Little or No Higgs

Higgsless models offer the possibility of electroweak symmetry breaking without the introduction of any explicit additional Higgs fields in the action within the extra dimensional framework. The various symmetries are broken via the application of appropriately chosen boundary conditions(BC) on the 5D gauge field wavefunctions. The role of the Goldstone bosons is played by the additional scalar component of the original 5D gauge fields which are ‘eaten’ to generate the massive 4D Kaluza-Klein(KK) tower of gauge fields. Unlike in orbifold models the would be zero mode can obtain a finite mass. The most phenomenologically sucessful framework for a model of this type has been presented in Refs.1,2. In these models one takes the usual Randall-Sundrum 5D warped setup3 and places a Left-Right Symmetric SU(2)L × SU(2)R × U(1)B−L gauge theory in the bulk with the SM fermions being localized to the Planck brane. (The LR symmetry helps insure that ρ parameter is very nearly unity.) The BC are chosen to break SU(2)R × U(1)B−L → U(1)Y on the Planck brane while simultaneously breaking SU(2)L × SU(2)R → SU(2)D, the diagonal subgroup, on the TeV brane leaving just U(1)Q. In addition, brane kinetic terms for the gauge fields corresponding to the unbroken generators on the two branes are also present: SU(2)D and U(1)B−L on the TeV brane and SU(2)L and U(1)Y on the Planck brane. The relative strength of these brane terms are described via a set of dimensionless parameters δD,B,L,Y , which, together with κ = gR/gL, the ratio of the gauge couplings, are the free parameters of this model. Other model parameters can be traded for MW,Z and GF in various combinations. There are a large number of constraints that such models must satisfy and there is a strong tension in this model between these various requirements: (i) agreement with the precision electroweak data (which, since we are performing a tree level analysis, we interpret to mean the tree level SM), (ii) the bounds on the masses and couplings of KK excitations from direct and indirect(i.e., contact interaction) searches and (iii) perturbative unitarity(PU) in W L W − L elastic scattering. Recall that without the Higgs the SM violates perturbative unitarity at a scale of √ s ≃ 1.7 TeV; here the KK’s must unitarize this scattering amplitude instead of the Higgs via s− and t−channel exchanges. Naively this amplitude grows like s but all such terms are cancelled due to gauge invariance when the various diagrams are combined thus leaving an s-like growth. The SM Higgs cancels these terms leaving an amplitude which has no power-like growth in s; here in this model the Higgs is absent so that this cancellation is non-trivial. In principle, the remaining amplitude that naively behaves as s can still be large. To see if an allowed model parameter space exists we2 generated via Monte Carlo techniques ∼ 5 · 10 sets of models deSLAC-PUB-10687