Higgsless electroweak symmetry breaking from theory space

We investigate unitarity of W+W- scattering in the context of theory space models of the form U(1) x [SU(2)](N) x SU(2)(N+1), which are broken down to U(1)(EM) by nonlinear Sigma fields, without the presence of a physical Higgs Boson. By allowing the couplings of the U(1) and the final SU(2)(N+1) to vary, we can fit the W and Z masses, and we find that the coefficient of the term in the amplitude that grows as E-2/m(W)(2) at high energies is suppressed by a factor of (N + 1)(-2). In the N + 1 --> infinity limit the model becomes a 5-dimensional SU(2) gauge theory defined on an interval, where boundary terms at the two ends of the interval break the SU(2) down to U(1)(EM). These boundary terms also modify the Kaluza-Klein (KK) mass spectrum, so that the lightest KK states can be identified as the W and Z bosons. The T parameter, which measures custodial symmetry breaking, is naturally small in these models. Depending on how matter fields are included, the strongest experimental constraints come from precision electroweak limits on the S parameter.