What Is A Neutrino?

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Although you may have never heard of them before, neutrinos make up 1/4 of all known fundamental particles. There are three different types, electron-, muon-, and tau-neutrinos.
The reason most people have never heard of neutrinos is because they pass through most matter like ghosts, only rarely interacting. Their elusiveness is a result of the fact that neutrinos are electrically neutral, and so are unaffected by the electro-magnetic force, and they also do not interact via the "strong nuclear force". They interact only through the "weak nuclear force" and gravity, the two weakest of the four forces. A neutrino can pass straight through the earth without hitting anything, and they don't pose a health risk to humans. This is actually pretty lucky for us, because neutrinos are everywhere around us, zipping past in all directions. If you hold out your hand, each second over a trillion neutrinos from the sun pass straight through it! In fact, if you could see each neutrino as a little black speck flying by, the world would look a lot like the picture on the right.
Neutrinos are created when heavy particles explode into smaller fragments. For example, an atom of carbon (11C), which has 6 protons and 5 neutrons, can decay into a neutrino (νe), a positron (e+, an anti-electron), and an atom of boron (11B) with 5 protons and 6 neutrons. Another example is neutron beta decay.

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