Georgi-Machacek Model Research Articles

Double Higgs boson production in the models with isotriplets

The enhancement of double Higgs boson production in the extensions of the Standard Model with extra isotriplets is studied. It is found that in see-saw type II model decays of new heavy Higgs can contribute to the double Higgs production cross section as much as Standard Model channels. In Georgi–Machacek model the cross section can be much larger since the custodial symmetry is preserved and the strongest limitation on triplet parameters is removed.

Suppression of H → VV decay channels in the Georgi–Machacek model

The H→ZZ decay mode is usually considered as one of the most promising ways to discover new heavy neutral scalar H. We show that in the Georgi–Machacek model it is possible to get large enhancement of double SM-like Higgs boson production due to H decays while ZZ and WW decay channels could be highly suppressed.

All the generalized Georgi-Machacek models

The Georgi-Machacek model adds two SU(2)_L-triplet scalars to the Standard Model in such a way as to preserve custodial SU(2) symmetry. We study the generalizations of the Georgi-Machacek model to SU(2)_L representations larger than triplets. Perturbative unitarity considerations limit the possibilities to models containing only SU(2)_L quartets, quintets, or sextets. These models are phenomenologically interesting because they allow the couplings of the 125 GeV Higgs boson to WW and ZZ to be larger than their values in the Standard Model. We write down the most general custodial SU(2)-preserving scalar potentials for these models and outline their phenomenology. We find that experimental and theoretical constraints on the fermiophobic custodial-fiveplet states present in each of the models lead to absolute upper bounds on the 125 GeV Higgs boson coupling strength to WW and ZZ.

GMSB with light stops

Gauge mediated supersymmetry breaking (GMSB) is an elegant mechanism to transmit supersymmetry breaking from the hidden to the MSSM observable sector, which solves the supersymmetric avor problem. However the smallness of the generated stop mixing requires superheavy stops to reproduce the experimental value of the Higgs mass. A possible way out is to extend the MSSM Higgs sector with singlets and/or triplets providing extra tree-level corrections to the Higgs mass. Singlets will not get any soft mass from GMSB and triplets will contribute to the parameter which could be an issue. In this paper we explore the second possibility by introducing extra supersymmetric triplets with hypercharges Y = (0; 1), with a tree-level custodial SU(2)L SU(2)R global symmetry in the Higgs sector protecting the parameter: a supersymmetric generalization of the Georgi-Machacek model, dubbed as supersymmetric custodial triplet model (SCTM). The renormalization group running from the messenger to the electroweak scale mildly breaks the custodial symmetry. We will present realistic low-scale scenarios (with the NLSP being a Bino-like neutralino or the right-handed stau) based on general (non-minimal) gauge mediation and consistent with all present experimental data. Their main features are: i) Light ( 1 TeV) stops; ii) Exotic couplings (H W Z and H W W ) absent in the MSSM and proportional to the triplets VEV, v ; and, iii) A possible (measurable) universality breaking of the Higgs couplings WZ = rWW=rZZ6 1.

Properties and searches of the exotic neutral Higgs bosons in the Georgi-Machacek model

The Georgi-Machacek model predicts the existence of four neutral Higgs bosons, one of which can be identified as the 125-GeV Higgs boson. The latest Higgs data favor the parameter space of small mixing angle α between the two custodial singlets of the model. The other two neutral Higgs bosons belong respectively to the custodial triplet and quintet. We study the general decay and production properties of these particles in the small-α scenario. Constraints on the SU(2) L triplet vacuum expectation value are obtained as a function of the exotic Higgs boson masses using latest ATLAS data of various search channels for additional neutral Higgs bosons.

Double Higgs production at LHC, see-saw type-II and Georgi-Machacek model

The double Higgs production in the models with isospin-triplet scalars is studied. It is shown that in the see-saw type II model the mode with an intermediate heavy scalar, $pp\to H+X\to 2h+X$, may have the cross section which is compatible with that in the Standard Model. In the Georgi-Machacek model this cross section could be much larger than in SM since the vacuum expectation value of the triplet can be large.

Supersymmetric custodial Higgs triplets and the breaking of universality

Higgs triplet models are known to have difficulties obtaining agreement with electroweak precision data and in particular constraints on the $\rho$ parameter. Either a global $SU(2)_L \otimes SU(2)_R$ symmetry has to be imposed on the scalar potential at the electroweak scale, as done in the well-known Georgi-Machacek (GM) model, or the triplet vacuum expectation values must be very small. We construct a supersymmetric model that can satisfy constraints on the $\rho$ parameter, even if these two conditions are not fulfilled. We supersymmetrize the GM model by imposing the $SU(2)_L \otimes SU(2)_R$ symmetry at a scale $\mathcal M$, which we argue should be at or above the messenger scale, where supersymmetry breaking is transmitted to the observable sector. We show that scales $\mathcal M$ well above 100 TeV and sizable contributions from the triplets to electroweak symmetry breaking can be comfortably accommodated. We discuss the main phenomenological properties of the model and demonstrate that the departure from custodial symmetry at the electroweak scale, due to radiative breaking, can show up at the LHC as a deviation in the `universal' relation for the Higgs couplings to $WW$ and $ZZ$. As a by-product of supersymmetry, we also show that one can easily obtain both large tree-level \emph{and} one loop corrections to the Higgs mass. This allows for stops that can be significantly lighter and with smaller mixing than those needed in the MSSM.

Indirect constraints on the Georgi-Machacek model and implications for Higgs boson couplings

We update the indirect constraints on the Georgi-Machacek model from $B$-physics and electroweak precision observables, including new constraints from $b \to s \gamma$ and $B^0_s \to \mu^+ \mu^-$. We illustrate the effect of these constraints on the couplings of the Standard Model-like Higgs boson by performing scans using the most general scalar potential, subject to vacuum stability and perturbativity constraints. We find that simultaneous enhancements of all the Higgs production cross sections by up to 39\% are still allowed after imposing these constraints. LHC rate measurements on the Higgs pole could be blind to these enhancements if unobserved non-standard Higgs decays are present.

Novel constraint on the parameter space of the Georgi-Machacek model with current LHC data

The same-sign diboson process $pp\to W^\pm W^\pm jj$ has been measured at the LHC using leptonic decay channels of the $W$ bosons, with production cross sections of two fiducial regions reported to be consistent with the standard model expectations within 1 sigma. These results constrain new physics models with a modified $W^+W^+W^-W^-$ vertex. We consider in particular the Georgi-Machacek model in which the quartic $W$ boson vertex is effectively modified due to mediations of new Higgs bosons in the model. The relevant gauge-gauge-scalar couplings are all proportional to the vacuum expectation value of the isospin triplets, which can be of $\mathcal{O}(10)$ GeV because of custodial vacuum alignment. Using the current 8-TeV data at the LHC, we exclude parameter space on the plane of the triplet vacuum expectation value and the new Higgs boson mass. The expected discovery reach at the 14-TeV LHC is also studied.

The decoupling limit in the Georgi-Machacek model

We study the most general scalar potential of the Georgi-Machacek model, which adds isospin-triplet scalars to the Standard Model (SM) in a way that preserves custodial SU(2) symmetry. We show that this model possesses a decoupling limit, in which the predominantly-triplet states become heavy and degenerate while the couplings of the remaining light neutral scalar approach those of the SM Higgs boson. We find that the SM-like Higgs boson couplings to fermion pairs and gauge boson pairs can deviate from their SM values by corrections as large as $\mathcal{O}(v^2/M_{\rm new}^2)$, where $v$ is the SM Higgs vacuum expectation value and $M_{\rm new}$ is the mass scale of the predominantly-triplet states. In particular, the SM-like Higgs boson couplings to $W$ and $Z$ boson pairs can decouple much more slowly than in two Higgs doublet models, in which they deviate from their SM values like $\mathcal{O}(v^4/M_{\rm new}^4)$. Furthermore, near the decoupling limit the SM-like Higgs boson couplings to $W$ and $Z$ pairs are always larger than their SM values, which cannot occur in two Higgs doublet models. As such, a precision measurement of Higgs couplings to $W$ and $Z$ pairs may provide an effective method of distinguishing the Georgi-Machacek model from two Higgs doublet models. Using numerical scans, we show that the coupling deviations can reach 10% for $M_{\rm new}$ as large as 800~GeV.

Electroweak phase transition in Georgi–Machacek model

The Georgi–Machacek model extends the standard model Higgs sector by adding two isospin triplet scalar fields and imposing global SU(2)R symmetry on them. A feature of the model is that the triplets can acquire a large vacuum expectation value without conflicting with the current experimental bound on the ρ parameter. We investigate the electroweak phase transition in the Georgi–Machacek model by evaluating the finite-temperature effective potential of the Higgs sector. The electroweak phase transition can be sufficiently strong in a large parameter space when the triplets acquire a vacuum expectation value of O(10) GeV, opening a possibility to realize successful electroweak baryogenesis.

Enhancements of weak gauge boson scattering processes at the CERN LHC

Current CERN Large Hadron Collider data on the 126 GeV standard model-like Higgs boson suggest the possibility of larger Higgs boson couplings with the weak gauge bosons, $g_{hVV}$, than those in the standard model. We use the Georgi-Machacek model as an explicit model to realize such a scenario. We find that the $g_{hVV}$ couplings can be larger than the standard model value by a factor of about 1.3 maximally in the parameter region consistent with the current Higgs boson search data and allowed by various other constraints. We then show how the modified $g_{hVV}$ couplings lead to enhancements in various weak boson scattering processes. This can be clearly observed as excesses in the transverse mass distributions at around 126 GeV and also the mass of heavy Higgs bosons.

Probing exotic Higgs sectors from the precise measurement of Higgs boson couplings

We study coupling constants of the standard model like Higgs boson with the gauge bosons $hZZ$ and $hWW$ and fermions $hf\bar{f}$ in the general Higgs sector which contains higher isospin representations with arbitrary hypercharge. In Higgs sectors with exotic Higgs representations, the $hZZ$ and $hWW$ coupling constants can be larger than those in the standard model. We calculate deviations in the Higgs boson couplings from standard model values in the model with a real or complex triplet field, the Georgi-Machacek model and the model with a septet scalar field. We also study deviations in the event rates of $h\to ZZ^*$, $h\to WW^*$, $h\to \gamma\gamma$, $h\to b\bar{b}$ and $h\to \tau\tau$ channels.

Triplet Higgs boson collider phenomenology after the LHC

ATLAS and CMS have discovered a Standard Model Higgs-like particle. One of the main discovery channels is the Higgs decay to two photons, which, at the moment, seems to be considerably enhanced over the Standard Model expectation. Models with additional charged matter coupling to the Higgs sector can enhance or decrease the Higgs to two photons branching ratio. We take this as motivation to confront the so-called Georgi-Machacek model of Higgs triplets with the results of recent searches for a SM Higgs boson performed at the LHC. We also investigate the model in regions of the allowed parameter space relevant for a SM-like phenomenology. The Georgi-Machacek model avoids tree-level issues of the T parameter, while offering a vastly modified Higgs phenomenology compared to the Standard Model. This comes at the price of introducing another fine-tuning problem related to electroweak precision measurements. We investigate the collider phenomenology of the Georgi-Machacek model in the light of existing collider constraints beyond any effective field theory approximation and contextualize our findings with electroweak precision constraints.

Testing the custodial symmetry in the Higgs sector of the Georgi-Machacek model

We study how the custodial symmetry in the Higgs sector of the Georgi-Machacek (GM) model can be tested at the LHC. As the minimal extension of the Higgs triplet model, in which tiny neutrino masses are generated via the Type-II Seesaw Mechanism, the GM model keeps the electroweak $\rho$ parameter at unity at tree level. In the GM model, there are 5-plet ($H_5$), 3-plet ($H_3$) and singlet ($H_1$) Higgs bosons under the classification of the custodial $SU(2)_V$ symmetry, in addition to the standard model-like Higgs boson ($h$). These new Higgs bosons have the following characteristic features at the tree level: (1) the masses of the Higgs bosons belonging to the same $SU(2)_V$ multiplet are degenerate; and (2) $H_5$ and $H_1$ couple to the electroweak gauge bosons but not SM quarks, whereas $H_3$ couples to the quarks but not the gauge bosons. We find that the $H_5$ production from the weak vector boson fusion process and the Drell-Yan process associated with $H_3$ are useful in testing the custodial symmetry of the Higgs sector at the LHC. In addition, these processes can also be used to discriminate from other models that contain singly-charged Higgs bosons and extra neutral Higgs bosons. We also investigate a possible enhancement in the $h\to \gamma\gamma$ as well as $h\to Z\gamma$ decays.

Revisiting theories with enhanced Higgs couplings to weak gauge bosons

Based on recent LHC Higgs analyses and in anticipation of future results we revisit theories where Higgs bosons can couple to weak gauge bosons with enhanced strength relative to the Standard Model value. Specifically, we look at the Georgi-Machacek model and its generalizations where higher ``spin'' representations of $SU(2{)}_{L}$ break electroweak symmetry while maintaining custodial $SU(2)$. In these theories, there is not only a Higgs-like boson but partner Higgs scalars transforming under representations of custodial $SU(2)$, leading to a rich phenomenology. These theories serve as a consistent theoretical and experimental framework to explain enhanced couplings to gauge bosons, including fermiophobic Higgses. We focus on the phenomenology of a neutral scalar partner to the Higgs, which is determined once the Higgs couplings are specified. Depending on the parameter space, this partner could have (i) enhanced fermion and gauge boson couplings and should be searched for at high mass ($>600\text{ }\text{ }\mathrm{GeV}$), (ii) suppressed couplings and could be searched for at lower masses, where the Standard Model Higgs has already been ruled out, and (iii) fermiophilic couplings, where it can be searched for in heavy Higgs and top resonance searches. In the first two regions, the partner also has substantial decay rates into a pair of Higgs bosons. We touch briefly on the more model-dependent effects of the nontrivial $SU(2{)}_{C}$ multiplets, which have exotic signals, such as a doubly charged Higgs. We also discuss how the loop induced effects of these scalars tend to reduce the Higgs decay rate to photons, adding an additional uncertainty when extracting the couplings for the Higgs boson.

Exploring Higgs triplet models via vector boson scattering at the LHC

We present the results of a study of Higgs triplet boson production arising in the littlest Higgs, left-right symmetric, and Georgi-Machacek models in the ${W}^{\ifmmode\pm\else\textpm\fi{}}{W}^{\ifmmode\pm\else\textpm\fi{}}$, ${W}^{\ifmmode\pm\else\textpm\fi{}}Z$, ${W}^{+}{W}^{\ensuremath{-}}$, and $ZZ$ channels at the LHC. We focus on the ``gold-plated'' purely leptonic decay modes and consider the irreducible electroweak, QCD, and $t$-quark backgrounds, applying a combination of forward-jet tagging, central-jet vetoing, and stringent leptonic cuts to suppress the backgrounds. We find that, given the constraints on the triplet vacuum expectation value (vev), considerable luminosity is required to observe Higgs triplet bosons in vector boson scattering. Observing a Higgs triplet at the LHC is most promising in the Georgi-Machacek model due to a weaker constraint on the triplet vev. In this model, we find that a Higgs triplet boson with a mass of 1.0(1.5) TeV can be observed at the LHC with an integrated luminosity as low as $41(119)\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ in the ${W}^{\ifmmode\pm\else\textpm\fi{}}{W}^{\ifmmode\pm\else\textpm\fi{}}$ channel and as low as $171(474)\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ in the ${W}^{\ifmmode\pm\else\textpm\fi{}}Z$ channel. Observation of Higgs triplet bosons in these channels would help identify the underlying theory.

Unitarity bounds in the Higgs model including triplet fields with custodial symmetry

We study bounds on Higgs-boson masses from perturbative unitarity in the Georgi-Machacek model, whose Higgs sector is composed of a scalar isospin doublet and a real and a complex isospin triplet field. This model can be compatible with the electroweak precision data without fine-tuning because of the imposed global $SU(2{)}_{R}$ symmetry in the Higgs potential, by which the electroweak rho parameter is unity at the tree level. All possible two-body elastic-scattering channels are taken into account to evaluate the $S$-wave amplitude matrix, and then the condition of perturbative unitarity is imposed on the eigenvalues to obtain constraint on the Higgs parameters. Masses of all scalar bosons turn out to be bounded from above, some of which receive more strict upper bounds as compared to that in the standard model (712 GeV). In particular, the upper bound of the lightest scalar boson, whatever it would be, is about 270 GeV.