Two of the three winners of the Nobel Prize in Physics for 2002 worked on the Solar Neutrino Problem. What is this problem? Has it been solved?

The two winners of the Nobel Prize in Physics for 2002 who worked on the Solar Neutrino Problem are Raymond Davis, who started measuring neutrinos from the Sun with a huge container of dry cleaning fluid in a mine in South Dakota, and Masatoshi Koshiba, who led the Kamiokande neutrino experiment in Japan that confirmed and extended the solar neutrino measurements.

The Sun, like most stars, is ultimately powered by fusion reactions the energy gained when simpler atomic nuclei like hydrogen combine to form heavier nuclei like helium. Fusion processes, which take place in the core of the Sun, produce a huge flux of neutrinos. These neutrinos can be detected on Earth using large underground detectors, such as those built by Davis and Koshiba. The neutrino flux has been measured to see if it agrees with theoretical calculations based upon our understanding of the workings of the Sun and the details of the Standard Model of particle physics. The measured flux is roughly one-half of that expected from theory (especially the work of John Bahcall, although checked by many others). This discrepancy between theory and observation is the "Solar Neutrino Problem."

The cause of the deficit was a mystery. Is the model of particle physics wrong? Is the model of the Solar interior wrong? Are the experiments in error?

Thanks to recent measurements, particularly those with the Super-Kamiokande detector in Japan and the Sudbury Neutrino Observatory in Canada, there is probably an answer the Sun’s nuclear fusion source of energy operates as predicted, but the neutrinos themselves seem to behave in a way unexpected by the Standard Model of particle physics. In the Standard Model, neutrinos are massless particles that travel at the speed of light. The three types of neutrino, called electron, muon, or tau neutrinos, were thought to be different particles, completely independent of each other. The new results strongly suggest that this part of the Standard Model is incorrect. Neutrinos have a small mass and can “oscillate” or change from one type of neutrino into another. The change actually occurs before the neutrinos escape from the Sun. The reason the earlier underground experiments were seeing a deficit of neutrinos is that they were not detecting all three types. The Sudbury results do measure all three types of neutrino, and their results match the theoretical predictions well.

As so often happens in science, solving one mystery reveals a new one. The new puzzle is how to correct the Standard Model of particle physics to include these new neutrino results.

Nobel Prize Information: http://focus.aps.org/story/v10/st18

Sudbury Neutrino Observatory: http://www.physicscentral.com/news/news-01-7.html

Solar Fusion and Neutrinos: http://www.tim-thompson.com/fusion.html

This week's question is provided by Dr. Dave Thompson. Dr. Thompson is an astrophysicist who studies gamma rays in the Laboratory for High Energy Astrophysics. He helped build, test, and analyze data from EGRET on the Compton Gamma Ray Observatory, and he is now helping build part of the Gamma Ray Large Area Space Telescope (GLAST), scheduled for launch in 2006. His particular scientific interest is gamma-ray pulsars.