gauge-Higgs Unification Scenario Research Articles

Proton decay prediction from a gauge-Higgs unification scenario in five dimensions

Proton decay prediction from a gauge-Higgs unification scenario in five dimensions

The implication of gauge–Higgs unification for the hierarchical fermion masses

The observed hierarchical charged fermion masses of three generations seem to imply that these masses are originally universal for three generations and then get exponential suppression with "quantized exponents" by some mechanism. We argue that such remarkable feature of hierarchical fermion mass spectrum may be naturally understood in the scenario of gauge-Higgs unification, where the universality of fermion masses is guaranteed by the fact that Higgs boson is originally gauge boson in this scenario and also the quantized exponents may be attributed to the well-known quantization condition of magnetic charge of the magnetic monopole placed inside the torus, as the extra dimensional space. Because of the presence of the magnetic monopole, we get a chiral theory and multiple Kaluza-Klein zero modes even if we introduce only a single 6-dimensional Weyl fermion. We present two types of three generation model, which succeed to realize the remarkable hierarchical mass spectrum.

Majorana neutrino masses in the scenario of gauge–Higgs unification

In this paper we consider possible mechanisms to generate small Majorana neutrino masses for active neutrinos in the scenario of gauge-Higgs unification, a candidate for physics beyond the standard model. We stress that it is non-trivial to find a gauge-invariant operator, responsible for the Majorana masses, which is the counterpart of the well-known SU(2)$_L \times$ U(1)$_Y$ invariant higher-mass-dimensional ($d = 5$) operator. As the first possibility we discuss the seesaw mechanism by assigning leptonic fields to the adjoint representation of the gauge group, so that a $d = 5$ gauge-invariant operator can be formed. It turns out that the mechanism leading to the small Majorana masses is the admixture of the Type I and Type III seesaw mechanisms. As the second possibility, we consider the case where the relevant operator has $d = 7$, by introducing a matter scalar belonging to the fundamental representation of the gauge group. Reflecting the fact that the mass dimension of the operator is higher than usually expected, the Majorana masses are generated by a "double seesaw mechanism."

125 GeV Higgs boson mass from 5D gauge-Higgs unification

In the context of a simple gauge-Higgs unification (GHU) scenario based on the gauge group SU(3)$\times$U(1)$^\prime$ in a 5-dimensional flat space-time, we investigate a possibility to reproduce the observed Higgs boson mass of around 125 GeV. We introduce bulk fermion multiplets with a bulk mass and a (half) periodic boundary condition. In our analysis, we adopt a low energy effective theoretical approach of the GHU scenario, where the running Higgs quartic coupling is required to vanish at the compactification scale. Under this "gauge-Higgs condition," we investigate the renormalization group evolution of the Higgs quartic coupling and find a relation between the bulk mass and the compactification scale so as to reproduce the 125 GeV Higgs boson mass. Through quantum corrections at the one-loop level, the bulk fermions contribute to the Higgs boson production and decay processes and deviate the Higgs boson signal strengths at the Large Hadron Collider (LHC) experiments from the Standard Model (SM) predictions. Employing the current experimental data which show the the Higgs boson signal strengths for a variety of Higgs decay modes are consistent with the SM predictions, we obtain lower mass bounds on the lightest mode of the bulk fermions.

Fermion dark matter in gauge-Higgs unification

We propose a Majorana fermion dark matter in the context of a simple gauge-Higgs Unification (GHU) scenario based on the gauge group SU(3)×U(1)′ in 5-dimensional Minkowski space with a compactification of the 5th dimension on S1/Z2 orbifold. The dark matter particle is identified with the lightest mode in SU(3) triplet fermions additionally introduced in the 5-dimensional bulk. We find an allowed parameter region for the dark matter mass around a half of the Standard Model Higgs boson mass, which is consistent with the observed dark matter density and the constraint from the LUX 2016 result for the direct dark matter search. The entire allowed region will be covered by, for example, the LUX-ZEPLIN dark matter experiment in the near future. We also show that in the presence of the bulk SU(3) triplet fermions the 125 GeV Higgs boson mass is reproduced through the renormalization group evolution of Higgs quartic coupling with the compactification scale of around 108 GeV.

Proton decay prediction from a gauge-Higgs unification scenario in five dimensions

The Higgs boson mass and top quark mass imply that the Higgs quartic coupling vanishes around the scale of $10^9 - 10^{13}$ GeV, depending on the precise value of the top quark mass. The vanishing quartic coupling can be naturally addressed if the Higgs field originates from a 5-dimensional gauge field and the 5th dimension is compactified at the scale of the vanishing Higgs quartic coupling, which is a scenario based on gauge-Higgs unification. We present a general prediction of the scenario on the proton decay process $p \to \pi^0 e^+$. In many gauge-Higgs unification models, the 1st generation fermions are localized towards an orbifold fixed point in order to realize the realistic Yukawa couplings. Hence, four-fermion operators responsible for the proton decay can appear with a suppression of the 5-dimensional Planck scale (not the 4-dimensional Planck scale). Since the 5-dimensional Planck scale is connected to the compactification scale, we have a correlation between the proton partial decay width and the top quark mass. We show that the future Hyper-Kamiokande experiment may discover the proton decay if the top quark pole mass is larger than about 172.5 GeV.

Few remarks on the Higgs boson decays in gauge-Higgs unification

In the scenario of gauge-Higgs unification, the origin of the Higgs boson is higher dimensional gauge boson. Reflecting its origin, very characteristic predictions are made of the Higgs boson interactions in this scenario. Especially, a remarkable claim has been made: the contribution of non-zero Kaluza-Klein modes to the Higgs decay $H \to Z \gamma$ exactly vanishes in the minimal SU(3) electro-weak unified model, at least at the one-loop level. In this brief report, in order to see whether this prediction is a general feature of the scenario or the consequence of the specific choice of the model, matter content or the order of perturbative expansion, we perform an operator analysis. We demonstrate that there does not exist any relevant operator, respecting the gauge symmetry SU(3) in the bulk. We also comment on the possibly important contribution to the photonic decay $H \to \gamma \gamma$ due to the non-zero Kaluza-Klein modes of light quarks.

Gauge-Higgs EW and grand unification

Four-dimensional Higgs field is identified with the extra-dimensional component of gauge potentials in the gauge-Higgs unification scenario. [Formula: see text] gauge-Higgs EW unification in the Randall–Sundrum warped space is successful at low energies. The Higgs field appears as an Aharonov–Bohm phase [Formula: see text] in the fifth dimension. Its mass is generated at the quantum level and is finite. The model yields almost the same phenomenology as the standard model for [Formula: see text], and predicts [Formula: see text] bosons around 6–10 TeV with very broad widths. The scenario is generalized to [Formula: see text] gauge-Higgs grand unification. Fermions are introduced in the spinor and vector representations of [Formula: see text]. Proton decay is naturally forbidden.

Trilinear gauge boson couplings in the gauge—Higgs unification

We examine trilinear gauge boson couplings (TGCs) in the context of the SU (3)W ⊗ U (1) � gauge–Higgs unification scenario. The TGCs play important roles in probes of the physics beyond the standard model, since they are highly restricted by the experiments. We discuss the mass spectrum of the neutral gauge boson with brane-localized mass terms carefully and find that the TGCs and ρ parameter may deviate from standard model predictions. Finally, we put a constraint on these observables and discuss the possible parameter space.

Deviation of Yukawa coupling in gauge-Higgs unification

We study the deviation of yukawa coupling in the gauge-Higgs unification scenario from the Standard Model one. Taking into account the brane mass terms necessary for generating the flavor mixing and removing the exotic massless fermions, we derive an analytic formula determining the KK mass spectrum and yukawa coupling. Applying the obtained results to the tau and bottom yukawa couplings, we numerically calculate the ratio of the yukawa couplings in the gauge-Higgs unification and in the Standard Model.

Is the 126 GeV Higgs boson mass calculable in gauge-Higgs unification?

We address the question of whether the recently observed Higgs mass |$M_{H} = 126$|GeV, of the order of the weak scale |$M_{W}$|⁠, is calculable as a finite value in the scenario of gauge–Higgs unification. In the scenario formulated on a flat five-dimensional space-time, the Higgs mass is calculable, being protected under the quantum correction by gauge invariance, though the predicted Higgs mass is generally too small compared with |$M_{W}$|⁠. In the six-dimensional SU(3) model, however, a suitable orbifolding is known to lead to a mass of the order of |$M_{W}$|⁠: |$M_{H} = 2M_{W}$| at the tree level, which has some similarity to the corresponding prediction by the minimal supersymmetric standard model, |$M_{H} \leq (\cos \beta ) M_{Z}$|⁠. We demonstrate first by a general argument and secondly by explicit calculations that, even though the quantum correction to the quartic self-coupling of the Higgs field is UV-divergent, its deviation from that of |$g^{2}$| is calculable, and therefore two observables, |$M_{H}^{2}$| and |$\Delta \equiv \left ( \frac {M_{H}}{2M_{W}}\right ) ^{2}-1$|⁠, are both calculable in the gauge–Higgs unification scenario. The implication of the precise value 126 GeV to the compactification scale and the bulk mass of the matter field in our model is also discussed.

H→Zγin gauge-Higgs unification

In our previous paper, we have investigated effects of a simple gauge-Higgs unification model on the diphoton signal events from the Higgs boson production at the Large Hadron Collider. We have found that in this model the effective Higgs-to-diphoton coupling can be enhanced by 1-loop corrections with Kaluza-Klein (KK) modes of bulk fields. This result can be an explanation for the observed excess of the signal strength in the Higgs to diphoton decay channel. In this paper, we investigate KK-mode effects on another Higgs boson decay mode, H to Z gamma. One naturally expects that the KK modes also contribute to the effective H-Z-gamma coupling and can cause some deviation for the H to Z gamma decay mode from the Standard Model prediction. Revealing a correlation between the KK-mode effects on the Higgs-to-diphoton and H-Z-gamma couplings is an interesting topic in terms of a possibility to discriminate a variety of models for physics beyond the Standard Model. We show a very striking result for the gauge-Higgs unification model, namely, the absence of KK-mode contributions to the H-Z-gamma coupling at the 1-loop level. This is a very specific and general prediction of the gauge-Higgs unification scenario.

Is the Higgs boson a sign of extra dimensions?

We introduce a four-dimensional cutoff in the scenario of gauge-Higgs unification to control the ultraviolet behavior. A one-loop effective potential for a Higgs field and the Higgs mass are obtained with the cutoff. We find an interrelation between the four-dimensional cutoff and the scale of extra dimensions, which is concretized through the Higgs mass. Combining this interrelation and the recently discovered Higgs boson at the LHC, we obtain an interesting constraint for the four-dimensional cutoff and the extra-dimensional scale.

Diphoton decay excess and 125 GeV Higgs boson in gauge-Higgs unification

In the context of the gauge-Higgs unification scenario in a 5-dimensional flat spacetime, we investigate Higgs boson production via gluon fusion and its diphoton decay mode at the LHC. We show that the signal strength of the Higgs diphoton decay mode observed at ATLAS and CMS, which is considerably larger than the Standard Model expectation, can be explained by a simple gauge-Higgs unification model with color-singlet bulk fermions to which a half-periodic boundary condition is assigned. The bulk fermions also play a crucial role in reproducing the observed Higgs boson mass around 125 GeV.

Anomalous Higgs interactions in gauge-Higgs unification

We discuss anomalous Higgs interactions in the scenario of gauge-Higgs unification. In the scenario Higgs originates from higher dimensional gauge field and has a physical meaning as AB phase or Wilson loop. As its inevitable consequence, physical observables are expected to be periodic in the Higgs field. In particular, the Yukawa coupling is expected to show some periodic and non-linear behavior as the function of the Higgs VEV. For a specific choice of the VEV, the Yukawa coupling of KK zero mode fermion even vanishes. On the other hand, the Yukawa coupling is originally provided by gauge interaction, which is linear in the Higgs field. We discuss how such two apparent contradiction about the non-linearity of the Yukawa coupling can be reconciled and at the same time how these two "pictures" give different predictions in the simplest framework of the scenario: SU(3) electroweak model in 5-dimensional flat space-time with orbifolding. The deviation of the Yukawa coupling from the standard model prediction is also calculated for arbitrary VEV. We study "$H$-parity", which guarantees the stability of the Higgs for a specific choice of the VEV. Also discussed is the Higgs interaction with $W^\pm$ and $Z^0$. It turns out that in our framework of flat space-time the interaction does not show deviation from the standard model prediction, except for the specific case of the VEV.

125 GeV Higgs, type III seesaw and gauge–Higgs unification

Recently, both the ATLAS and CMS experiments have observed an excess of events that could be the first evidence for a 125 GeV Higgs boson. This is a few GeV below the (absolute) vacuum stability bound on the Higgs mass in the Standard Model (SM), assuming a Planck mass ultraviolet (UV) cutoff. In this Letter, we study some implications of a 125 GeV Higgs boson for new physics in terms of the vacuum stability bound. We first consider the seesaw extension of the SM and find that in type III seesaw, the vacuum stability bound on the Higgs mass can be as low as 125 GeV for the seesaw scale around a TeV. Next we discuss some alternative new physics models which provide an effective ultraviolet cutoff lower than the Planck mass. An effective cutoff Λ≃1011 GeV leads to a vacuum stability bound on the Higgs mass of 125 GeV. In a gauge–Higgs unification scenario with five-dimensional flat spacetime, the so-called gauge–Higgs condition can yield a Higgs mass of 125 GeV, with the compactification scale of the extra-dimension being identified as the cutoff scale Λ≃1011 GeV. Identifying the compactification scale with the unification scale of the SM SU(2) gauge coupling and the top quark Yukawa coupling yields a Higgs mass of 121±2 GeV.

Flavor mixing in gauge-Higgs unification

We discuss flavor mixing and resultant flavor changing neutral current processes in the SU(3) ⊗ SU(3)color gauge-Higgs unification scenario. To achieve flavor violation is a challenging issue in the scenario, since the Yukawa couplings are originally higher dimensional gauge interactions. We argue that the presence of Z 2-odd bulk masses of fermions plays a crucial role as the new source of flavor violation. Although introducing brane-localized mass terms in addition to the bulk masses is necessary to realize flavor mixing, if the bulk masses were universal among generations, the flavor mixing and flavor changing neutral current processes are known to disappear. We also discuss whether natural flavor conservation is realized in the scenario. It is shown that the new source of flavor violation leads to flavor changing neutral current processes at the tree level due to the exchange of non-zero Kaluza-Klein gauge bosons. As a typical example we calculate the rate of \( {K^0} - {\overline K^0} \) mixing due to the non-zero Kaluza-Klein gluon exchange at the tree level. The obtained result for the mass difference of neutral kaon is suppressed by the inverse powers of the compactification scale. By comparing our prediction with the data we obtain the lower bound of the compactification scale as a function of one unfixed parameter of the theory, which is of \( \mathcal{O}\left( {10} \right) \) TeV, except for some extreme cases. We argue that the reason to get such rather mild lower bound is the presence of “GIM-like” mechanism, which is a genuine feature of GHU scenario.

Gauge-Higgs dark matter

When the anti-periodic boundary condition is imposed for a bulk field in extradimensional theories, independently of the background metric, the lightest component in the anti-periodic field becomes stable and hence a good candidate for the dark matter in the effective 4D theory due to the remaining accidental discrete symmetry. Noting that in the gauge-Higgs unification scenario, introduction of anti-periodic fermions is well-motivated by a phenomenological reason, we investigate dark matter physics in the scenario. As an example, we consider a five-dimensional SO(5)$\timesU(1)_X$ gauge-Higgs unification model compactified on the $S^1/Z_2$ with the warped metric. Due to the structure of the gauge-Higgs unification, interactions between the dark matter particle and the Standard Model particles are largely controlled by the gauge symmetry, and hence the model has a strong predictive power for the dark matter physics. Evaluating the dark matter relic abundance, we identify a parameter region consistent with the current observations. Furthermore, we calculate the elastic scattering cross section between the dark matter particle and nucleon and find that a part of the parameter region is already excluded by the current experimental results for the direct dark matter search and most of the region will be explored in future experiments.

Symmetry and Z2-Orbifolding Approach in Five-Dimensional Lattice Gauge Theory

In a lattice gauge-Higgs unification scenario using a Z_2-orbifolded extra-dimension, we find a new global symmetry in a case of SU(2) bulk gauge symmetry. It is a global symmetry on sites in a fixed point with respect to Z_2-orbifolding, independent of the bulk gauge symmetry. It is shown that the vacuum expectation value of a Z_2-projected Polyakov loop is a good order parameter of the new symmetry. The effective theory on lattice is also discussed.

Neutron electric dipole moment in the gauge-Higgs unification

We study the neutron electric dipole moment (EDM) in a five-dimensional $SU(3)$ gauge-Higgs unification compactified on ${M}^{4}\ifmmode\times\else\texttimes\fi{}{S}^{1}/{Z}_{2}$ space-time including a massive fermion. We point out that to realize the $CP$ violation is a nontrivial task in the gauge-Higgs unification scenario and argue how the $CP$ symmetry is broken spontaneously by the vacuum expectation value of the Higgs, the extra space component of the gauge field. We emphasize the importance of the interplay between the vacuum expectation value of the Higgs and the ${Z}_{2}$-odd bulk mass term to get physically the $CP$ violation. We then calculate the one-loop contributions to the neutron EDM as the typical example of the $CP$ violating observable and find that the EDM appears already at the one-loop level, without invoking the three-generation scheme. We then derive a lower bound for the compactification scale, which is around 2.6 TeV, by comparing the contribution due to the nonzero Kaluza-Klein modes with the experimental data.