Higgsless Research Articles

Emergent Higgsless Superconductivity

We present a new Higgsless model of superconductivity, inspired from anyon superconductivity but P- and T-invariant and generalizable to any dimension. While the original anyon superconductivity mechanism was based on incompressible quantum Hall fluids as average field states, our mechanism involves topological insulators as average field states. In D space dimensions it involves a (D-1)-form fictitious pseudovector gauge field which originates from the condensation of topological defects in compact low-energy effective BF theories. There is no massive Higgs scalar as there is no local order parameter. When electromagnetism is switched on, the photon acquires mass by the topological BF mechanism. Although the charge of the gapless mode (2) and the topological order (4) are the same as those of the standard Higgs model, the two models of superconductivity are clearly different since the origins of the gap, reflected in the high-energy sectors are totally different. In 2D this type of superconductivity is explicitly realized as global superconductivity in Josephson junction arrays. In 3D this model predicts a possible phase transition from topological insulators to Higgsless superconductors.

Higgsless superconductivity from topological defects in compact BF terms

We present a new Higgsless model of superconductivity, inspired from anyon superconductivity but P- and T-invariant and generalisable to any dimension. While the original anyon superconductivity mechanism was based on incompressible quantum Hall fluids as average field states, our mechanism involves topological insulators as average field states. In D space dimensions it involves a (D−1)-form fictitious pseudovector gauge field which originates from the condensation of topological defects in compact low-energy effective BF theories. In the average field approximation, the corresponding uniform emergent charge creates a gap for the (D−2)-dimensional branes via the Magnus force, the dual of the Lorentz force. One particular combination of intrinsic and emergent charge fluctuations that leaves the total charge distribution invariant constitutes an isolated gapless mode leading to superfluidity. The remaining massive modes organise themselves into a D-dimensional charged, massive vector. There is no massive Higgs scalar as there is no local order parameter. When electromagnetism is switched on, the photon acquires mass by the topological BF mechanism. Although the charge of the gapless mode (2) and the topological order (4) are the same as those of the standard Higgs model, the two models of superconductivity are clearly different since the origins of the gap, reflected in the high-energy sectors are totally different. In 2D this type of superconductivity is explicitly realised as global superconductivity in Josephson junction arrays. In 3D this model predicts a possible phase transition from topological insulators to Higgsless superconductors.

New light onWWscattering at the LHC withWjet tagging

After the recent discovery of a 125 GeV Higgs-like particle at the Large Hadron Collider (LHC), it is crucial to examine its role in unitarizing high energy W_LW_L scattering, which may reveal its possible deviation from a Standard Model Higgs. We perform an updated study on WW scattering in the semileptonic channel at the LHC, improved by the recently developed W jet tagging method. The resultant statistical significance of a Strongly-Interacting Light Higgs (SILH) model is about 20% larger than that based on the conventionally "gold-plated" dileptonic channel, while 200% more signal events are retained. This allows the discovery of an SILH model if the signal strength is 100% (10%) of a pure Higgsless model, using about 40 (3000) fb^{-1} data at the 14 TeV LHC. Meanwhile, the excellent sensitivity to the anomalous Higgs-W boson coupling makes semileptonic WW scattering an important complement to precision measurements at the Higgs resonance.

Electroweak symmetry breaking beyond the Standard Model

In this paper, two key issues related to electroweak symmetry breaking are addressed. First, how fine-tuned different models are that trigger this phenomenon? Second, even if a light Higgs boson exists, does it have to be necessarily elementary? After a brief introduction, the fine-tuning aspects of the MSSM, NMSSM, generalized NMSSM and GMSB scenarios shall be reviewed, then the little Higgs, composite Higgs and the Higgsless models shall be compared. Finally, a broad overview will be given on where we stand at the end of 2011.

One-loop calculation of the oblique S parameter in higgsless electroweak models

We present a one-loop calculation of the oblique S parameter within Higgsless models of electroweak symmetry breaking and analyze the phenomenological implications of the available electroweak precision data. We use the most general effective Lagrangian with at most two derivatives, implementing the chiral symmetry breaking SU(2)_L x SU(2)_R -> SU(2)_{L+R} with Goldstones, gauge bosons and one multiplet of vector and axial-vector massive resonance states. Using the dispersive representation of Peskin and Takeuchi and imposing the short-distance constraints dictated by the operator product expansion, we obtain S at the NLO in terms of a few resonance parameters. In asymptotically-free gauge theories, the final result only depends on the vector-resonance mass and requires M_V > 1.8 TeV (3.8 TeV) to satisfy the experimental limits at the 3 \sigma (1\sigma) level; the axial state is always heavier, we obtain M_A > 2.5 TeV (6.6 TeV) at 3\sigma (1\sigma). In strongly-coupled models, such as walking or conformal technicolour, where the second Weinberg sum rule does not apply, the vector and axial couplings are not determined by the short-distance constraints; but one can still derive a lower bound on S, provided the hierarchy M_V < M_A remains valid. Even in this less constrained situation, we find that in order to satisfy the experimental limits at 3\sigma one needs M_{V,A} > 1.8 TeV.

SINGLE PRODUCTION OF THE GAUGE BOSONS VIA eγ COLLISION AT THE HIGH ENERGY e+e- LINEAR COLLIDERS

The eγ collider is better suited to study single production of the gauge bosons. In this paper, we first consider the contributions of the three-site Higgsless (3SHL) model and the left–right twin Higgs (LRTH) model to the standard model (SM) processes eγ→eZ and eγ→νeW, and then study single production of the new neutral and charged gauge bosons predicted by these two models via eγ collision in the future high energy e+e- linear collider experiments. We find that the correction effects of 3SHL model on the processes eγ→eZ and eγ→νeW might be detected in the future, while this is not the case for the LRTH model. The process eγ→eZ' can give rise to different signatures in the 3SHL and LRTH models and thus can be used to distinguish these two kinds of new physics models at the high energy e+e- linear colliders.

Theory and LHC phenomenology of classicalon decays

Abstract It has been recently proposed by Dvali et al. [1] that high energy scattering in non-renormalizable theories, like the higgsless Standard Model, can be unitarized by the formation of classical configurations called classicalons. In this work we argue that clas- sicalons should have analogs of thermodynamic properties like temperature and entropy and perform a model-independent statistical mechanical analysis of classicalon decays. We find that, in the case of massless quanta, the decay products have a Planck distribution with an effective temperature $$ {\text{T}}\sim {1}/{{\text{r}}_{*}} $$ , where r ∗ is the classicalon radius. These results, in particular a computation of the decay multiplicity, N ∗, allow us to make the first collider analysis of classicalization. In the model for unitarization of WW scattering by classical- ization of longitudinal Ws and Zs we get spectacular multi-W/Z final states that decay into leptons, missing energy and a very high multiplicity (at least 10) of jets. We find that for the classicalization scale, $$ {M_{ * }} = \upsilon = {246}\, {\text{GeV}}({{\text{M}}_{ * }} = {\text{1TeV}}) $$ discovery should be possible in the present 7 TeV (14 TeV) run of the LHC with about 10 fb−1 (100 fb−1) data. We also consider a model to solve the hierarchy problem, where the classicalons are configurations of the Higgs field which decay into to multi-Higgs boson final states. We find that, in this case, for M ∗ = 500 GeV (M ∗ = 1 TeV), discovery should be possible in the top fusion process with about 10 fb−1 (100 fb−1) data at 14 TeV LHC.

Exploring T and S parameters in Vector Meson Dominance models of strong electroweak symmetry breaking

Abstract We revisit the electroweak precision tests for Higgsless models of strong EWSB. We use the Vector Meson Dominance approach and express S and T via couplings characterizing vector and axial spin-1 resonances of the strong sector. These couplings are constrained by the elastic unitarity and by requiring a good UV behavior of various form- factors. We pay particular attention to the one-loop contribution of resonances to T (beyond the chiral log), and to how it can improve the fit. We also make contact with the recent studies of Conformal Technicolor. We explain why the second Weinberg sum rule never converges in these models, and formulate a condition necessary for preserving the custodial symmetry in the IR.

Flavor structure of the three-site Higgsless model

We study the flavor structure of the three-site Higgsless model and evaluate the constraints on the model arising from flavor physics. We find that current data constrain the model to exhibit only minimal flavor violation at tree level. Moreover, at the one-loop level, by studying the leading chiral logarithmic corrections to chirality-preserving Delta F = 1 and Delta F = 2 processes from new physics in the model, we show that the combination of minimal flavor violation and ideal delocalization ensures that these flavor-changing effects are sufficiently small that the model remains phenomenologically viable.

Discovery prospects of an almost fermiophobicW′in the three-site Higgsless model at the LHC

In extensions of the standard model with compactified extra dimensions, perturbative unitarity of the longitudinal gauge bosons is maintained through the contribution of heavy Kaluza-Klein excitations of the gauge fields, without the necessity of introducing a Higgs field. The three-site Higgsless model represents a minimal approach in this respect, containing just one extra set of heavy gauge bosons ${Z}^{\ensuremath{'}}/{W}^{\ensuremath{'}\ifmmode\pm\else\textpm\fi{}}$ in the spectrum. While the ${Z}^{\ensuremath{'}}$ can have robust couplings to SM fermions and, hence, may be detected within the first $1--20\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ of LHC data ($\sqrt{s}=14\text{ }\text{ }\mathrm{TeV}$), the coupling of the ${W}^{\ensuremath{'}}$ to light fermions is suppressed and depends on the model parameters. Expanding on previous parton-level studies, we determine discovery thresholds of the ${W}^{\ensuremath{'}}$ in $s$-channel Drell-Yan production at the LHC for masses ${m}_{{W}^{\ensuremath{'}}}=380$, 500, and 600 GeV, combining analyses of the semileptonic final states $\ensuremath{\ell}\ensuremath{\ell}jj$, $\ensuremath{\ell}\ensuremath{\nu}jj$ and the leptonic final state $\ensuremath{\ell}\ensuremath{\nu}\ensuremath{\ell}\ensuremath{\ell}$ ($\ensuremath{\ell}=e,\ensuremath{\mu}$) including fast detector simulation.

W′production at the LHC in the four-site Higgsless model

We study the phenomenology of the four-site Higgsless model, based on the $SU(2{)}_{L}\ifmmode\times\else\texttimes\fi{}SU(2{)}_{1}\ifmmode\times\else\texttimes\fi{}SU(2{)}_{2}\ifmmode\times\else\texttimes\fi{}U(1{)}_{Y}$ gauge symmetry, at present colliders. The model predicts the existence of two neutral and four charged extra gauge bosons, ${Z}_{1,2}$ and ${W}_{1,2}^{\ifmmode\pm\else\textpm\fi{}}$. In this paper, we focus on the charged gauge sector. We first derive limits on ${W}_{1,2}$-boson masses and couplings to SM fermions from direct searches at the Tevatron. We then estimate at the 7 TeV LHC the exclusion limits with the actual $L=1\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ and the discovery potential with the expected $L=10\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$. In contrast to the minimal (or three-site) Higgsless model which predicts almost fermiophobic extra gauge bosons, the next-to-minimal (or four-site) Higgsless model recovers sizable ${W}_{1,2}$-boson couplings to ordinary matter, expressing the nonfermiophobic multiresonance inner nature of extra-dimensional theories. Owing to this feature, we find that in one year from now the new heavy gauge bosons, ${W}_{1,2}^{\ifmmode\pm\else\textpm\fi{}}$, could be discovered in the final state with an electron and large missing transverse energy at the 7 TeV LHC for ${W}_{1,2}$-boson masses in the TeV region, depending on model parameters.

LHC limits on the top-Higgs in models with strong top-quark dynamics

LHC searches for the standard model Higgs Boson in WW or ZZ decay modes place strong constraints on the top-Higgs state predicted in many models with new dynamics preferentially affecting top quarks. Such a state couples strongly to top-quarks, and is therefore produced through gluon fusion at a rate enhanced relative to the rate for the standard model Higgs boson. A top-Higgs state with mass less than 300 GeV is excluded at 95% CL if the associated top-pion has a mass of 150 GeV, and the constraint is even stronger if the mass of the top-pion state exceeds the top-quark mass or if the top-pion decay constant is a substantial fraction of the weak scale. These results have significant implications for theories with strong top dynamics, such as topcolor-assisted technicolor, top-seesaw models, and certain Higgsless models.

Drell-Yan production ofZ′in the three-site Higgsless model at the LHC

In the Higgsless models, there are extra gauge bosons which keep the perturbative unitarity of a longitudinally polarized gauge boson. The three-site Higgsless model is a minimal Higgsless model and contains three extra gauge bosons, $W^{\prime \pm}$ and Z'. In this paper, we report the discovery potential of the Z' gauge boson via Drell-Yan production with Z'(mass=380, 500, 600 GeV) $\rightarrow WW \rightarrow \ell\nu qq$ ($\ell=e$, $\mu$) at the LHC ($\sqrt{s}$=14 TeV).

Gluon excitations intt¯production at hadron colliders

We argue that a relatively light massive gluon with mass <= 1 TeV, small purely axial couplings to light quarks and sizable vector and axial couplings to the top quark can reproduce the large forward-backward asymmetry observed at the Tevatron without conflicting with the t tbar and the dijet invariant mass distributions measured at the Tevatron and the LHC. We show that realistic Higgsless models with warped extra dimensions naturally fulfil all the necessary ingredients to realize this scenario. While current data is unable to discover or exclude these heavy gluons with masses 850 GeV, they should be observed at the (7 TeV) LHC with a luminosity of the order of 300 pb^{-1}.

Improved analysis of the bounds from the electroweak precision tests on the four-site model

We present a new complete analysis of the electroweak precision observables within the recently proposed 4-site Higgsless model, which is based on the SU(2)_L x SU(2)_1 x SU(2)_2 x U(1)_Y gauge symmetry and predicts six extra gauge bosons, W_{1,2} and Z_{1,2}. Within the epsilon_i (i=1,2,3,b) parametrization, we compute for the first time the EWPT bounds via a complete numerical algorithm going beyond commonly used approximations. Both epsilon_{1,3} impose strong constraints. Hence, it is mandatory to consider them jointly when extracting EWPT bounds and to fully take in to account the correlations among the electroweak precison measurements. The phenomenological consequence is that the extra gauge bosons must be heavier than 250 GeV. Their couplings to SM fermions, even if bounded, might be of the same order of magnitude than the SM ones. In contrast to other Higgsless models, the 4-site model is not fermiophobic. The new gauge bosons could thus be discovered in the favoured Drell-Yan channel already during the present run of the LHC experiment.

Electroweak precision observables at one loop in Higgsless models

We study the viability of generic Higgsless models at low energies when compliance with electroweak precision observables and unitarity constraints up to the TeV scale are imposed. Our analysis shows that consistency with $S$ and $T$ can be achieved at the one-loop level even with a single light vector state, ${m}_{V}\ensuremath{\lesssim}500\text{ }\text{ }\mathrm{GeV}$. However, this scenario turns out to be strongly disfavored when direct resonance searches at the Tevatron are also taken into account. We show that a fully consistent picture can be obtained if an axial state is introduced. Interestingly, ${m}_{V}$ is still predicted to be light (below 1 TeV) while typical values of ${m}_{A}$ span over the window $1.2{m}_{V}\ensuremath{\le}{m}_{A}\ensuremath{\le}1.4{m}_{V}$. Our results for the vector channel are rather robust and well within the reach of present-day colliders, while the axial channel is more loosely constrained.

A comprehensive approach to new physics simulations

We describe a framework to develop, implement and validate any perturbative Lagrangian-based particle physics model for further theoretical, phenomenological and experimental studies. The starting point is FeynRules, a Mathematica package that allows to generate Feynman rules for any Lagrangian and then, through dedicated interfaces, automatically pass the corresponding relevant information to any supported Monte Carlo event generator. We prove the power, robustness and flexibility of this approach by presenting a few examples of new physics models (the Hidden Abelian Higgs Model, the general Two-Higgs-Doublet Model, the most general Minimal Supersymmetric Standard Model, the Minimal Higgsless Model, Universal and Large Extra Dimensions, and QCD-inspired effective Lagrangians) and their implementation/validation in FeynArts/FormCalc, CalcHep, MadGraph/MadEvent, and Sherpa.

Z′production at the LHC in the four-site Higgsless model

We study the phenomenology of the neutral gauge sector of the four-site Higgsless model, based on the $SU(2{)}_{L}\ifmmode\times\else\texttimes\fi{}SU(2{)}_{1}\ifmmode\times\else\texttimes\fi{}SU(2{)}_{2}\ifmmode\times\else\texttimes\fi{}U(1{)}_{Y}$ gauge symmetry, at present colliders. The model predicts the existence of two neutral and four charged extra gauge bosons, ${Z}_{1,2}$ and ${W}_{1,2}^{\ifmmode\pm\else\textpm\fi{}}$. 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 $L=10\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$, and at the 7 TeV LHC with $L=1\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$. In contrast to other Higgsless theories characterized by almost fermiophobic extra gauge bosons, the four-site model allows sizable ${Z}_{1,2}$-boson couplings to standard model fermions. Owing to this feature, we find that in the next 2 years the extra ${Z}_{1,2}$ bosons could be discovered in the favored Drell-Yan channel at the 7 TeV LHC for ${Z}_{1,2}$ masses in the TeV region, depending on model parameters.

Measurements of neutral vector resonance in Higgsless models at the LHC

In Higgsless models, new vector resonances appear to restore the unitarity of the W_L W_L scattering amplitude without the Higgs boson. In the ideal delocalized three site Higgsless model, one of large prodcution cross section of the neutral vector resonance (Z') at the Large Hadron Collider is the W-associated production, pp \to Z'W \to WWW. Although the dileptonic decay channnel, l\nu l'\nu 'jj, is experimentally clean to search for the Z' signals, it is difficult to reconstruct the Z' invariant mass due to the two neutrinos in the final state. We study collider signatures of Z' using the M_{T2}-Assisted On-Shell (MAOS) reconstruction of the missing neutrino momenta. We show the prospect of the Z' mass determination in the channel, l\nu l'\nu 'jj, at the Large Hadron Collider.

Phenomenology of a light scalar: The dilaton

We make use of the language of nonlinear realizations to analyze electroweak symmetry breaking scenarios in which a light dilaton emerges from the breaking of a nearly conformal strong dynamics and compare the phenomenology of the dilaton to that of the well-motivated light composite Higgs scenario. We argue that---in addition to departures in the decay/production rates into massless gauge bosons mediated by the conformal anomaly---characterizing features of the light dilaton scenario are enhancements in off-shell events at high invariant mass involving two longitudinally polarized vector bosons and a dilaton, and tree-level flavor violating processes. Accommodating both electroweak precision measurements and flavor constraints appears especially challenging in the ambiguous scenario in which the Higgs and the dilaton fields strongly mix. We show that warped higgsless models of electroweak symmetry breaking are explicit and tractable realizations of this limiting case. The relation between the naive radion profile often adopted in the study of holographic realizations of the light dilaton scenario and the actual dynamical dilaton field is clarified in the Appendix.