Question: How does Super-Kamiokande work?

Susan Cartwright answered on 15 Jun 2015:

Super-Kamiokande, known to its friends as Super-K, is a water Cherenkov detector. This means that it detects charged particles that are travelling faster than the speed of light in water.

“Wait!” I hear you cry. “Nothing can travel faster than the speed of light!” This is true, but it’s incomplete – what is really true is that nothing can travel faster than the speed of light IN A VACUUM. When travelling through something other than a vacuum, light slows down according to the refractive index of the material: water has a refractive index of about 4/3, so light in water is travelling at about 3/4 of the speed of light in a vacuum.

Particles other than light are not slowed down by the refractive index, so a particle that is travelling (perfectly legally) at say 90% of the speed of light in a vacuum is travelling faster than the speed of light in water. This has an effect very like what happens when a plane travels faster than the speed of sound in air – with the plane, you get a sonic boom; with the particle, you get an “optical boom”: a coherent wave front of blue light (called Cherenkov radiation), travelling outwards from the track of the particle in a cone-shaped pattern. The water tank of Super-K is lined with very sensitive light detectors called photomultiplier tubes (PMTs), which pick up this blue light. The amount of light and the time it arrived at each PMT is recorded and used to reconstruct the cone, which tells you the direction that the particle was travelling in (from the axis of the cone) and its energy (from the amount of light).

This only works for charged particles, because only charged particles emit or absorb photons (the photon is the carrier of the electromagnetic force). So Super-K cannot (unfortunately) detect neutrinos directly. Instead, we detect cases where an incoming neutrino has interacted with an atom or electron in the water and caused the emission of a charged particle (electron or muon) travelling faster than the speed of light in water. As neutrinos are very weakly interacting, this happens only very rarely: Super-K detects only a tiny fraction of the neutrinos that pass through it – most of them just go straight through leaving no trace.

A problem with this is that cosmic rays, caused by energetic particles from space interacting with the top of the atmosphere, are mostly muons at ground level. These can cause Cherenkov radiation in Super-K, and would drown out our signal if we didn’t do something about them. In fact we do two things: the first is simply that Super-K is not on the surface, it’s in a zinc mine underneath a mountain, so there is a lot of rock on top of us to absorb most of the cosmic ray muons. The second is that, as well as the inward-facing PMTs surrounding the central part of the detector, there are outward-facing PMTs picking up light from a thin “jacket” of water, called the outer detector, that surrounds the central core. A charged particle coming in emits Cherenkov light in this outer detector which is picked up by the outward-facing tubes, whereas a neutrino coming in produces no light until it interacts and is converted into a charged electron or muon. Therefore, by rejecting any events that have Cherenkov light in the outer detector, we can remove any remaining cosmic rays that have made it through the rock.