New particles continue to be discovered. Sometimes a new particle implies the existence of a new quark. Just this happened in the November Revolution in 1974 when the J/psi particle was found simultaneously by Sam Ting at Brookhaven and Burton Richter at the Stanford Linear Accelerator. The J/psi can only be explained as a bound state of a charm and anticharm quarks -- hence proving the existence of a new type of quark. Similarly, the bottom quark was infered from the discovery of the upsilon particle by Leon Lederman in 1977 at Fermilab. Finally, the top quark was found in 1994 at Fermilab. Measurements taken at CERN in Geneva lead us to expect that no more types of quarks exist (unless something really strange happens!).
