Today’s colloquium was Stuart Freedman on the latest results from KamLAND, one of the neutrino detection experiments. The experiment is basically a gigantic vat of liquid scintillator—an oil convenient for producing photons from exotic particles passing through—surrounded by high-efficiency photon detectors. Neutrinos are produced in huge quantities by the sun and nuclear reactors, but they rarely interact with matter, so to observe them one needs to construct a very large detector and wait for a while.
I’ve always enjoyed following the neutrino experiments, since they came online about when I started to study physics, and since then they have made steady progress understanding this particle. It’s a nice example of the incremental progress of science. Around my senior year in high school the story was “We’ve been assuming neutrinos are massless, but it’s been suggested they do have mass and experiments are being constructed to look for it.” (That was the year I went to IPhO, which was held in Sudbury, Canada, a town whose only distinction was that the Sudbury Neutrino Observatory was being built there, so we heard a lot on this subject.) Over the next few years the line became “Neutrinos might have mass,” then “Neutrinos probably have mass (but we don’t know what it is)”. And in today’s colloquium, the word was:
- Neutrinos totally have mass. There are three different varieties of neutrinos, named according to the lepton they’re associated with in weak-force interactions: for example, the basic nuclear beta decay produces an electron and an electron neutrino. But KamLAND looked at neutrinos produced in this way by nuclear reactors, and found that neutrinos that start out as electron neutrinos oscillate between this type and the other two types (the mu and tau neutrinos). This can happen only if neutrinos are massive.
- But we don’t know what it is. Measurements on neutrino oscillations only tell you the relationship between the masses of the three types of neutrinos, and not the masses themselves. There are estimates of the actual masses based on this, but they are not very precise.
- The electron, mu, and tau neutrinos are not mass eigenstates. Rather than having a single mass itself, the electron neutrino is actually a kind of mixture (technically, a superposition) of three neutrinos, each of which does have its own mass. The mass eigenstates have been creatively named ν1, ν2, and ν3. It’s known approximately how much of each mass eigenstate is present in the electron, mu, and tau mixtures, but not how the masses are arranged—so ν3 could be the heaviest or the lightest.
- There’s still an undetermined parameter in neutrino mixing. It’s a complex phase, and relates to a symmetry breaking? This is one of those things I’d be more informed about if I’d ever taken a course on the Standard Model.
Freedman also spent some time on another angle of this experiment, in geophysics rather than fundamental physics. (I know I have some geophysicists reading, so you can correct me if I get this wrong.) There’s a discrepancy between various estimates of the heat produced by the Earth, and one hypothesis (which is apparently not widely credited) is that the core of the Earth contains a natural nuclear reactor. Since KamLAND is built to detect neutrinos from man-made reactors, it could in principle look for one at the center of the planet as well. Except that KamLAND is (deliberately) built really close to a number of reactors in Japan, and any geophysical signal would be absolutely swamped by the signal from power plants. So in practice it looks like another detector would have to be built somewhere else to do this experiment.