Contents

# Contents

## 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” of fermionic particles.

The first generation contains the

type of fundamental particleFirst generationSecond generationThird generation
up-type quarkup quark?charm quarktop quark
down-type quarkdown quark?strange quark?bottom quark
charged leptonelectronmuontau?
neutrinoelectron neutrino?muon neutrino?tau neutrino?

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.

## References

### General

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