Yup, quite a few particles have been discovered in accelerator-based
experiments over the years. Actually, some of them, including our first
glimpse of the second generation, the muon, were first spotted in
Most of the 200+ particles are not "elementary" but are composed
of constituent parts. We now know that protons are made up of three
quarks -- two "up" and one "down", while neutrons are made of two downs
and an up. You can put three up-type quarks together and get a Delta++.
Pions are made of a quark and an antiquark bound together with the
strong force. Stir in three generations of quarks and you can make lots
of interesting combinations.
Exotic particles of all three generations are made in collisions
of cosmic rays and the molecules in the atmosphere. We also have found
out that neutrinos, once thought to be massless, slowly change from one
generation to another and back again, and so neutrinos of all three
generations populate the universe. Manmade accelerators are not the
only places where these things exist, but they provide the best places
to study these particles because we can produce enough of them to
measure their properties accurately and produce them in controlled
environments where we can understand signals and backgrounds (where
signals are evidences for the particle we want to study and backgrounds
are the other particles whose interactions sometimes produces data that
are indistinguishable from signals). With an accelerator you can do all
kinds of good experiments and use control samples, while with cosmic
rays you only get one kind of mixture of events at an unknown rate and
energy spectrum and sample composition.
As for not knowing "why" the second and third generations exist, I
happily agree, and even would say that we don't even know "why" any
particle ought to exist. One can make an argument that a system with
just one or two generations of particles has to obey "CP symmetry"
(swapping particles for antiparticles and reversing the signs of the
x,y, and z axes), making matter and antimatter behave in necessarily
symmetric ways. Our universe has matter in it but no antimatter, and we
deduce that CP symmetry doesn't govern the interactions of the
particles in the universe. A three-generation set is the minimum which
allows CP symmetry to be broken in the quarks, but we suspect that even
this isn't enough to explain the preponderance of matter over
antimatter in the universe.
The universe is filled with even stranger stuff still, things we
haven't yet made in the laboratory (or haven't been able to separate
from the backgrounds). Mysterious dark matter particles float around,
interacting with ordinary matter only gravitationally, affecting galaxy
distributions, and even how fast stars rotate around galactic cores.
In the very early univese, all generations of particles were
common, as enough energy was available to freely produce all of them.
At short times after the big bang, compared to the lifetimes of these
particles, you can even think of these particles as "stable" (as
compared to the age of the universe when it was very young). They
contributed to the evolution of the early universe just as much as our
more mundane particles we observe today.
(published on 10/22/2007)