Generation (particle physics)

Known elementary particles of matter
	electric charge	Generation 1		Generation 2		Generation 3
Leptons	−1	electron	${\ displaystyle {\ boldsymbol {e}} \,}$	Muon	${\ displaystyle {\ boldsymbol {\ mu}} \,}$	Tauon	${\ displaystyle {\ boldsymbol {\ tau}} \,}$
Leptons	uncharged ( neutrinos )	Electron neutrino	${\ displaystyle {\ boldsymbol {\ nu}} _ {e} \,}$	Muon neutrino	${\ displaystyle {\ boldsymbol {\ nu}} _ {\ mu} \,}$	Tauon neutrino	${\ displaystyle {\ boldsymbol {\ nu}} _ {\ tau} \,}$
Quarks	+ ² ⁄ ₃	Up	${\ displaystyle {\ boldsymbol {u}}}$	Charm	${\ displaystyle {\ boldsymbol {c}}}$	Top	${\ displaystyle {\ boldsymbol {t}}}$
Quarks	- ¹ ⁄ ₃	Down	${\ displaystyle {\ boldsymbol {d}}}$	Strange	${\ displaystyle {\ boldsymbol {s}}}$	Bottom	${\ displaystyle {\ boldsymbol {b}}}$

In particle physics , the twelve known elementary matter particles are often divided into three generations of four particles each plus their antiparticles (the term “generation” 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 tauon ), an associated neutrino and two quarks . All atoms of the periodic table and the structures built up from them ( molecules , cells , ...) are made up of the particles of the first generation: the protons and neutrons forming the atomic nucleus consist of up and down quarks, the electrons of the atomic shell are themselves elementary particles of the first generation.

Relationship between generations

Decay of a free muon into its neutrino and generation 1 particles

Crowds

The second and third generation are often viewed approximately as copies of the first generation with greater mass and otherwise identical properties of the particles. The mass ratios of the particles from different generations do not follow any known scheme.

The assumption that particles of higher generation are copies of lower generations with only larger masses is practical, but actually only approximately correct. On the one hand, the masses of the neutrinos are not yet known (2018). On the other hand, the relationship between the generations is complicated by the electroweak symmetry breaking . As a result of this symmetry breaking, the quarks' own mass states are no longer identical to their interaction states of the weak interaction , but are linked to one another by the CKM matrix . That is, the mass eigenstate of a quark of a generation is a mixture of the interaction states of the quarks of the same type of all generations and vice versa.

No such mixing takes place for the charged leptons; instead, the theorem of lepton universality applies in the Standard Model . It says that the electrically charged leptons (with the exception of the masses) are exact copies of each other and thus a muon behaves exactly like an electron with regard to its interactions with other particles. Differences only arise due to the higher mass, but can be drastic - the muon, for example, unlike the electron, is not stable.

stability

With the exception of the almost massless neutrinos, free particles of the second and third generation can decay into particles of a lower generation via a W boson due to their higher mass compared to the first generation . It must be noted, however, that in the standard model the lepton family number is a conserved quantity of the weak interaction. This means that for each decaying charged lepton of a generation, a neutrino of the same generation must be generated during this decay. The muon has a lifetime of 10 ⁻⁵ seconds, the tauon one of 10 ⁻¹³ seconds.

Number of generations

The existence of a fourth, fifth or higher generation with corresponding elementary particles that have not yet been proven experimentally cannot be ruled out, but no indications are known for further generations. A well-known experimental test for the possible presence of further generations is an investigation of the lifetime of the Z boson at the LEP . Since the neutrino masses of the first three generations are very small, one could assume that the mass of a fourth generation neutrino is less than half the mass of the Z boson. In this case, the Z boson could decay into a fourth generation neutrino and an antineutrino, which would reduce the life of the Z boson. By comparing the measured lifespan with calculations, the existence of a fourth generation with a neutrino mass less than 40 GeV / c² could be ruled out with a probability of 98 percent.

Individual evidence

↑ D. Decamp: Determination of the number of light neutrino species . In: Physics Letters B . 231, No. 4, 1989, p. 519. doi : 10.1016 / 0370-2693 (89) 90704-1 .

[1] D. Decamp: Determination of the number of light neutrino species . In: Physics Letters B . 231, No. 4, 1989, p. 519. doi : 10.1016 / 0370-2693 (89) 90704-1 .