The Generations of Matter
Notice that both quarks and leptons exist in 3 distinct sets.
We call each of these sets a generation of matter particles.
A generation is a set of one of each charge type of quark and lepton.
Each generation tends to be heavier than the previous set.
All visible matter in the universe is made
from the first generation of matter particles: up and down
quarks, and electrons.
Second and third generation particles are unstable, and
decay into first generation particles.
It is for this reason that all the stable matter in the
universe is made from first generation particles.
We thank Fermi National Laboratory for the use of their "Generations of Matter" image.