Neutrino

The neutrino is an extremely light elementary matter particle. It has a half-integer spin and is therefore a fermion. The masses of neutrinos are very small compared to most other particles, almost but not quite non-existent. Since it is an electrically neutral lepton, the neutrino interacts neither by way of the strong nor the electromagnetic force, but only through the weak force and gravitation.

Because the cross section in weak nuclear interactions is very small, neutrinos can pass through matter almost unhindered. For typical neutrinos produced in a main sequence star (energy of a few MeV), it would take approximately one light year (~1016m) of lead to block half of them. Detection of neutrinos can therefore be challenging, requiring large detection volumes or high intensity artificial neutrino beams.

High-energy neutrinos

Neutrinos with higher energy are larger. They can be 15,000 larger than low-energy counterparts.