Generation (particle physics)

In particle physics, the elementary particles of matter known twelve are often divided into three so-called generations of four particles plus their antiparticles. ( The term generation here has nothing to do with, for example, "mother " and "daughter" states in decay processes. )

Each generation consists of an electrically charged lepton ( electron, muon or τ - lepton ), an associated neutrino and two quarks. All atoms of the periodic table and built up from them structures (molecules, cells, ... ) are constructed from the particles of the first generation: the atomic nucleus forming protons and neutrons are made of up quarks and down quarks, the electrons of the atomic shell are even elementary the first generation.

Relationship between the generations

Masses

The second and third generation are often regarded approximately as a first-generation copies with greater mass and otherwise identical properties of the particles. Behaves according to the standard model of particle physics (SM ) a muon with respect to its interactions with other particles essentially as an electron. Differences come into existence only by the higher mass, but can be drastic - the muon is not, for example, in contrast to the electron stable. The mass ratios of the particles from different generations do not follow a familiar pattern.

The assumption that higher generation particles are copies of the lower generations with only larger masses is often practical, but in fact only approximately correct. Firstly, the masses of the neutrinos are simply not known. On the other hand, the relation of the generations to one another is complicated by the electroweak symmetry breaking. In this symmetry breaking mix fields (more precisely, interaction eigenstates ) of different generation to new observable particles ( mass eigenstates ).

Stability

With the exception of the nearly massless neutrinos free particles of the second and third generation can spontaneously decay via a W - boson in a smaller particle generation due to their higher compared to the first generation of mass. The muon and the τ - lepton, whose only known decay possibility is this process that have lifetimes in the microsecond range and smaller.

Number of generations

The existence of a fourth, fifth, ... with corresponding generation not yet experimentally proven elementary particles can not be excluded, but there are no indications for future generations known. A well-known experimental test for the possible presence of further generations is a study of the life of the Z boson at LEP. Since the neutrino masses of the first three generations are very low, you might assume the mass of a neutrino of the fourth generation was less than half the mass of the Z boson. In this case, the Z boson could decay into a neutrino and an antineutrino of the fourth generation, which would reduce the lifetime of the Z boson. By comparing the measured lifetime with calculations, the existence of a fourth generation with a neutrino mass was less than 40 GeV / c ² be excluded with 98 percent probability.