Avi Hauser

The sensitivity of physical amplitudes to the observed isospin symmetry breaking in the fermion mass matrix is analyzed in the standard model of electroweak interactions and in theories where the symmetry breaking is dynamical. We consider the non linear sigma model coupled to an SU(2) x U(1) Yang Mills theory and to fermions as a limit of the strongly interacting Higgs sector, and then we make use of it as a model-independent description of the dynamical Higgs theories. The strengths of the different interactions induced in the non linear sigma model are discussed, in particular those that break isospin symmetry. New sources of isospin breaking are found in the technicolour models that we consider as representative of the dynamical Higgs theories. In particular there is one interaction that contributes directly to the (rho) parameter with a strength that is linear in the effective Yukawa coupling of the heaviest ordinary fermion. A functional integral formalism is adopted to try to deal with the case of a fourth generation of fermions with masses much heavier than the known fermions. Mixed contributions of Higgs fields and fermions to the (rho) parameter are computed.