Leptons

A lepton is an elementary particle that interacts with electromagnetism, weak force and gravity, but is not affected by the strong force. It has ½ spin and is fermionic particle. Most famous leptons are electron and (electron) neutrino, but there are four more types called flavours. All of them are divided into two classes - charged and neutral leptons. Charged leptons are electron, muon and tau, while neutral leptons are electron neutrino, muon neutrino and tau neutrino. 

Additionally, they are categorised by generations equivalent to the categorisation of quarks. In the first generation, there are electron and corresponding electron neutrino, together with up quark and down quark. Muon and muon neutrino are in the second generation, with charm quark and strange quark. Leptons of the third generation are tau and tau neutrino, joined by bottom quark and top quark. Only leptons of the first generation are stable and thus can be found in normal matter, just like quarks of this generation. Leptons of higher generations have correspondingly higher mass, just as it is case with quarks of higher generations. We are not sure if this applies to neutrinos as well, since we only know that they do have a mass, but it is so small, that we haven’t measure it for either of the three variants. Leptons of different generations differ in mass and flavour, but their interactions are identical. Taus and muons can only be found in high energy collisions in particle accelerators and after cosmic rays hit Earth’s atmosphere, but they quickly decay into electrons.

You could have guessed that charged leptons have electric charge, while neutral leptons do not. This is why negatively charged, electron-like leptons interact with other charged particles, like protons. In atoms, electrons are bound to protons by electromagnetism and are directly tied to all of atom’s chemical properties. Neutrinos, however, interact only rarely with matter, since they are very small and travel at the speed near the speed of light. They are passing through you this very moment and don’t affect you in any way.

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