NEW PARTICLE SEARCHES AT LEP

Considerable effort has been expended by the LEP collaborations since the start of data taking on the search for new particles and phenomena. Two early results provided evidence that only three families of quarks and leptons exist in nature: the width of the Z established to high precision the existence of only three neutrinos and a direct search for particles belonging to a fourth family proved negative. The search for the mass mechanism of the Standard Model has taken up much effort. Different models predict the existence of one or more Higgs bosons, of supersymmetric particles or of effects arising from compositeness; so far no evidence has been found for any such particles. The paper describes the principles, techniques and results of these searches by the four LEP collaborations. Improvements to some of the published limits, obtained by combining the separate results of the collaborations, are presented.