Composite Higgs Models Research Articles

Towards top-down holographic composite Higgs: minimal coset from maximal supergravity

Within the context of top-down holography, we study a one-parameter family of regular background solutions of maximal gauged supergravity in seven dimensions, dimensionally reduced on a 2-torus. The dual, four-dimensional confining field theory realises the global (spontaneous as well as explicit) symmetry breaking pattern SO(5) → SO(4). We compute the complete mass spectrum for the fluctuations of the 128 bosonic degrees of freedom of the five-dimensional gravity theory, which correspond to scalar, pseudoscalar, vector, axial-vector, and tensor bound states of the dual field theory, and includes particles with exotic SO(4) quantum numbers. We confirm the existence of tachyonic instabilities near the boundaries of the parameter space.We discuss the interplay between explicit and spontaneous symmetry breaking. The SO(5)/SO(4) coset might provide a first step towards the realisation of a calculable framework and ultraviolet completion of minimal composite Higgs models, if the four pseudo-Nambu-Goldstone bosons are identified with the real components of the Higgs doublet in the standard model (SM), and a subgroup of SO(4) with the SU(2) × U(1) SM gauge group. We exhibit an example with an additional localised boundary term that mimics the effect of a weakly-coupled external sector.

Phenomenology of unusual top partners in composite Higgs models

We consider a particular composite Higgs model which contains SU(3) color octet top partners besides the usually considered triplet representations. Moreover, color singlet top partners are present as well which can in principle serve as dark matter candidates. We investigate the LHC phenomenology of these unusual top partners. Some of these states could be confused with gluinos predicted in supersymmetric models at first glance.

Enhancing the Large Hadron Collider sensitivity to charged and neutral broad resonances of new gauge sectors

In scenarios beyond the Standard Model (BSM) characterised by charged (W′) or neutral (Z′) massive gauge bosons with large width, resonant mass searches are not very effective, so that one has to exploit the tails of the mass distributions measured at the Large Hadron Collider (LHC). In this case, the LHC sensitivity to new physics signals is influenced significantly by systematic uncertainties associated with the Parton Distribution Functions (PDF), particularly in the valence quark sector relevant for the multi-TeV mass region. As a BSM framework featuring such conditions, we consider the 4-Dimensional Composite Higgs Model (4DCHM), in which multiple W′ and Z′ broad resonances are present, with strongly correlated properties. By using the QCD tool xFitter, we study the implications on W′ and Z′ searches in Drell-Yan (DY) lepton decay channels that follow from the reduction of PDF uncertainties obtained through combining high-statistics precision measurements of DY lepton-charge and forward-backward asymmetries. We find that the sensitivity to the BSM states is greatly increased with respect to the case of base PDF sets, thereby enabling one to set more stringent limits on (or indeed discover) such new particles, both independently and in correlated searches.

Tera- Z stage at future colliders and light composite axionlike particles

The Tera-Z phase of future $e^+ e^-$ colliders, FCC-ee and CepC, is a goldmine for exploring $Z$ portal physics. We focus on axion-like particles (ALPs) that can be produced via $Z$ decays with a monochromatic photon. As a template model, we consider composite Higgs models with a light pseudo-scalar that couples through the Wess-Zumino-Witten term to the electroweak gauge bosons. For both photophilic and photophobic cases, we show that the Tera-Z can probe composite scales up to $100$s of TeV, well beyond the capability of the LHC and current precision physics. Our results also apply to generic ALPs and, in particular, severely constrain models that explain the muon $g-2$ anomaly.

E6 GUT and Baryon Asymmetry Generation in the E6CHM

Grand unified theories (GUTs) may result in the E6-inspired composite Higgs model (E6CHM) at low energies, almost stabilizing the electroweak scale. We consider an orbifold GUT in 6 dimensions in which the E6-gauge group is broken to the gauge symmetry of the standard model (SM) while different multiplets of the SM fermions come from different 27-plets. The strongly coupled sector of the E6CHM is confined on the brane where E6 is broken down to its SU(6) subgroup. Near the scale of f≳5TeV, this approximate SU(6) symmetry is expected to be further broken down to its SU(5) subgroup, which contains the SM-gauge group. Such a breakdown leads to a set of pseudo-Nambu–Goldstone bosons (pNGBs) that includes an SM-like Higgs doublet. The approximate gauge coupling unification in the E6CHM takes place at high energies when the right-handed top quark is a composite fermion. To ensure anomaly cancellation, the weakly coupled sector of this model contains extra exotic matter beyond the SM. We discuss the mechanism of the generation of matter–antimatter asymmetry within the variant of the E6CHM in which the baryon number and CP invariance are violated.

Singlet channel scattering in a composite Higgs model on the lattice

We present the first calculation of the scattering amplitude in the singlet channel beyond QCD. The calculation is performed in SU(2) gauge theory with N_f=2 fundamental Dirac fermions and based on a finite-volume scattering formalism. The theory exhibits a SU(4) rightarrow Sp(4) chiral symmetry breaking pattern that is used to design minimal composite Higgs models currently tested at the LHC. Our results show that, for the range of underlying fermion mass considered, the lowest flavour singlet state is stable.

Applying gauge/gravity duality to Composite Higgs models

The AdS/CFT correspondence and its generalization to gauge/gravity dualities provide a very useful approach into solving strongly coupled systems. We put this at work for the strongly coupled sector of Composite Higgs models. We work out relations between masses of proposed states in Composite Higgs. As a cross check we compare these results to existing lattice calculations for which we find good agreement.

Holography of Strongly Coupled Gauge Theories

The "periodic table" of strongly coupled gauge theories remains only sketchily understood. Holography has developed to the point where bottom up constructions can describe the spectrum of individual gauge theories (based on assumptions of their running) including quarks in different representations and higher dimension operators. I highlight the method with a "perfected" version of an AdS dual of QCD and results for composite higgs models with two representations of quarks. The method raises questions about the degree to which energy scales can be split in generic gauge theories including whether confinement and chiral symmetry breaking are linked.

Flavorful composite Higgs model: Connecting the B anomalies with the hierarchy problem

We present a model which connects the neutral current B anomalies with composite Higgs models. The model is based on the minimal fundamental composite Higgs model with $SU(4)/Sp(4)$ coset. The strong dynamics spontaneously break the symmetry and introduce five Nambu-Goldstone bosons. Four of them become the Standard Model Higgs doublet and the last one, corresponding to the broken local $U(1)'$ symmetry, is eaten by the gauge boson. This leads to an additional TeV-scale $Z'$ boson, which can explain the recent B anomalies. The experimental constraints and allowed parameter space are discussed in detail.

Composite flavon-Higgs models

We consider a composite Higgs model based on the $SU(6)/Sp(6)$ coset, where an $U(1)$ subgroup of $Sp(6)$ is identified as the flavor symmetry. A complex scalar field $s$, which is a pseudo-Nambu-Goldstone boson of the broken symmetry, carries a flavor charge and plays the role of a flavon field. The $U(1)_F$ flavor symmetry is then broken by a VEV of the flavon field, which leads to a small parameter and generates the mass hierarchy between the top and bottom quarks. A light flavon below the TeV scale can be naturally introduced, which provides a fully testable model for the origin of flavor hierarchy. A light flavon also leads to substantial flavor changing neutral currents, which are strongly constrained by the flavor precision tests. The direct search of additional scalar bosons can also be conducted in HL-LHC and future hadron colliders.

Small instantons and the strong CP problem in composite Higgs models

We show that QCD instantons can generate large effects at small length scales in the ultraviolet in standard composite Higgs models that utilize partial compositeness. This has important implications for possible solutions of the strong $CP$ problem in these models. First we show that in the simplest known UV completions of composite Higgs models, if an axion is also present, it can have a mass much larger than the usual QCD axion. Even more remarkable is the case where there are no axions, but the strong $CP$ problem can be solved by generating the up quark mass entirely from the contribution of instantons thus reviving the massless up-quark solution for these models. In both cases no additional field content is required apart from what is required to realize partial compositeness.

Loop-generated neutrino masses in composite Higgs models

We present a composite scotogenic model for neutrino masses, which are generated via loops of ℤ2-odd composite scalars. We consider three different approaches to the couplings of the neutrinos (including three right-handed singlets) and the composite sector: ETC-like four-fermion interactions, fundamental partial compositeness and fermion partial compositeness. In all cases, the model can feature sizeable couplings and remain viable with respect to various experimental constraints if the three ℤ2-odd right-handed neutrinos have masses between the TeV and the Planck scales. Additionally, the lightest ℤ2-odd composite scalar may play the role of Dark Matter, either via thermal freeze-out or as an asymmetric relic. This mechanism can be featured in a variety of models based on vacuum misalignment. For concreteness, we demonstrate it in a composite two-Higgs scheme based on the coset SU(6)/Sp(6).

Composite Higgs revealed in Higgs pair photo-production at future colliders

The next generation electron-positron colliders are designed for precision studies of the Standard Model and its extensions, in particular in the Higgs sector. We consider the potential for discovery of composite Higgs models in Higgs pair production through photon collisions. This process is loop-generated, thus it provides access to all Higgs couplings and can show new physics effects in polarized and unpolarized cross-sections starting at relatively low collider energies. It is, therefore, relevant for all electron-positron colliders planned or in preparation. Sizeable deviations from the Standard Model predictions are present in a general class of composite Higgs models, as couplings of one or more Higgs bosons to fermions, or fermionic and scalar resonances, modify the destructive interference present in the Standard Model. In particular, large effects are due to the new quartic coupling of the Higgs to tops and to the presence of a light scalar resonance.

Baryogenesis from the weak scale to the grand unification scale

We review the current status of baryogenesis with emphasis on electroweak baryogenesis and leptogenesis. The first detailed studies were carried out for SU(5) GUT models where CP-violating decays of leptoquarks generate a baryon asymmetry. These GUT models were excluded by the discovery of B+L violating sphaleron processes at high temperatures. Yet a new possibility emerged, electroweak baryogenesis. Here sphaleron processes generate a baryon asymmetry during a strongly first-order phase transition. This mechanism has been studied in many extensions of the Standard Model. However, constraints from the LHC and from low-energy precision experiments exclude most of the known models, leaving composite Higgs models of electroweak symmetry breaking as an interesting possibility. Sphaleron processes are also the basis of leptogenesis, where CP-violating decays of heavy right-handed neutrinos generate a lepton asymmetry which is partially converted to a baryon asymmetry. This mechanism is closely related to the one of GUT baryogenesis, and simple estimates based on GUT models can explain the order of magnitude of the observed baryon-to-photon ratio. In the one-flavour approximation an upper bound on the light neutrino masses has been derived which is consistent with the cosmological upper bound on the sum of neutrino masses. For quasi-degenerate right-handed neutrinos the leptogenesis temperature can be lowered from the GUT scale down to the weak scale, and CP-violating oscillations of GeV sterile neutinos can also lead to successfull leptogenesis. Significant progress has been made in developing a full field theoretical description of thermal leptogenesis, which demonstrated that interactions with gauge bosons of the thermal plasma play a crucial role. Finally, we discuss recent ideas how the seesaw mechanism and B-L breaking at the GUT scale can be probed by gravitational waves.

UV completed composite Higgs model with heavy composite partners

We study electroweak symmetry breaking in minimal composite Higgs models $SU(4)/Sp(4)$ with purely fermionic UV completions based on a confining Hypercolor gauge group and find that the extra Higgs potential from the underlying preon mass can destruct the correlation between the mass of Higgs and composite partners. Thus the composite partners can be very heavy for successful electroweak symmetry breaking without enhancing the separation between the new physical scale and Higgs VEV. So this kind of model can be easily realized by ordinary strong dynamics theories without artificial assumptions and more likely consistent with lattice simulations. The UV completion of partial compositeness predicts a light singlet Goldstone boson which interacts with QCD and electroweak gauge bosons through Wess-Zumino-Witten terms. It can be produced through gluon fusion at LHC and decay into gauge boson pairs. We briefly discuss its phenomenology and derive its bounds from LHC searches.

Contact interactions and top-philic scalar dark matter

We investigate the phenomenology of a scalar top-philic dark matter candidate when adding a dimension-five contact interaction term, as motivated by possible underlying extensions of the Standard Model such as composite Higgs models. We show that the presence of contact interactions can have a major impact on the dark matter relic density as well as on its direct and indirect detection prospects, while the collider phenomenology of the model is unaffected. This underlines the complementarity of collider and cosmological constraints on dark matter models.

A natural composite Higgs via universal boundary conditions

We present a novel realization of a composite Higgs, which can naturally produce top partners above the current LHC bounds without increasing the tuning above 10%. The essential ingredients are softened breaking of the Higgs shift symmetry as well as maximal symmetry, which turn out to perfectly complement each other. The 5D realization of this model is particularly simple: universal UV and IR boundary conditions for the bulk fermions containing the SM fields will cure the problems of existing holographic composite Higgs models and provide a complete viable model for a naturally light Higgs without much tuning.

Nearly Conformal Composite Higgs Model.

We analyze a composite Higgs model based on the confining SU(3) gauge theory with N_{f}=8 Dirac fermions in the fundamental representation. This gauge theory has been studied on the lattice and shown to be well described by a dilaton effective field theory (EFT). Here we modify the EFT by assigning standard-model quantum numbers such that four of the composite pseudo-Nambu-Goldstone boson (pNGB) fields form the standard-model Higgs doublet, by coupling it to the top quark and by adding to the potential a term that triggers electroweak symmetry breaking. The model contains a pNGB Higgs boson, a set of heavier pNGBs, and an approximate dilaton in the same mass range. We study the phenomenology of the model and discuss the amount of tuning required to ensure consistency with current direct and indirect bounds on new physics, highlighting the role of the dilaton field.

Convergent Bayesian global fits of 4D composite Higgs models

Models in which the Higgs boson is a composite pseudo-Nambu-Goldstone boson offer attractive solutions to the Higgs mass naturalness problem. We consider three such models based on the minimal SO(5) → SO(4) symmetry breaking pattern, and perform convergent global fits on the models under a Bayesian framework in order to find the regions of their parameter spaces that best fit a wide range of constraints, including recent Higgs measurements. We use a novel technique to analyse the fine-tuning of the models, quantifying the tuning as the Kullback-Leibler divergence from the prior to the posterior probability on the parameter space. Each model is found to be able to satisfy all constraints at the 3σ level simultaneously. As a by-product of the fits, we analyse the collider phenomenology of our models in these viable regions. In two of the three models, we find that the gg → H → γγ cross section is less than ∼90% that predicted by the SM, which is already in slight tension with experiment and could potentially be ruled out in the future high-luminosity run of the LHC. In addition, the lightest fermions F arising from the new strong dynamics in these models are seen in general to lie above ∼1.1 TeV, with the F → tW+ and F → overline{b}{W}^{+} decays offering particularly promising channels for probing these models in future collider searches.

Low-energy constant L10 in a two-representation lattice theory

We calculate the low-energy constant $L_{10}$ in a two-representation SU(4) lattice gauge theory that is close to a composite-Higgs model. From this we obtain the contribution of the new strong sector to the $S$ parameter. This leads to an upper bound on the vacuum misalignment parameter $\xi$ which is similar to current estimates of this bound. Our result agrees with large-$N_c$ scaling expectations, within large systematic uncertainties.