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Extremely light particle produced by the Sun

A solar neutrino is a neutrino originating from nuclear fusion in the Sun's core, and is the most common type of neutrino passing through any source observed on Earth at any particular moment. Neutrinos are elementary particles with extremely small rest mass and a neutral electric charge. They only interact with matter via weak interaction and gravity, making their detection very difficult. This has led to the now resolved solar neutrino problem. Much is now known about solar neutrinos, but research in this field is ongoing.

The timeline of solar neutrinos and their discovery dates back to the 1960s, beginning with the two astrophysicists John N. Bahcall and Raymond Davis Jr. The experiment, known as the Homestake experiment, named after the town in which it was conducted (Homestake, South Dakota), aimed to count the solar neutrinos arriving at Earth. Bahcall, using a solar model he developed, came to the conclusion that the most effective way to study solar neutrinos would be via the chlorine argon reaction. Using his model, Bahcall was able to calculate the number of neutrinos expected to arrive at Earth from the Sun. Once the theoretical value was determined, the astrophysicists began pursuing experimental confirmation. Davis developed the idea of taking hundreds of thousands of liters of perchloroethylene, a chemical compound made up of carbon and chlorine, and searching for neutrinos using a chlorine argon detector. This theory Pontecorvo had would make sense in accounting for the discrepancy between the experimental and theoretical results that persisted.

Pontecorvo was never able to prove his theory, but he was on to something with his thinking. In 2002, results from an experiment conducted 2100 meters underground at the Sudbury Neutrino Observatory proved and supported Pontecorvo's theory and discovered that neutrinos released from the Sun can in fact change form or flavor because they are not completely massless. This discovery of neutrino oscillation solved the solar neutrino problem, nearly 40 years after Davis and Bahcall began studying solar neutrinos.

The Super Kamiokande is a 50,000 ton water Cherenkov detector underground. The primary uses for this detector in Japan in addition to neutrino observation is cosmic ray observation as well as searching for proton decay. In 1998, the Super Kamiokande was the site of the Super Kamiokande experiment which led to the discovery of neutrino oscillation, the process by neutrinos change their flavor, either to electron, muon or tau. The Super Kamiokande experiment began in 1996 and is still active. In the experiment, the detector works by being able to spot neutrinos by analyzing water molecules and detecting electrons being removed from them which then produces a blue Cherenkov light, which is produced by neutrinos. Therefore, when this detection of blue light happens it can be inferred that a neutrino is present and counted.

The Sudbury Neutrino Observatory (SNO), a underground observatory in Sudbury, Canada, is the other site where neutrino oscillation research was taking place in the late 1990s and early 2000s. The results from experiments at this observatory along with those at Super Kamiokande are what helped solve the solar neutrino problem. The SNO is also a heavy water Cherenkov detector and designed to work the same way as the Super Kamiokande. The Neutrinos when reacted with heavy water produce the blue Cherenkov light, signaling the detection of neutrinos to researchers and observers.

[[File:Solar neutrino flux spectrum. png|thumb|right|300px|The greatest number of solar neutrinos are direct products of the proton–proton reaction (tall, dark blue curve on the left). They have a low energy – only reaching up to 400 keV. There are several other significant production mechanisms, with energies up to 18 MeV. From the Earth, the amount of neutrino flux at Earth is around 7·1010 particles·cm−2·s −1. The number of neutrinos can be predicted with great confidence by the standard solar model, but the number of neutrinos detected on Earth versus the number of neutrinos predicted are different by a factor of a third, which is the solar neutrino problem. Solar models additionally predict the location within the Sun's core where solar neutrinos should originate, depending on the nuclear fusion reaction which leads to their production. Future neutrino detectors will be able to detect the incoming direction of these neutrinos with enough precision to measure this effect. thumb|300px|Theoretical curves of survival probability of solar neutrinos that arrive on day (orange, continuous) or on night (purple, dashed), as a function of the energy of the neutrinos. Also shown the four values of the energy of the neutrinos at which measurements have been performed, corresponding to four different branches of the proton–proton chain. The energy spectrum of solar neutrinos is also predicted by solar models. It is essential to know this energy spectrum because different neutrino detection experiments are sensitive to different neutrino energy ranges. The Homestake experiment used chlorine and was most sensitive to solar neutrinos produced by the decay of the beryllium isotope 7Be. The Sudbury Neutrino Observatory is most sensitive to solar neutrinos produced by 8B. The detectors that use gallium are most sensitive to the solar neutrinos produced by the proton–proton chain reaction process, however they were not able to observe this contribution separately. The observation of the neutrinos from the basic reaction of this chain, proton–proton fusion in deuterium, was achieved for the first time by Borexino in 2014. In 2012 the same collaboration reported detecting low energy neutrinos for the proton–electron–proton (pep reaction) that produces 1 in 400 deuterium nuclei in the Sun. The detector contained 100 metric tons of liquid and saw on average 3 events each day (due to C production) from this relatively uncommon thermonuclear reaction. In 2014, Borexino reported a successful direct detection of neutrinos from the pp reaction at a rate of 144±33/day, consistent with the predicted rate of 131±2/day that was expected based on the standard solar model prediction that the pp reaction generates 99% of the Sun's luminosity and their analysis of the detector's efficiency. And in 2020, Borexino reported the first detection of CNO cycle neutrinos from deep within the solar core. Note that Borexino measured neutrinos of several energies; in this manner they have demonstrated experimentally, for the first time, the pattern of solar neutrino oscillations predicted by the theory. Neutrinos can trigger nuclear reactions. By looking at ancient ores of various ages that have been exposed to solar neutrinos over geologic time, it may be possible to interrogate the luminosity of the Sun over time, which, according to the standard solar model, has changed over the eons as the (presently) inert byproduct helium has accumulated in its core.

Wolfgang Pauli was the first to suggest the idea of a particle such as the neutrino existing in our universe in 1930. He believed such a particle to be completely massless. This was the belief amongst the astrophysics community until the solar neutrino problem was solved. Frederick Reines, from the University of California at Irvine, and Clyde Cowan were the first astrophysicists to detect neutrinos in 1956. They won a Nobel Prize in Physics for their work in 1995.
thumb|Raymond Davis Jr receives the Medal of Science from President George W. Bush. Raymond Davis and John Bahcall are the pioneers of solar neutrino studies. While Bahcall never won a Nobel Prize, Davis along with Masatoshi Koshiba won the Nobel Prize in Physics in 2002 after the solar neutrino problem was solved for their contributions in helping solve the problem. Pontecorvo, known as the first astrophysicist to suggest the idea neutrinos have some mass and can oscillate, never received a Nobel Prize for his contributions due to his passing in 1993. Arthur B. McDonald, a Canadian physicist, was a key contributor in building the Sudbury Neutrino Observatory (SNO) in the mid 1980s and later became the director of the SNO and leader of the team that solved the solar neutrino problem. Processes at these low energies consisted vital information that told researchers about the solar metallicity. The results from this research yielded significantly different findings compared to past research in terms of the overall flux spectrum. The search was completed using data from exposure from the Borexino experiment's second phase which consisted of data over 1291.5 days (3.54 years). The results yielded that the electron recoil spectrum shape was as expected with no major changes or deviations from it.