Top quark, the end of the particle line?
Well, we're not actually sure that it is. But we do know a couple of things. To begin with, the world seems to be arranged in `generations'. Thus all of normal matter consists of (up and down quarks, electrons and electron-type neutrinos). We know of two other generations (charm and strange quarks, muons and muon-type neutrinos) and (top and bottom quarks, taus and tau-type neutrinos). The particles in these two generations are very unstable (except for the neutrinos) and they decay very quickly. They only existed at the moment of creation and in large accelerators like the Tevatron. People at the moment of creation and in large accelerators like the Tevatron. People at the two large collider detectors (CDF and D0) are looking for a fourth generation. Typically, this is by looking for another quark (the so-called b' quark) or an electron-like thing (a heavy lepton). These scientists have published results which show that we have no evidence for the b' quark. It may exist, but if it does, it's heavy.
We have some (small) evidence that there are only three generations. There are 4 experiments in Europe (at CERN, a Fermilab-like place on the Swiss-France border). They have shown that there are no more than 3 light neutrinos. We currently believe that the three neutrino-types we have now are very light. In order to keep our theories neat and tidy, we believe that each generation must have an associated neutrino. No fourth neutrino type--no next generation. If there is a fourth generation neutrino, it must be very heavy. This is possible, but it would mean we need to think more about our understanding of our theories.
To complicate things, we at Fermilab (and at big competing experiments in Europe (due to start in 2005)) are looking for a whole new `zoo' of particles, predicted by a theory called supersymmetry (or SUSY). Whether or not these particles exist is anyone's guess. But there are a number of very smart people that argue that they should. The experimental physicists here at Fermilab are more inclined to wait and see.
Finally, since you asked my opinion (and realize this is no more than that), I have some additional ideas. The generations are very regular...they all look alike. Historically, when we have seen something like this, it implied that there were smaller particles which had some rules of how they could combine. The easiest example of this is chemistry. In the 1700-1800's, people were looking at different elements. They found that some were reactive, some weren't and so on. Of those that were reactive, some of them combined in similar ways. A very smart chap by the name of Mendeleev organized them in a particular type of chart called the periodic table. (You can look in a chemistry book and see what I mean.) We now know that the reason that these elements were similar had to do with the fact that there were electrons inside each atom. There were rules of how the electrons could orbit the atom, which meant that as the patterns repeated themselves, you would have similar elements.
We have seen similar examples of patterns of particles implied smaller particles along with the rules that governed them. I predict that there are particles, smaller than quarks, which are governing the generations. We look for direct evidence of things smaller than quarks here at Fermilab. So far,no luck. But we keep looking!
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