Deep-dive paper
Direct evidence for neutrino flavor transformation from neutral-current interactions
The Sudbury Neutrino Observatory measured all three neutrino flavours from the Sun and recovered the total flux stellar models predict.
Why it matters
For decades only about a third of the expected solar neutrinos were seen, casting doubt on whether the flux was really there. SNO showed the flux is complete — the missing two-thirds had simply changed flavour.
Method
A kilotonne of heavy water enabled three separate reaction channels, one of which counts all flavours equally, allowing the total flux and the electron-flavour fraction to be measured at once.
Mechanism
Electron neutrinos born in the solar core convert to other flavours en route. A flavour-blind neutral-current channel sees the sum, while charged-current channels see only the electron part.
Results
The total measured flux matched the standard solar model, while the electron fraction was suppressed — direct proof of flavour transformation and a full-strength solar flux.
Significance
It resolved the solar-neutrino problem and confirmed that the ambient neutrino flux from the Sun is real and undiminished, also recognised in the 2015 Nobel Prize.
Formula-term mapping
Reinforces Φ(E,θ,t): the incident flux is quantitatively as large as first-principles astrophysics predicts, not a depleted trickle.