|Type||Generations of matter|
|neutral||electron neutrino||muon neutrino||tau neutrino|
In particle physics, a generation or family is a division of the elementary particles. Between generations, particles differ by their flavour quantum number and mass, but their interactions are identical.
There are three generations according to the Standard Model of particle physics. Each generation is divided into two types of leptons and two types of quarks. The two leptons may be classified into one with electric charge −1 (electron-like) and neutral (neutrino-like); the two quarks may be classified into one with charge −1⁄3 (down-type) and one with charge +2⁄3 (up-type). The basic features of quark-lepton generation or families, such as their masses and mixings etc., can be described by some of the proposed family symmetries.
Each member of a higher generation has greater mass than the corresponding particle of the previous generation, with the possible exception of the neutrinos (whose small but non-zero masses have not been accurately determined). 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 has a mass of 1777 MeV/c2 (almost twice as heavy as a proton). This mass hierarchy causes particles of higher generations to decay to the first generation, which explains why everyday matter (atoms) is 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 such as cosmic rays or particle accelerators. The term generation was first introduced by Haim Harari in Les Houches Summer School, 1976.
Neutrinos of all generations stream throughout the universe but rarely interact with other matter. 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.
Fourth and further generations are considered unlikely by many (but not all) theoretical physicists. Some 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 at CERN's Large Electron–Positron Collider (LEP). 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′).
The lower bound for a fourth generation of quark (b′, t′) masses is currently at 1.4TeV from experiments at the LHC.
The lower bound for a fourth generation neutrino () mass is currently at about 60GeV. (Millions of times larger than the upper bound for the other 3 neutrino masses).
The lower bound for a fourth generation charged lepton () mass is currently 100GeV and proposed upper bound of 1.2TeV from unitarity considerations. 
If the Koide formula continues to hold, the masses of the fourth generation charged lepton would be 44GeV (ruled out) and b′ and t′ should be 3.6TeV and 84TeV respectively. (The maximum energy of protons at the LHC is about 6TeV).
The origin of multiple generations of fermions, and the particular count of 3, is an unsolved problem of physics. String theory provides a cause for multiple generations, but the particular number depends on the details of the compactification or the D-brane intersections.
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- Boosting searches for fourth-generation quarks (2019)
- |Revisiting constraints on fourth generation neutrino masses (2010)
- [https://arxiv.org/abs/1204.3550 | Large mass splittings for fourth generation fermions allowed by LHC Higgs boson exclusion (2012)]