More particles [in physics]
To produce more massive particles requires higher-energy particle collisions, and today's accelerators only allow one to search up to masses of perhaps 200 GeV. (Sufficiently stable particles could have survived from the early universe, and a few cosmic ray interactions in principle give higher energies—but are normally too rare to be useful.) I am not sure whether in my approach one should expect an infinite series of progressively more massive particles. The example of nonplanarity might suggest not, but even in the class 4 cellular automata discussed in Chapter 6 it is not clear whether fundamentally different progressively larger structures will appear forever. In quantum field theory particles of any mass can always in principle exist for short times in virtual form. But normally their effects decrease like powers of their mass—making them hard to measure. In two kinds of cases, however, this does not happen: one is so-called anomalies, the other interactions with the Higgs field, in which couplings are proportional to mass. In the minimal Standard Model it turns out to be impossible to get quarks or leptons with masses much above about 200 GeV without destabilizing the vacuum (a fact pointed out by David Politzer and me in 1979). But with more complicated models one can avoid this constraint. In supersymmetric models—and string theory—there are typically also all sorts of other types of particles, assumed to have high masses since they have not been observed. There is evidence against any more than the three known generations of quarks and leptons in that the decay process Z^0 -> ν*OverBar[ν] has a rate that rather accurately agrees with what is expected from just three types of low-mass neutrinos.