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. 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).
| First generation | Second generation | Third generation | |
|---|---|---|---|
| Lepton | Electron | Muon | Tau |
| Neutrino | Electron neutrino | Muon neutrino | Tau neutrino |
| Down-type quark | Down quark | Strange quark | Bottom quark |
| Up-type quark | Up quark | Charm quark | Top quark |
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/c2, the second-generation muon has a mass of 106 MeV/c2, and the third-generation tau lepton has a mass of 1777 MeV/c2 (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 the arguments against the possibility of a fourth generation 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/c2) 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. In such searches, fourth-generation particles are denoted by the same symbols as third-generation ones with an added prime (e.g. b′ and t′). Given the unlikeliness of any such particles being discovered, no other names have been seriously proposed.
[edit] Fundamentality of second and third generation particles
There is some debate as to whether the muon and tau particles are actually fundamental in a strict sense, or whether they are better described as excited states of an electron.[1][2][3][4] Richard Feynman is quoted in Genius, the life and science of Richard Feynman as questioning the fundamentality of all of the leptons (including the electron), explaining:
- My feeling is that the standard model is a "perfect thing", and making small modifications of it (well, other than neutrino masses or, for that matter, making changes to the masses or couplings of any of the various elementary particles) is not possible. Especially in the area of eliminating muons as fundamental particles. But I should also admit that I don't think that the muons are fundamental. I think all the quarks and leptons are composite.
Physicists have yet to reconcile the notion of the higher generation leptons as excited states of the electron with the theory that the electron is a point charge. However, other theories as to the nature of an electron include the possibility that an electron is a charged conducting surface, with a surface tension to prevent it from flying apart under the repulsive forces of the charge.[5]
It is hoped that a comprehensive understanding of the relationship between the generations of the leptons may eventually explain the ratio of masses of the fundamental particles, and shed further light on the nature of mass generally, from a quantum perspective.[6]
[edit] References
- ^ An Extensible Model of the Electron (P.A.M. Dirac, in support of an excited state theory)
- ^ Gravitational Measurements, Fundamental Metrology, and Constants (Venzo, in support of an excited state theory)
- ^ The aetheron as the source of a new conservation law (Sardin, in support of an excited state theory)
- ^ Concepts of Particle Physics (Gottfried & Weisskopf, in opposition to an excited state theory)
- ^ An Extensible Model of the Electron (P.A.M. Dirac)
- ^ A "Muon Mass Tree" with α-quantized Lepton, Quark and Hadron Masses (Malcolm H. MacGregor)
