In particle physics, a generation (or family) 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 classified into one with electric charge −1 (electron-like) and one neutral (neutrino); the two quarks may be classified 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 tauon has a mass of 1,777 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.^{[citation needed]}

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.^[1]

[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 45 GeV/c²) has been ruled out by measurements of the widths of the Z boson (LEP, CERN).^[2] Nonetheless, searches at high-energy colliders for particles from a fourth generation continue, but as yet no evidence has been observed.^[3][4] 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′).

[edit] References

1. ^ Mac Gregor, M.H. (2006). "A 'Muon Mass Tree' with α-quantized Lepton, Quark, and Hadron Masses". arΧiv:hep-ph/0607233 [hep-ph]. 
2. ^ D. Decamp (1989). "Determination of the number of light neutrino species". Physics Letters B 231 (4): 519. doi:10.1016/0370-2693(89)90704-1. 
3. ^ C. Amsler et al. (2008). "Review of Particle Physics: b′ (4th Generation) Quarks, Searches for". Physics Letters B (Particle Data Group) 667 (1): 1–1340. http://pdg.lbl.gov/2008/listings/q008.pdf. 
4. ^ C. Amsler et al. (2008). "Review of Particle Physics: t′ (4th Generation) Quarks, Searches for". Physics Letters B (Particle Data Group) 667 (1): 1–1340. http://pdg.lbl.gov/2008/listings/q009.pdf.