Dynamical electroweak symmetry breaking

In theories of dynamical electroweak symmetry breaking, the electroweak interactions are broken to electromagnetism by the vacuum expectation value of a fermion bilinear. These theories may thereby avoid the introduction of fundamental scalar particles, of which we have no examples in nature. In this note, we review the status of experimental searches for the particles predicted in technicolor, topcolor, and related models.     

On electroweak symmetry breaking in the littlest Higgs model

In SU(5)/SO(5) little Higgs models radiative corrections give rise to SU(2)_L×U(1)_Y symmetry breaking. In this work we start a program for a detailed determination of the relevant terms of the effective Higgs potential by computing the contribution of the t, b and T quarks at the one-loop level, as a starting point for a higher-loop computation. In spite of the fact that some two-loop level contributions are well known to be important, we use our preliminary one-loop result to illustrate that, by demanding the effective potential to reproduce exactly the standard model Higgs potential, and in particular the relation m_H ²=2λv²=2μ², it will be possible to set new constraints on the parameter space of the littlest Higgs model when the computation of all the relevant contributions to the effective Higgs potential is completed.     

Radiative electroweak symmetry breaking revisited

In the absence of a tree-level scalar-field mass, renormalization-group methods permit the explicit summation of leading-logarithm contributions to all orders of the perturbative series within the effective potential for SU(2)xU(1) electroweak symmetry. This improvement of the effective potential function is seen to reduce residual dependence on the renormalization mass scale. The all-orders summation of leading-logarithm terms involving the dominant three couplings contributing to radiative corrections is suggestive of a potential characterized by a plausible Higgs boson mass of 216 GeV. However, the tree potential's local minimum at phi=0 is restored if QCD is sufficiently strong.     

Towards a realistic model of Higgsless electroweak symmetry breaking

We present a 5D gauge theory in warped space based on a bulk SU(2)L x SU(2)R x U(1)(B-L) gauge group where the gauge symmetry is broken by boundary conditions. The symmetry breaking pattern and the mass spectrum resemble that in the standard model (SM). To leading order in the warp factor the rho parameter and the coupling of the Z (S parameter) are as in the SM, while corrections are expected at the level of a percent. From the anti-de Sitter (AdS) conformal field theory point of view the model presented here can be viewed as the AdS dual of a (walking) technicolorlike theory, in the sense that it is the presence of the IR brane itself that breaks electroweak symmetry, and not a localized Higgs on the IR brane (which should be interpreted as a composite Higgs model). This model predicts the lightest W, Z, and gamma resonances to be at around 1.2 TeV, and no fundamental (or composite) Higgs particles.     

Warped electroweak breaking without custodial symmetry

We propose an alternative to the introduction of an extra gauge (custodial) symmetry to suppress the contribution of KK modes to the T   parameter in warped theories of electroweak breaking. The mechanism is based on a general class of warped 5D metrics and a Higgs propagating in the bulk. The metrics are nearly AdS in the UV region but depart from AdS in the IR region, towards where KK fluctuations are mainly localized, and have a singularity outside the slice between the UV and IR branes. This gravitational background is generated by a bulk stabilizing scalar field which triggers a natural solution to the hierarchy problem. Depending on the model parameters, gauge-boson KK modes can be consistent with present bounds on EWPT for mKK?1 TeV$m_{\mathit{KK}}?1\text{TeV}$ at 95% CL. The model contains a light Higgs mode which unitarizes the four-dimensional theory. The reduction in the precision observables can be traced back to a large wave function renormalization for this mode.     

Higgsless approach to electroweak symmetry breaking

Higgsless models are an attempt to achieve a breaking of the electroweak symmetry via boundary conditions at the end-points of a fifth dimension compactified on an interval, as an alternative to the usual Higgs mechanism. There is no physical Higgs scalar in the spectrum and the perturbative unitarity violation scale is delayed via the exchange of massive spin-1 KK resonances. The correct mass spectrum is reproduced in a model in warped space, which inherits a custodial symmetry from a left–right gauge symmetry in the bulk. Phenomenological challenges as well as collider signatures are presented. From the AdS/CFT perspective, this model appears as a weakly coupled dual to walking technicolour models. To cite this article: C. Grojean, C. R. Physique 8 (2007).     

Natural electroweak breaking from a mirror symmetry

We present "twin Higgs models," simple realizations of the Higgs boson as a pseudo Goldstone boson that protect the weak scale from radiative corrections up to scales of order 5-10 TeV. In the ultraviolet these theories have a discrete symmetry which interchanges each standard model particle with a corresponding particle which transforms under a twin or a mirror standard model gauge group. In addition, the Higgs sector respects an approximate global symmetry. When this global symmetry is broken, the discrete symmetry tightly constrains the form of corrections to the pseudo Goldstone Higgs potential, allowing natural electroweak symmetry breaking. Precision electroweak constraints are satisfied by construction. These models demonstrate that, contrary to the conventional wisdom, stabilizing the weak scale does not require new light particles charged under the standard model gauge groups.     

Natural electroweak symmetry breaking in generalised mirror matter models

It has recently been pointed out that the mirror or twin Higgs model is more technically natural than the standard model, thus alleviating the “little” hierarchy problem. In this Letter we generalise the analysis to models with an arbitrary number of isomorphic standard model sectors, and demonstrate that technical naturalness increases with the number of additional sectors. We consider two kinds of models. The first has N standard model sectors symmetric under arbitrary permutations thereof. The second has p left-chiral standard model sectors and p right-chiral or mirror standard model sectors, with p  -fold permutation symmetries within both and a discrete parity transformation interchanging left and right. In both kinds of models the lightest scalar has an invisible width fraction 1/N$1/N$, which will provide an important means of experimentally testing this class of models.     

Dynamical electroweak symmetry breaking via fermion condensates inSU(5) model

TheSU(4) symmetry scheme, used in a previous paper for the study of the 2v double decay, is applied to the study of the 0v double decay. Differences and similarities between the 2 processes are emphasized. The sensitivity to short-range correlations is discussed.     

Radiative electroweak symmetry breaking in the extra dimensions scenarios

We study radiative spontaneous electroweak symmetry breaking in the non-supersymmetric extra dimension scenarios of the standard model extension proposed by Antoniadis et al., Dienes et al. and Pomarol et al. In the framework of the multi-scale effective theory, by using the renormalization group method with an up-to-down viewpoint, we find that the effects of Kaluza-Klein excitations of bosons of the standard model can change the sign of the Higgs mass term of the standard model from positive to negative and break the electroweak symmetry. The critical scale for the electroweak phase transition to occur depends on the compactification scale (say 1.6 (2.0) TeV if the compactification scale is assumed to be 0.8 (1.5) TeV or so), and is insensitive to the mass of the Higgs particle. This radiative spontaneous symmetry breaking mechanism can work naturally in the extra dimension scenarios, and neither new particle contents beyond the standard model from the supersymmetry nor technicolor are necessary. Received: 7 January 2002 / Revised version: 2 March 2002 / Published online: 7 June 2002