Extra families, Higgs spectrum, and oblique corrections - art. no. 053004

The standard model accommodates, but does not explain, three families of le prous and quarks, while various extensions suggest extra matter families. T he oblique corrections from extra chiral families with relatively light (we ak-scale) masses, M(f)similar toH, are analyzed and used to constrain the n umber of extra families and their spectrum. The analysis is motivated, in p art, by recent N=2 supersymmetry constructions, but is performed in a model -independent way. It is shown that the correlations among the contributions to the three oblique parameters, rather than the contribution to a particu lar one, provide the most significant bound. Nevertheless, a single extra c hiral family with a constrained spectrum is found to be consistent with pre cision data without requiring any other new physics source. Models with thr ee additional families may also be accommodated but only by invoking additi onal new physics, most notably, a two-Higgs-doublet extension. The interpla y between the spectra of the extra fermions and the Higgs boson(s) is analy zed in the case of either one or two Higgs doublets, and its implications a re explored. In particular, the precision bound on the standard model-like Higgs boson mass is shown to be significantly relaxed in the presence of an extra relatively light chiral family.