Deviations from Low-energy Theorem for V_L V_L Scattering due to Pseudo-Goldstone Bosons

Possible deviations from a low-energy theorem for the scattering of strongly interacting longitudinally polarized $W$ and $Z$ bosons are discussed within a particular scheme of electroweak symmetry breaking. The scheme (suggested earlier by other authors in a slightly different context) is based on spontaneous breakdown of a SU(4) symmetry to custodial SU(2) subgroup. The physical spectrum of such a model contains a set of relatively light pseudo-Goldstone bosons whose interactions with vector bosons modify the low-energy theorem proven for a ``minimal'' symmetry-breaking sector. The Goldstone-boson manifold $SU(4)/SU(2)$ is not a symmetric space. In this context it is observed that, on the other hand, there is a large class of models of electroweak symmetry breaking, involving groups $G$ and $H$ such that the $G/H$ is a symmetric space and the corresponding rich multiplets of pseudo-Goldstone bosons do not influence the canonical low-energy theorem. For the scheme considered here, the relevant interactions are described in terms of an effective chiral Lagrangian and tree-level contributions of the pseudo-Goldstone boson exchanges to the vector boson scattering are computed explicitly. A comparison with the Standard Model is made.