Ever heard of neutrinos? Not neutrons, but neutrinos. They’re part of the Standard model of particle physics theory. There isn’t just a single kind of them – they come in a pack of 3 (as all of the fermions):
- Electron neutrino
- Muon neutrino
- Tau neutrino

They are so called Leptons, which have no electrical charge, but (a bit of) mass.

Whenever a neutrino (which really doesn’t happen that often) collides with a charged particle (e.g. proton or electron) in a medium, where the speed of light is slower than in vacuum (e.g. in water), it is absorbed and partially re-emitted as so called Cherenkov radiation. This kind of radiation is also generated within nuclear reactors and is visible as blue light (opposed to the green glow known from movies).