[gus_massa]

[I was going to write an explanation, but these sentences of Wikipedia article ar better.] From https://en.wikipedia.org/wiki/Super-Kamiokande

> A neutrino interaction with the electrons or nuclei of water can produce a charged particle that moves faster than the speed of light in water, which is slower than the speed of light in vacuum. This creates a cone of light known as Cherenkov radiation, which is the optical equivalent to a sonic boom. The Cherenkov light is projected as a ring on the wall of the detector and recorded by the PMTs.