Have the solar neutrinos that once went missing been found?
The energy of the sun has long puzzled physicists and astronomers. It wasn't until the 1930s that scientists realized that the sun's energy comes from nuclear fusion in its core. The theory of nuclear fusion successfully explained various observational characteristics of the sun, but there was one phenomenon that did not agree with theoretical predictions, which puzzled astronomers for many years.
According to the laws of nuclear fusion, if the sun's core was indeed undergoing large-scale thermonuclear reactions, then a large number of neutrinos would be produced. Neutrinos are a very peculiar particle that does not carry a charge and, according to the Standard Model, which describes the interactions of fundamental particles, have a zero resting mass. In addition, neutrinos have the characteristic of not participating in electromagnetic interactions or strong interactions within atomic nuclei, making them extremely penetrating and difficult to "capture".
To verify the theory of nuclear fusion in the sun's core, scientists had to devise special methods to measure the actual number of solar neutrinos. To this end, scientists cleverly set up "traps". In a gold mine 1,500 meters deep in South Dakota, USA, American physicist Davis led the first important experiment in history to measure solar neutrinos. However, the actual results measured in 1968 showed that the actual number of solar neutrinos was only one-third of the predicted theoretical value. A large number of solar neutrinos had gone missing!
Later, physicists improved their measurement methods. The Kamioka Neutrino Detector Experiment led by Japanese scientist Masatoshi Koshiba confirmed a similar phenomenon. For a time, the mystery of the disappearance of solar neutrinos became a hot topic of discussion among scientists. Where did the problem lie? Did astronomers make a mistake? Does the sun's energy not come from nuclear fusion? Or was physicists' theory about neutrinos wrong?
People first thought of modifying the standard solar model. However, the results of helioseismology showed that the solar model was not wrong. No matter how the solar model was adjusted, it could not conform to the actual measurement results of neutrinos. Any adjustment to the model would only lead to more contradictions in other aspects.
Scientists were forced to attempt to modify the theory of neutrinos and proposed the concept of "neutrino oscillation". Currently, three types of neutrinos are known: electron neutrinos, μ neutrinos, and τ neutrinos. The theory of neutrino oscillation states that different types of neutrinos can convert into each other. Therefore, some of the electron neutrinos emitted by the sun may have been converted into other types of neutrinos during their journey. Since current measurement methods can only detect electron neutrinos, this created the illusion of a shortage of solar neutrinos. However, this theory also meant that the resting mass of neutrinos was not zero, which challenged many fundamental concepts of particle physics and required rigorous scientific experiments to verify.
In 1998, the Super-Kamioka Detector in Japan first discovered definitive evidence of the real existence of neutrino oscillation. In the experiment, it was confirmed that μ neutrinos had indeed converted into τ neutrinos. In 2001, the Sudbury Neutrino Observatory in Canada published new measurement results, detecting all three types of neutrinos emitted by the sun, of which only 35% were electron neutrinos. The total flux of the three types of neutrinos agreed well with the predictions of the standard solar model, thus completely solving the previously observed shortage of solar neutrinos.
In 2002, Davis, who led the first neutrino measurement experiment, and Koshiba, the leader of the Kamioka Detector, jointly won the Nobel Prize in Physics. Their contributions to fundamental particle physics will forever be remembered by future generations alongside the mysterious mystery of neutrinos.