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 and neutral ; the two quarks may be classified into one with charge − and one with charge +. 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.
Overview
Each member of a higher generation has greater mass than the corresponding particle of the previous generation, with the possible exception of the neutrinos. For example, the first-generation electron has a mass of only, the second-generation muon has a mass of, and the third-generation tau has a mass of . This mass hierarchy causes particles of higher generations to decay to the first generation, which explains why everyday matter 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 explainthe ratio of masses of the fundamental particles, and shed further light on the nature of mass generally, from a quantum perspective.
Fourth generation
Fourth and further generations are considered unlikely by many 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 has been ruled out by measurements of the widths of the Z boson at CERN's Large Electron–Positron Collider. 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. The lower bound for a fourth generation of quark 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.. 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 and b′ and t′ should be 3.6TeV and 84TeV respectively..
