Generation (particle physics)

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In particle physics, a generation is a division of the elementary particles. Between generations, particles differ only by their mass and flavor; all interactions and quantum numbers are identical. There are three generations according to the Standard Model of particle physics.

Each generation is divided into two leptons and two quarks. The two leptons may be divided into one with electric charge −1 (electron-like) and one neutral (neutrino); the two quarks may be divided into one with charge −1/3 (down-type) and one with charge +2/3 (up-type).

The first generation consists of the electron, electron neutrino and the down and up quarks.

The second generation consists of the muon, muon neutrino and the strange and charm quarks.

The third generation consists of the tau lepton, tau neutrino and the bottom and top quarks.

Each member of a higher generation has greater mass than the corresponding particle of the previous generation. For example: the first-generation electron has a mass of only 0.511 MeV, the second-generation muon has a mass of 106 MeV, and the third-generation tau lepton has a mass of 1777 MeV (almost twice as heavy as a proton).

All ordinary atoms are made of particles from the first generation. Electrons surround a nucleus made of protons and neutrons, which contain up and down quarks. The second and third generations of charged particles do not occur in normal matter and are only seen in extremely high-energy environments. Neutrinos of all generations stream throughout the universe but rarely interact with normal matter.

[edit] Possibility of a fourth generation

Within the Standard Model, fourth and further generations have been ruled out by theoretical considerations. Some of these are based on the subtle modifications of precision electroweak observables that extra generations would induce; such modifications are strongly disfavored by measurements. Furthermore, a fourth generation with a light neutrino (one with a mass less than about 40 GeV) has been ruled out by measurements of the widths of the Z boson (LEP, CERN)^{[citation needed]}. Nonetheless, searches at high-energy colliders for particles from a fourth generation continue, but as yet no evidence has been observed.