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Each generation contains 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); the two quarks may be classified into one with charge − 1 ⁄ 3 (down-type) and one with charge + 2 ⁄ 3 (up-type).
In particle physics, the family symmetries or horizontal symmetries are various discrete, global, or local symmetries between quark-lepton families or generations.In contrast to the intrafamily or vertical symmetries (collected in the conventional Standard Model and Grand Unified Theories) which operate inside each family, these symmetries presumably underlie physics of the family flavors.
Higher generation leptons and quarks are presumed to be excited states of first generation leptons and quarks, but those states are not specified. The simple RM does not provide an explanation of the mass-spectrum of the leptons and quarks. Baryon number (B) and lepton number (L) are not conserved, but the quantity B − L is conserved.
τ) by the interaction eigenstate, whereas for the quarks we define the flavor (up, down, etc.) by the mass state. We can switch between these states using the CKM matrix for the quarks, or the PMNS matrix for the neutrinos (the charged leptons on the other hand are eigenstates of both mass and flavor).
The quark–lepton complementarity (QLC) is a possible fundamental symmetry between quarks and leptons. First proposed in 1990 by Foot and Lew, [1] it assumes that leptons as well as quarks come in three "colors". Such theory may reproduce the Standard Model at low energies, and hence quark–lepton symmetry may be realized in nature.
Half of the fermions are leptons, three of which have an electric charge of −1 e, called the electron (e −), the muon (μ −), and the tau (τ −); the other three leptons are neutrinos (ν e, ν μ, ν τ), which are the only elementary fermions with neither electric nor color charge. The remaining six particles are quarks (discussed below).
In addition, leptons carry weak isospin, T 3, which is − 1 / 2 for the three charged leptons (i.e. electron, muon and tau) and + 1 / 2 for the three associated neutrinos. Each doublet of a charged lepton and a neutrino consisting of opposite T 3 are said to constitute one generation of leptons.
All quarks are assigned a baryon number of 1 / 3 . Up, charm and top quarks have an electric charge of + 2 / 3 , while the down, strange, and bottom quarks have an electric charge of − 1 / 3 . Antiquarks have the opposite quantum numbers. Quarks are spin- 1 / 2 particles, and thus fermions. Each quark or antiquark ...