Inparticle physics, ageneration orfamily is a division of theelementary particles. Between generations, particles differ by theirflavour quantum number andmass, but theirelectric and strong interactions are identical.
There are three generations according to theStandard Model of particle physics. Each generation contains two types ofleptons and two types ofquarks. The two leptons may be classified into one withelectric charge −1 (electron-like) and neutral (neutrino); 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 proposedfamily symmetries.
| Fermion categories | Elementary particlegeneration | |||
|---|---|---|---|---|
| Type | Subtype | First | Second | Third |
| Quarks (colored) | down-type | down | strange | bottom |
| up-type | up | charm | top | |
| Leptons (color-free) | charged | electron | muon | tau |
| neutral | electron neutrino | muon neutrino | tau neutrino | |
Each member of a higher generation has greater mass than the corresponding particle of the previous generation, with the possible exception of theneutrinos (whose small but non-zeromasses have not been accurately determined). For example, the first-generationelectron has a mass of only0.511 MeV/c2, the second-generationmuon has a mass of106 MeV/c2, and the third-generationtau has a mass of1777 MeV/c2 (almost twice as massive as theproton). Thismass hierarchy[1]causes particles of higher generations to decay to the first generation, which explains why everydaymatter (atoms) is made of particles from the first generation only. Electrons surround anucleus made ofprotons andneutrons, 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 ascosmic rays orparticle accelerators. The termgeneration was first introduced byHaim Harari inLes Houches Summer School, 1976.[2][3]
Neutrinos of all generations stream throughout the universe but rarely interact with other matter.[4]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.[5]
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 precisionelectroweak observables that extra generations would induce; such modifications are strongly disfavored by measurements. There are functions used to generalize terms for introduction in a new quark that is an isosinglet and is responsible for generatingFlavour-Changing-Neutral-Currents' (FCNC) at tree level in the electroweak sectors.[6][7]
Nonetheless, searches at high-energy colliders[8] 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′ andt′).
A fourth generation with a 'light' neutrino (one with a mass less than about45 GeV/c2) was ruled out by measurements of the decay widths of theZ boson atCERN'sLarge Electron–Positron Collider (LEP) as early as 1989.[9] The lower bound for a fourth generation neutrino (ν'τ) mass as of 2010 was at about 60 GeV (millions of times larger than the upper bound for the other 3 neutrino masses).[10] As of 2024, no evidence of a fourth-generation neutrino has ever been observed inneutrino oscillation studies either. Because even in the third generation (tau) neutrinoντ, mass is extremely small (makingντ the only third-generation particle that outside highly most energetic conditions will not readily decay), a fourth-generation neutrinoν'τ that observes the general rules for the known 3 neutrino generations should both be easily within current particle accelerators' energy levels, and occur during the regular and highly predictable switching-of-generations (oscillation) neutrinos perform.
If theKoide formula continues to hold, the masses of the fourth generation charged lepton would be 44 GeV (ruled out) andb′ andt′ should be 3.6 TeV and 84 TeV respectively (The maximum possible energy for protons in theLHC is about 6 TeV). The lower bound for a fourth generation of quark (b′,t′) masses as of 2019 was at 1.4 TeV from experiments at the LHC.[11] The lower bound for a fourth generation charged lepton (τ') mass in 2012 was 100GeV, with a proposed upper bound of 1.2 TeV from unitarity considerations.[12]
The origin of multiple generations of fermions, and the particular count of3, is anunsolved problem of physics.String theory provides a cause for multiple generations, but the particular number depends on the details of thecompactification of theD-brane intersections. Additionally,E8grand unified theories in 10 dimensionscompactified on certainorbifolds down to 4 D naturally contain 3 generations of matter.[13] This includes manyheterotic string theory models.
In standard quantum field theory, under certain assumptions, a single fermion field can give rise to multiple fermion poles with mass ratios of aroundeπ ≈ 23 ande2π ≈ 535 potentially explaining the large ratios of fermion masses between successive generations and their origin.[1]
The existence of precisely three generations with the correct structure was at least tentatively derived from first principles through a connection with gravity.[14] The result implies a unification of gauge forces intoSU(5). The question regarding the masses is unsolved, but this is a logically separate question, related to the Higgs sector of the theory.
ABSTRACT: We revisit the current experimental bounds on fourth-generation Majorana neutrino masses, including the effects of right handed neutrinos. Current bounds from LEP‑II are significantly altered by a global analysis. We show that the current bounds on fourth generation neutrinos decaying toe W andμ W can be reduced to about 80 GeV (from the current bound of 90 GeV), while a neutrino decaying toτ W can be as light as 62.1 GeV. The weakened bound opens up a neutrino decay channel for intermediate mass Higgs, and interesting multi-particle final states forHiggs and fourth generation lepton decays.