If the next question is “What do they make foghorns out of?” you’re ready for this slice of quantum phenomena subtitled “Does your neutrino lose its flavour on the bed-post overnight?”
Neutrinos, which interact so weakly with normal matter that even the best detectors only manage to capture small handfuls of interactions, come in three “flavours”: electron, muon, and tau, and they oscillate between these flavours.
The SuperKamiokande detector, which comprises 50,000 tonnes of water and 11,000 photomultiplier tubes, is specific to electron neutrinos, spotting the tiny amount of Cherenkov radiation emitted when a neutrino scores a direct hit on an atom in the tank. These interactions are rare, which is why the experiments are so long-lasting.
With enough data, however, something interesting emerged: when it’s night-time at SuperKamiokande, the detector observes 3.2 per cent more electron neutrinos than during the day. In other words, when the detector is on the sun-side of Earth, the neutrinos passing through it are very slightly skewed towards muon and tau flavours, while at night-time, there’s slightly more electron flavours for the detector to observe.
Which, at the very least, justifies going on to a HyperKamiokande detector.
(With thanks to Lonnie Donegan.)