While there have been many experiments searching for additional generations that have proved negative and many theoretical arguments against more, this is still an open question.

Peter Higgs (1929-), a British physicist, proposed a mechanism that gives rise to the masses of the quarks, leptons, and the two weak-interaction bosons, the W and Z. The mechanism successfully predicted the mass of the Z boson, but it requires the existence of a particle, called the Higgs boson. Despite many searches, the Higgs boson has not yet been discovered. Physicists hope that the Large Hadron Collider (LHC) that started operation in 2010 will accelerate protons to high enough energies that the Higgs will be found.