Question: What is a neutrino?

Steve Marsden answered on 13 Mar 2016:

They are fundamental particles. They have tiny masses, even in comparison to other fundamental particles like electrons. The key property that makes them different from other particles is that they very rarely interact with matter. The sun emits a huge number of neutrinos. A majority of these that ‘hit’ the Earth, fly straight through and come out the other side.

There are experiments which attempt to study them. Because neutrinos will usually pass through the detector without interacting, the detector needs to be huge (so more neutrinos pass through it), and run for very long periods in order for a sizeable number measurements to be made.

We still don’t completely understand some of their properties. Most notably, their masses are so small we’ve not been able to measure them (though we do know that they aren’t massless). Also, as far as we can tell there are three types (or ‘flavours’) of neutrino. These can change into each other. We’re still experimenting on them to try and understand how this happens.