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.
History and background
Homestake experiment
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. The process was conducted very far underground, hence the decision to conduct the experiment in Homestake as the town was home to the Homestake Gold Mine. By conducting the experiment deep underground, Bahcall and Davis were able to avoid cosmic ray interactions which could affect the process and results. The entire experiment lasted several years as it was able to detect only a few chlorine to argon conversions each day, and the first results were not yielded by the team until 1968. To their surprise, the experimental value of the solar neutrinos present was less than 20% of the theoretical value Bahcall calculated. At the time, it was unknown if there was an error with the experiment or with the calculations, or if Bahcall and Davis did not account for all variables, but this discrepancy gave birth to what became known as the solar neutrino problem.
Further experimentation Davis and Bahcall continued their work to understand where they may have gone wrong or what they were missing, along with other astrophysicists who also did their own research on the subject. Many reviewed and redid Bahcall's calculations in the 1970s and 1980s, and although there was more data making the results more precise, the difference still remained. Davis even repeated his experiment changing the sensitivity and other factors to make sure nothing was overlooked, but he found nothing and the results still showed "missing" neutrinos. By the end of the 1970s, the widely expected result was the experimental data yielded about 39% of the calculated number of neutrinos. In 1969, Bruno Pontecorvo, an Italo-Russian astrophysicist, suggested a new idea that maybe we do not quite understand neutrinos like we think we do, and that neutrinos could change in some way, meaning the neutrinos that are released by the sun changed form and were no longer neutrinos the way neutrinos were thought of by the time they reached Earth where the experiment was conducted. This theory Pontecorvo had would make sense in accounting for the discrepancy between the experimental and theoretical results that persisted.
Solution to solar neutrino problem
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.
Neutrino observatories
Super-Kamiokande
The Super-Kamiokande is a 50,000 ton water Cherenkov detector 2,700 meters (8,900 ft) 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 the blue Cherenkov light emitted by electrons removed from water molecules 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
The Sudbury Neutrino Observatory (SNO), a 2,100 m (6,900 ft) 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.
Borexino
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