Everything about Generation Particle Physics totally explained
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).
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/c
2, the second-generation muon has a mass of 106 MeV/c
2, and the third-generation tau lepton has a mass of 1777 MeV/c
2 (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 don't 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.
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/c
2) has been ruled out by measurements of the widths of the
Z boson (
LEP,
CERN). Nonetheless, searches at high-energy colliders for particles from a fourth generation continue, but as yet no evidence has been observed. If eventually discovered, the proposed names for the fourth generation of quarks are
audio and
video.
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're better described as excited states of an electron. 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) isn't 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.
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.
Further Information
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