Neutrino

Neutrinos are elementary particles of matter, which participate only in the weak interaction that makes them uniquely valuable as astronomical messengers. Neutrinos are produced as a result of certain types of nuclear reactions such as those that take place in the Sun, in nuclear reactors, or when cosmic rays hit the atmosphere. Neutrinos can travel across the universe without interference, they are not deflected by interstellar magnetic fields and are not absorbed by intervening matter. However, it makes cosmic neutrinos extremely difficult to detect. Neutrinos can interact via the neutral (the exchange of a Z boson) or charged (the exchange of a W boson) current weak interactions.

Neutrino detectors must be generally placed deep underground or water to escape the backgrounds caused, for example, the cosmic rays produce many muons which penetrate deeply into the earth.