We investigate by means of a variational approach the effective potential of a 5D U(1) scalar model at finite temperature and compactified on S^1 and S^1/Z_2 as well as the corresponding 4D model obtained through a trivial dimensional reduction. We a
re particularly interested in the behaviour of the thermal masses of the scalar field with respect to the Wilson line phase and the results obtained are compared with those coming from a one-loop effective potential calculation. We also explore the nature of the phase transition.
We reconsider a model of two relativistic particles interacting via a multiplicative potential, as an example of a simple dynamical system with sectors, or branches, with different dynamics and degrees of freedom.The presence or absence of sectors de
pends on the values of rest masses. Some aspects of the canonical quantization are described. The model could be interpreted as a bigravity model in one dimension.
We study the phenomenology of the neutral gauge sector of the four-site Higgsless model, based on the SU(2)_L x SU(2)_1 x SU(2)_2 x U(1)_Y gauge symmetry, at present colliders. The model predicts the existence of two neutral and four charged extra ga
uge bosons, Z_{1,2} and W^pm_{1,2}. We expand and update a previous study, by concentrating on the neutral sector. We derive new limits on Z_{1,2}-boson masses and couplings from recent direct searches at the Tevatron. We moreover estimate the discovery potential at the Tevatron with a project luminosity L=10 fb^{-1}, and at the 7 TeV LHC with L=1 fb^{-1}. In contrast to other Higgsless theories characterized by almost fermiophobic extra gauge bosons, the four-site model allows sizeable Z_{1,2}-boson couplings to SM fermions. Owing to this feature, we find that in the next two years the extra Z_{1,2}-bosons could be discovered in the favoured Drell-Yan channel at the 7 TeV LHC for Z_{1,2} masses in the TeV region, depending on model parameters.
We consider the continuum limit of a moose model corresponding to a generalization to N sites of the Degenerate BESS model. The five dimensional formulation emerging in this limit is a realization of a RS1 type model with SU(2)_L x SU(2)_R in the bul
k, broken by boundary conditions and a vacuum expectation value on the infrared brane. A low energy effective Lagrangian is derived by means of the holographic technique and corresponding bounds on the model parameters are obtained.
We recompute the invisible Higgs decay width arising from Higgs-graviscalar mixing in the ADD model, comparing the original derivation in the non-diagonal mass basis to that in a diagonal mass basis. The results obtained are identical (and differ by
a factor of 2 from the original calculation) but the diagonal-basis derivation is pedagogically useful for clarifying the physics of the invisible width from mixing. We emphasize that both derivations make it clear that a direct scan in energy for a process such as $WWto WW$ mediated by Higgs plus graviscalar intermediate resonances would follow a {it single} Breit-Wigner form with total width given by $Gamma^{tot}=Gamma_h^{SM}+Gamma_{invisible}$. We also compute the additional contributions to the invisible width due to direct Higgs to graviscalar pair decays. We find that the invisible width due to the latter is relatively small unless the Higgs mass is comparable to or larger than the effective extra-dimensional Planck mass.
We show how translational invariance can be broken by the vacuum that drives the spontaneous symmetry breaking of extra-dimensional extensions of the Standard Model, when delta-like interactions between brane and bulk scalar fields are present. We ex
plicitly build some examples of vacuum configurations, which induce the spontaneous symmetry breaking, and have non trivial profile in the extra coordinate.
