Electron-type neutrinos (νe) are produced in the core of the Sun but they change into muon-type or tau-type neutrinos (νμ/ντ) by a phenomenon called “neutrino oscillation” on their way to the earth. Because the probabilities for interaction with matter for νe and νμ/ντ, is smaller than that for νe, the observed flux at earth is smaller than would be expected if there were no neutrino oscillations.
An indication of a difference between the daytime and nighttime solar neutrino fluxes has recently been observed by Super-Kamiokande (SK) using solar neutrino data accumulated since 1996. A paper detailing this result has been published in the journal Physical Review Letters (PRL), where it was selected as an “Editor's Suggestion.” This is the first direct observation of the effects of matter on neutrino oscillations.
When solar neutrinos arrive at the surface of the earth roughly 32% are νe and 68% are νμ/ντ. During the nighttime, though, solar neutrinos travel through the earth and some fraction of the νμ/ντ change back into νe. In this sense the νe are regenerated in the earth (see the figure below). Thus the observed solar neutrino flux during the nighttime is larger than during the daytime. Though the difference is only about a few percent, the world’s largest solar neutrino detector, Super-Kamiokande, has successfully observed it.
Neutrino oscillation research is progressing quite rapidly and future projects like “Hyper-Kamiokande” plan to address questions such as, “Why is there so much matter but no anti-matter in the universe?” A thorough understanding of the matter effect is one of the key ingredients for such research and the SK result contributes directly to that understanding. SK will further improve its solar neutrino measurements and plans to study the matter effect using the solar neutrino energy spectrum in the future.
This result has been reported in the media as follows