Magnetic Screening in the High Temperature Phase of the Standard Model
  Non-perturbative effects in the high-temperature phase of the electroweak theory are characterized by a magnetic screening length. Its size influences the range of validity of perturbation theory, and it also determines the critical Higgs boson mass where the first-order phase transition changes to a crossover. We propose a gauge-invariant definition of the magnetic screening length and discuss its role in several gauge-dependent and gauge-invariant correlation functions. 
W.Buchm\"uller (DESY) and O.Philipsen (U. Heidelberg)