arXiv subjects

Yuki Adachi

Publications and source records attributed to Yuki Adachi.

At least 19 recordsLinked to original sources

arXiv2026

Conjugate Boundary Conditions, Kaluza-Klein Fermions, and an Extended Seesaw Model

In this paper, we discuss the conjugate boundary condition (CBC), which has recently been studied as a way of realizing a Majorana fermion within a compactified five-dimensional theory ($\mathcal M^4\otimes S^1/Z_2$). The Majorana fermion which plays a crucial role in the seesaw scenario arises as a zero mode (lowest mode) by imposing the CBC. Although each nonzero Kaluza-Klein (KK) mode is also naively expected to be described by two Majorana fermions, we show by direct calculation that they can be combined into a Dirac spinor in the free theory or up to the level of the quadratic term in the Lagrangian. This difference comes from the fact that an accidental U(1) symmetry exists in the KK mode sector, though such a symmetry does not appear in the zero mode sector. We also point out that the compatibility between the CBC and the axial U(1) transformation plays a crucial role in the diagonalization of the KK mode. We also investigate interactions compatible with both the CBC and the chiral orbifold projection. We find that a nontrivial bulk interaction between a CBC fermion and a chiral-orbifold fermion is allowed. As an application, we construct an extended seesaw scenario by utilizing both boundary conditions and such nontrivial bulk interactions. The resulting seesaw scenario has a different property in contrast with the conventional extended seesaw scenario; namely, the above new non-trivial interactions cause lepton number violation (LNV), while the Majorana mass term in our model does not violate lepton number.

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arXiv2025

Dyons in higher-dimensional gauge theories

We discuss the 't Hooft-Polyakov (TP) monopole and then dyon in the framework of higher dimensional gauge theories, such as gauge-Higgs unification models. First, we point out that the Bogomol'nyi-Prasad-Sommerfield (BPS) monopole is nothing but a self-dual gauge field in the 4-dimensional (4D) space including the extra dimension, which is argued to lead to a consequence that the mass of the BPS monopole $M_{\rm TPM}$ and therefore the vacuum expectation value (VEV) of the Higgs field are topologically quantized. In literatures, there exist related arguments on the calorons, which may be understood to be a composition of a pair of constituent monopole and anti-monopole, with each constituent carrying fractional topological charge, while the net topological charge carried by the caloron is unity. From the viewpoint of the caloron, our conclusion of the quantized monopole mass corresponds to the special case, where only a single monopole exists that carries the net topological charge. Next, the argument is generalized to the case of dyon. The mass of the BPS dyon, $M_{\rm BPS}$, is still proportional to the quantized Higgs VEV, though it also depends on a parameter $μ$, denoting the ratio of the electric and magnetic charges of the dyon. In the 5D gauge theories the Chern-Simons term is induced at the quantum level, which, after the extra space component of the gauge field is replaced by its VEV, produces the $θ$ term. Then, through the Witten effect we reach to an interesting conclusion that the parameter $μ$ and therefore $M_{\rm BPS}$ are discretized. In addition, we propose a numerical method to obtain the field configurations and the mass of the non-BPS dyons by use of ``modified" gradient flow equations.

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arXiv2023

Analytic construction of sphaleron-like solution invoking higher dimensional gauge theory

We perform analytic construction of a sphaleron-like solution in the 4-dimensional (4D) space-time invoking the framework of 5D SU(2) gauge theory. By the sphaleron-like solution we mean a static finite energy solution to the equation of motion, which carries the Chern-Simons number $N_\text{CS}=\frac{1}{2}$. Since we are interested in the static solution in the low-energy effective theory, we focus on the part of the action which contains only the gauge fields in the 4D space (not space-time), $(A_{i}, A_{y}) (i = 1,2,3)$ and keep only the Kaluza-Klein zero modes of these fields. Interestingly, the self-duality condition in this 4D space is known to be nothing but the BPS condition for the 't Hooft-Polyakov monopole, once the extra-space component $A_y$ is identified with the adjoint scalar, needed for the monopole solution. Thus, the sphaleron-like solution is based on the BPS monopole embedded in the higher dimensional space-time, which may be interpreted as a self-dual gauge field. By use of the lesson we learn in the case of the instanton in ordinary 4D space-time, we achieve the sphaleron-like configuration of $A_{i}$, which carries $N_\text{CS} = \frac{1}{2}$. As a characteristic feature of this construction invoking higher dimensional gauge theory, in clear contrast to the case of the ordinary BPS monopole, the VEV of the adjoint scalar is topologically fixed, and therefore the mass of the sphaleron-like solution is determined to be $M_\text{sp} = \frac{4π}{g_{4}^{2}}\frac{1}{R}$ ($g_{4}$: 4D gauge coupling constant, $R$: the radius of the circle as the extra space). We also argue that the sphaleron-like solution may be regarded as a saddle point of the energy in the space of static field configurations.

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arXiv2022

On the vacuum structure of gauge-Higgs unification models

In this paper, we discuss the vacuum structure of the gauge-Higgs unification theory, which is one of the attractive candidates of physics beyond the standard model. This scenario has a remarkable feature, namely it has infinitely degenerate vacua due to the characteristic periodic potential of the Higgs filed, to be identified with the extra space component of the higher dimensional gauge field. We address a question, whether to form the superposition of such degenerate vacua, like the $θ$-vacuum in QCD, is necessary or not, in order to realize the true vacuum state of the theory. We derive gauge field configuration which describes the transition between neighboring vacua, like the instanton (or anti-instanton) solution in QCD, and the corresponding Euclidean action in two models. In a simplified 2-dimensional U(1) model, the derived configuration to describe the transition is shown to have finite Euclidean action, and accordingly the "$θ$-vacuum" and the resultant "$θ$-term" are formulated. In a realistic 5D U(1) model, however, the gauge field configuration to describe the transition is shown to have infinite Euclidean action and therefore the tunneling probability between the degenerate vacua vanishes. Thus the superposition of the degenerate vacua is not necessary.

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arXiv2021

The strong CP problem and higher dimensional gauge theories

We discuss a natural scenario to solve the strong CP problem in the framework of the higher dimensional gauge theory. An axion-like field $A_y$ has been built-in as the extra-space component of the higher dimensional gauge field. The coupling of $A_y$ with gluons is attributed to the radiatively induced "Chern-Simons" (CS) term. We adopt a toy model with some unknown gauge symmetry U(1)$_X$. The CS term is obtained in two ways: first by a concrete 1-loop calculation and next by use of the Fujikawa's method to deal with the chiral anomaly in 4D space-time. The obtained results are identical, which implies that the radiative correction to the CS term is "1-loop exact" and is also free from UV-divergence even though the theory itself is non-renormalizable. As a novel feature of this scenario, such obtained CS term is no longer linear in the field $A_y$ as in the usually discussed CS term in 5D space-time but a periodic function of $A_y$, since $A_y$ has a physical meaning as the Wilson-loop phase. We argue how such novel feature of this scenario causes the modification of the ordinary solutions of the strong CP problem based on the axion fields.

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arXiv2019

Strong First Order Electroweak Phase Transition in Gauge-Higgs Unification at Finite Temperature

We analyze the electroweak phase transition at finite temperature in a model of gauge-Higgs unification where the fermion mass hierarchy including top quark mass, a viable electroweak symmetry breaking and an observed Higgs mass are successfully reproduced. To study the phase transition, we derive the general formula of the 1-loop effective potential at finite temperature by using the $ζ$ function regularization method. It is remarkable that the functions determining the Kaluza-Klein mass spectrum have only to be necessary in calculations. This potential can be applicable to any higher dimensional theory in flat space where one extra spatial dimension is compactified. Applying to our model of gauge-Higgs unification, the strong first phase transition compatible with 125 GeV Higgs mass is found to happen.

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arXiv2018

Triple Higgs Boson Coupling in Gauge-Higgs Unification

We consider the triple coupling of the Higgs boson in the context of the gauge-Higgs unification scenario. We show that the triple coupling of the Higgs boson in this scenario generically deviates from SM prediction since the Higgs potential in this scenario has a periodicity. We calculate the coupling in the five-dimensional $SU(3)$ x $U(1)_X$ gauge-Higgs unification model and obtain 70% deviation from the SM prediction.

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arXiv2018

Revisiting Electroweak Symmetry Breaking and Higgs Mass in Gauge-Higgs Unification

We propose a new model of 5D $SU(3)\otimes U(1)_X$ gauge-Higgs unification with a successful electroweak symmetry breaking and a realistic Higgs boson mass. In our model, the representations of the fermions are very simple, the ${\bf 3}, \overline{\bf 3}$ and $\bf \overline{15}$ representations of $SU(3)$ gauge group. Employing the anti-periodic boundary conditions for $\bf \overline{15}$ reduces massless exotic fermions and simplifies the brane localized mass terms. We calculate the 1-loop Higgs potential in detail and find that a realistic electroweak symmetry breaking and the observed Higgs mass are obtained.

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arXiv2016

Trilinear gauge boson couplings in the gauge-Higgs unification

We examine trilinear gauge boson couplings (TGCs) in the context of the $SU(3)_W\otimes U(1)'$ gauge-Higgs unification scenario. The TGCs play important roles in the probes of the physics beyond the standard model, since they are highly restricted by the experiments. We discuss 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 from these observables and discuss the possible parameter space.

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arXiv2015

Deviation of yukawa coupling and Higgs decay in gauge-Higgs Unification

We study the deviation of yukawa coupling in the gauge-Higgs unification scenario from the Standard Model one. Applying the obtained results to the tau and bottom yukawa couplings, we numerically calculate the signal strength of $gg \to H \to \bar b b, \bar τ τ$in the gauge-Higgs unification.

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arXiv2015

$H \to b \bar{b}, τ\barτ$ in Gauge-Higgs Unification

We study the deviation of Higgs production through the gluon fusion process and the decay of Higgs boson to the bottom quark and the tau lepton in gauge-Higgs unification from the Standard Model prediction. We find that the signal strength of $gg \to H \to b\bar{b}, τ\barτ$ is necessarily suppressed comparing to the SM predictions due to the dominant suppression of the Higgs production via gluon fusion.

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arXiv2015

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.

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arXiv2014

mu to 3e and mu to e Conversion in Gauge-Higgs Unification

The two dominant processes of lepton flavor violation, mu to 3e and mu to e conversion in atomic nuclei caused by neutral KK gauge boson exchanges at tree level, are studied in the context of five dimensional gauge-Higgs unification scenario. The key point of the flavor violation is a fact that the bulk masses and the brane ones cannot be simultaneously diagonalized. We estimate the branching ratio of each processes and obtain the lower bound of compactification scale around weak scale from the current experimental data. We discuss the reasons why the final result is not so severe although the large mixing in the lepton sector seems to give large lepton flavor violation processes.

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arXiv2014

$μ$ term and supersymmetry breaking from six dimensional theory

We propose a new next-to-minimal supersymmetric standard model (NMSSM) which is on a six-dimensional spacetime compactified on a $T^2/Z_3$ orbifold. In this model, three gauge singlet fields $N, S_1$ and $S_2$ in addition to the minimal supersymmetric standard model (MSSM) fields are introduced. These fields are localized at some fixed points except for the singlet $N$ and the gauge fields. The $μ$ parameter is provided from the vacuum expectation value (vev) of $N$. The $F$ terms get vevs simultaneously, and the gauginos mediate the supersymmetry breaking to the MSSM sector. Both of these parameters are strongly suppressed due to the profile of $N$. Thus these parameters induced from those of the order of the so-called GUT scale can become close to the electroweak scale without unnatural fine tuning.

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arXiv2012

CP Violation due to Flavor Mixing in Gauge-Higgs Unification

We discuss CP violation in the five dimensional $SU(3) \otimes SU(3)_{\rm color}$ gauge-Higgs unification scenario in which the fifth dimension is compactified on an orbifold $S^1 /Z_2$. It is shown that CP-violating phase appears even in the two generation case in contrast with the fact that at least three generations are required to break CP symmetry in the Standard Model. As our prediction, we obtain the phenomenological constraint on the compactification scale by comparing the typical CP-violating observables, namely $\varepsilon_K$ parameter and the mass difference of two neutral $K$ mesons $Δm_K$ with experimental data.

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arXiv2012

B^0-\bar B^0 Mixing in Gauge-Higgs Unification

We discuss flavor mixing and resulting Flavor Changing Neutral Current (FCNC) in a five dimensional SU(3)_color \otimes SU(3) \otimes U'(1) gauge-Higgs unification. Flavor mixing is realized by the fact that the bulk and brane localized mass terms are not diagonalized simultaneously. As the concrete FCNC processes, we calculate the rate of B^0_d-\bar B^0_d mixing and B^0_s-\bar B^0_s mixing due to the exchange of non-zero Kaluza-Klein gluons at the tree level. We obtain a lower bound on the compactification scale of order O(TeV) by comparing our prediction on the mass difference of neutral B meson with the recent experimental data.

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arXiv2011

D^0-\bar D^0 Mixing in Gauge-Higgs Unification

We discuss flavor mixing and resulting Flavor Changing Neutral Current (FCNC) in the SU(3) \otimes SU(3)_color gauge-Higgs unification. As the FCNC process we calculate the rate of D^0-\bar D^0 mixing due to the exchange of non-zero Kaluza-Klein gluons at the tree level. Flavor mixing is argued to be realized by the fact that the bulk mass term and brane localized mass term is not diagonalized simultaneously unless bulk masses are degenerate. It is shown that automatic suppression mechanism is operative for the FCNC processes of light quarks. We therefore obtain a lower bound on the compactification scale of order \cal O(TeV) by comparing our prediction on the mass difference of neutral D meson with the recent experimental data, which is much milder than what we naively expect assuming only the decoupling of non-zero Kaluza-Klein gluons.

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arXiv2011

Flavor Mixing in Gauge-Higgs Unification

We discuss flavor mixing and resultant flavor changing neutral current processes in the SU(3) \otimes SU(3)_\text{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 - \bar{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 {\cal O}(10) 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.

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