Electroweak symmetry breaking after LEP1 and LEP2

Abstract

In a generic 'universal' theory of electroweak symmetry breaking, non fine-tuned heavy new physics affects the low-energy data through four parameters, which include and properly extend the generally insufficient S and T. Only by adding the LEP2 data to the global electroweak fit, can all these four form factors be determined and deviations from the SM be strongly constrained. Several of the recently proposed models (little Higgs, gauge bosons in extra dimensions or Higgsless models in 5D) are recognized to be 'universal' in a straightforward way after a proper definition of the effective vector boson fields. Among various applications, we show that proposed Higgsless models in 5D, when calculable, do not provide a viable description of electroweak symmetry breaking in their full range of parameters.