning fluid (perchloroethylene) were utilized in the experiment. It was housed a mile deep in the caverns of the Homestake Gold Mine in South Dakota, which was an operational mine at the time and is currently used for various projects, including the Deep Underground Neutrino Experiment, which will continue neutrino research. John Bahcall, Davis’s scientific collaborator, had estimated the number of neutrinos that would arrive from the sun and convert one of the chlorine atoms in the detector to an argon atom. However, just about a third of the neutrinos appeared to have arrived. Davis’ experiment, Bahcall’s calculations, and the current model of the sun were all ruled out, as was their understanding of neutrinos. Some scientists, including as Bruno Pontecorvo, suggested that the neutrino model was the source of the inaccuracy, while many others were unconvinced.

The Kamiokande experiment in Japan in 1989 further contributed to the confusion. More neutrinos were discovered in the pure water detector than in Davis’ experiment, about half of what was predicted. However, there was still the issue of the missing neutrinos. Low-energy neutrinos were also missing in the GALLEX experiment in Italy and the SAGE experiment in Russia.

Researchers increasingly looked to novel physics beyond the Standard Model to explain the neutrino shortage as solar observations improved and the solar model was validated. Data from two newer experiments provided the breakthrough. In 1996, the Super-Kamiokande experiment, an enhanced version of the Kamiokande experiment, began observations, and in 1999, the Sudbury Neutrino Observatory in Canada joined. The leaders of these two initiatives were awarded the Nobel Prize in Physics in 2015 for discovering the solution to the solar neutrino problem: neutrino oscillations. Approximately two-thirds of the electron neutrinos emitted by the sun changed flavor on their way to Earth, arriving as muon or tau neutrinos. Evidence that neutrinos changed types also revealed that they have mass, a surprising finding that the Standard Model did not predict.

We still have a lot to learn from solar neutrinos. Scientists can, for example, compare how solar neutrinos moving through space vs neutrinos going through denser places like Earth. These studies provide insight into the neutrino oscillation phenomena.

Solar neutrinos can also reveal direct information about our sun’s core. Neutrinos produced in the sun’s core have an unexpected effect: they arrive on Earth before light from the sun (formed in the same reaction) does. This isn’t due to the fact that neutrinos can’t travel faster than light. Because neutrinos contact with matter so infrequently, they can immediately escape from the sun’s dense core, whereas photons (light particles) bounce about before breaking free. Because of this feature, the Borexino Experiment in Italy discovered that the sun emits the same amount of energy today as it did 100,000 years ago.