nLab generation of fundamental particles

Idea

The standard model of particle physics has the curious property that its content of fermions appears in three sets of particles that share all properties except that their rest mass increases drastically from one set to the next. These are called the three “generations” “families” of fermionic particles.

flavors of fundamental fermions in the standard model of particle physics:
generation of fermions	1st generation	2nd generation	3d generation
quarks ($q$)
up-type	up quark ($u$)	charm quark ($c$)	top quark ($t$)
down-type	down quark ($d$)	strange quark ($s$)	bottom quark ($b$)
leptons
charged	electron	muon	tauon
neutral	electron neutrino	muon neutrino	tau neutrino
bound states:
mesons	light mesons: pion ($u d$) ρ-meson ($u d$) ω-meson ($u d$) f1-meson a1-meson	strange-mesons: ϕ-meson ($s \bar s$), kaon, K*-meson ($u s$, $d s$) eta-meson ($u u + d d + s s$) charmed heavy mesons: D-meson ($u c$, $d c$, $s c$) J/ψ-meson ($c \bar c$)	bottom heavy mesons: B-meson ($q b$) ϒ-meson ($b \bar b$)
baryons	nucleons: proton $(u u d)$ neutron $(u d d)$

Any reason for this striking pattern presently remains mysterious.

While the particle content of any one single generation would naturally be explained by GUT models, in terms of a single irreducible representation of a simple gauge group containing that of the standard model of particle physics, this does not explain why there are three almost identical copies of this.

One suggestion to explain this phenomenon appears in intersecting D-brane models. See there at Generations of fermions.

Related concepts

• flavour anomaly

• fourth generation of fundamental particles

References

General

See also

• Wikipedia, Generation (particle physics)

In string theory

On model building with realistic Yukawa couplings and fermion masses in an MSSM Pati-Salam GUT model with 3 generations of fermions realized on intersecting D6-branes KK-compactified on a toroidal orbifold $T^6\sslash (\mathbb{Z}_2 \times \mathbb{Z}_2)$ are claimed in

• Ching-Ming Chen, Tianjun Li, Van Eric Mayes, Dimitri Nanopoulos, A Realistic World from Intersecting D6-Branes, Phys.Lett.B665:267-270, 2008 (arXiv:hep-th/0703280, doi:10.1016/j.physletb.2008.06.024)

• Ching-Ming Chen, Tianjun Li, Van Eric Mayes, Dimitri Nanopoulos, Realistic Yukawa Textures and SUSY Spectra from Intersecting Branes, Phys.Rev.D77:125023, 2008 (arXiv:0711.0396)

• Van Eric Mayes, All Fermion Masses and Mixings in an Intersecting D-brane World (arXiv:1902.00983)

• Jordan Gemmill, Evan Howington, Van E. Mayes, One String to Rule Them All: Neutrino Masses and Mixing Angles (arXiv:1907.07106)