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) × [SU(2)] N × SU(2) N+1, which are broken down to U(1) EM by non-linear Σ 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→∞ 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.