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 2 W 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.
Recently proposed chiral-color models predict the existence of a massive color octet of vector bosons, the axigluon. In this paper we investigate axigluon production in hadronic collisions. We compute the single-jet inclusive cross section and the two-jet invariant-mass distribution for the CERN collider, the Fermilab Tevatron, and the Superconducting Super Collider. We use CERN data to exclude axigluon masses between 125 and 275 GeV, subject to a mild constraint on the width.
Extra-dimensional Higgsless models with electroweak symmetry breaking through boundary conditions generically have difficulties with electroweak precision constraints, when the fermions are localized to the "branes" in the fifth dimension. In this paper we show that these constraints can be relaxed by allowing the light fermions to have a finite extent into the bulk of the fifth dimension. The T and U electroweak parameters can be naturally suppressed by a custodial symmetry, while the S parameter can be made to vanish through a cancellation, if the leakage into the bulk of the light gauge fields and the light left-handed fermion fields are of the same size. This cancellation is possible while allowing realistic values for the first two generations of fermion masses, although special treatment is probably required for the top quark. We present this idea here in the context of a specific continuum theory-space model; however, it can be applied to any five-dimensional Higgsless model, either with a flat or a warped background.
The main challenge faced by Higgsless models of electroweak symmetry breaking is to reconcile the experimental constraints imposed by the precision electroweak data and the top quark phenomenology with the unitarity constraints imposed by longitudinal gauge boson scattering amplitudes. In this paper we expand on previous work, giving details of how delocalized fermions can be used to adjust the S parameter to zero, while keeping the T and U parameters naturally suppressed. We also show that it is possible to obtain the top quark mass, without affecting the delay of unitarity violation of the W + W − → W + W − scattering amplitude, by separating the mass scales of the fermion sector (1/R f ) from that of the gauge sector (1/R g ). The fermion sector scale 1/R f is only weakly constrained by unitarity of the tt → W + W − scattering amplitude; thus the ratio R g /R f can be quite large, and the top mass can be easily achieved. Anomalous right-handed couplings involving the third generation quarks also avoid constraints from experimental data if 1/R f is sufficiently large.
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