arXiv subjects

Yugo Abe

Publications and source records attributed to Yugo Abe.

14 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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arXiv2024

UV stability of 1-loop radiative corrections in higher-derivative scalar field theory

We consider the theory of a higher-derivative (HD) real scalar field $\phi$ coupled to a complex scalar $\sigma$, the coupling of the $\phi$ and $\sigma$ being given by two types, $\lambda_{\sigma\phi}\sigma^\dagger \sigma\phi^{2}$ and $\xi_{\sigma\phi}\sigma^\dagger \sigma\left(\partial_{\mu}\phi\right)^{2}$. We evaluate $\phi$ one-loop corrections $\delta V(\sigma)$ to the effective potential of $\sigma$, both the contribution from the positive norm part of $\phi$ and that from the {\it negative norm part} (ghost). We show that $\delta V(\sigma_{\rm cl})$ at $\sigma_{\rm cl}\to \infty$, where $\sigma_{\rm cl}$ is a classical value of $\sigma$, is positive, implying the stability of $\delta V(\sigma_{\rm cl})$ by the HD 1-loop radiative corrections at high energy.

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arXiv2022

High-energy properties of the graviton scattering in quadratic gravity

We obtain the matter-graviton scattering amplitude in the gravitational theory of quadratic curvature, which has $R_{\mu\nu}^2$ term in the action. Unitarity bound is not satisfied because of the existence of negative norm states, while an analog of unitarity bound for $S$-matrix unitarity holds due to the cancelation among the positive norm states and negative norm ones in the unitarity summation in the optical theorem. The violation of unitarity bound is a counter example of Llewellyn Smith's conjecture on the relation between tree-level unitarity and renormalizability. We have recently proposed a new conjecture that an analog of the unitarity bound for $S$-matrix unitarity gives the equivalent conditions to those for renormalizability. We show that the gravitational theory of quadratic curvature is a nontrivial example consistent with our conjecture.

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arXiv2020

Perturbative $S$-matrix unitarity ($S^{\dagger}S=1$) in $R_{\mu \nu} ^2$ gravity

We show that in the quadratic curvature theory of gravity, or simply $R_{\mu \nu} ^2$ gravity, the tree-level unitariy bound (tree unitarity) is violated in the UV region but an analog for $S$-matrix unitarity ($SS^{\dagger} = 1$) is satisfied. This theory is renormalizable, and hence the failure of tree unitarity is a counter example of Llewellyn Smith's conjecture on the relation between them. We have recently proposed a new conjecture that $S$-matrix unitarity gives the same conditions as renormalizability. We verify that $S$-matrix unitarity holds in the matter-graviton scattering at tree level in the $R_{\mu \nu} ^2$ gravity, demonstrating our new conjecture.

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arXiv2018

Left-right symmetry, orbifold $S^1/Z_2$, and radiative breaking of $U(1)_{\rm R} \times U(1)_{\rm B-L}$

We study the origin of electroweak symmetry under the assumption that $SU(4)_{\rm C} \times SU(2)_{\rm L} \times SU(2)_{\rm R}$ is realized on a five-dimensional space-time. The Pati-Salam type gauge symmetry is reduced to $SU(3)_{\rm C} \times SU(2)_{\rm L} \times U(1)_{\rm R} \times U(1)_{\rm B-L}$ by orbifold breaking mechanism on the orbifold $S^1/Z_2$. The breakdown of residual gauge symmetries occurs radiatively via the Coleman-Weinberg mechanism, such that the $U(1)_{\rm R} \times U(1)_{\rm B-L}$ symmetry is broken down to $U(1)_{\rm Y}$ by the vacuum expectation value of an $SU(2)_{\rm L}$ singlet scalar field and the $SU(2)_{\rm L} \times U(1)_{\rm Y}$ symmetry is broken down to the electric one $U(1)_{\rm EM}$ by the vacuum expectation value of an $SU(2)_{\rm L}$ doublet scalar field regarded as the Higgs doublet. The negative Higgs squared mass term is originated from an interaction between the Higgs doublet and an $SU(2)_{\rm L}$ singlet scalar field as a Higgs portal. The vacuum stability is recovered due to the contributions from Kaluza-Klein modes of gauge bosons.

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arXiv2018

S-matrix Unitarity and Renormalizability in Higher Derivative Theories

We investigate the relation between the $S$-matrix unitarity ($SS^{\dagger}=1$) and the renormalizability, in theories with negative norm states. The relation has been confirmed in many theories, such as gauge theories, Einstein gravity and Lifshitz-type non-relativistic theories by analyzing the unitarity bound, which follows from the $S$-matrix unitarity and the norm positivity. On the other hand, renormalizable theories with a higher derivative kinetic term do not necessarily satisfy the unitarity bound essentially because the unitarity bound does not hold due to the negative norm states. In these theories, it is not clear if the $S$-matrix unitarity provides a nontrivial constraint related to the renormalizability. In this paper we introduce scalar field models with a higher derivative kinetic term and analyze the $S$-matrix unitarity. We have positive results of the relation.

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arXiv2018

Matter scattering in $R_{μν}^2$ gravity and unitarity

We investigate the ultraviolet (UV) behavior of two-scalar elastic scattering with graviton exchanges in higher curvature gravity theory. In the Einstein gravity, matter scattering is shown not to satisfy tree unitarity at high energy. Among a few possible directions to cure unitarity (i.e. UV completion of Einstein gravity), string theory, modified gravity, inclusion of high-mass/high-spin states, we take $R_{μν}^2$ gravity coupled to matter. We show that the matter scattering with graviton interactions satisfies the unitarity bound at high energy, in contrast with the Einstein gravity. The difference in unitarity property of the two gravity theories is due to that in the UV behavior of the propagator and is probably connected to that in another UV property, namely renormalizability property of the two.

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arXiv2018

Gravity loop corrections to the standard model Higgs in Einstein gravity

We study one-loop quantum gravity corrections to the standard model Higgs potential $V(ϕ)$ $\grave{\rm a}$ la Coleman-Weinberg and examine the stability question of $V(ϕ)$ in the energy region of Planck mass scale, $μ\simeq M_{\rm Pl}$ ($M_{\rm Pl}=1.22\times10^{19}{\rm GeV}$). We calculate the gravity one-loop corrections to $V(ϕ)$ in Einstein gravity by using the momentum cut-off $Λ$. We have found that even small gravity corrections compete with the standard model term of $V(ϕ)$ and affect the stability argument of the latter part alone. This is because the latter part is nearly zero in the energy region of $M_{\rm Pl}$.

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arXiv2017

Heavy neutrino mixing in the T2HK, the T2HKK and an extension of the T2HK with a detector at Oki Islands

We study the discovery potential for the mixing of heavy isospin-singlet neutrinos in extensions of the Tokai-to-Kamioka (T2K) experiment: The Tokai-to-Hyper-Kamiokande (T2HK), the Tokai-to-Hyper-Kamiokande-to-Korea (T2HKK), and a plan of adding a new detector at Oki Islands to the T2HK. We parametrize the mixing of heavy neutrinos in terms of a non-unitary mixing matrix acting on active flavors. It is shown that in the T2HK and T2HKK, the sensitivity to heavy neutrino mixing deteriorates for some values of $CP$-violating phases in the standard and the non-unitary mixing matrices, but the deterioration is drastically mitigated if a detector at Oki Islands is added to the T2HK. We also consider the feasibility of measuring the mass hierarchy and the standard $CP$-violating phase $δ_{CP}$ in the presence of heavy neutrino mixing, by fitting experimental data with the standard oscillation parameters only, i.e., under the assumption of unitary mixing. It is revealed that such measurement can be performed to some accuracy in the T2HKK and the extension of the T2HK with a detector at Oki Islands, owing to the fact that data with largely varying $L/E$ are collected in these experiments, while it cannot be in the T2HK.

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arXiv2017

Dark energy from gravitational corrections

We study physics concerning the cosmological constant problem in the framework of effective field theory and suggest that a dominant part of dark energy can originate from gravitational corrections of vacuum energy, under the assumption that the classical gravitational fields do not couple to a large portion of the vacuum energy effectively, in spite of the coupling between graviton and matters at a microscopic level. Our speculation is excellent with terascale supersymmetry.

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arXiv2016

Conjugate boundary condition, hidden particles, and gauge-Higgs inflation

We propose an idea that hidden particles can be separated according to gauge quantum numbers from the visible ones by the difference of boundary conditions on extra dimensions. We formulate 5-dimensional gauge theories yielding conjugate boundary conditions besides ordinary ones on $S^1/Z_2$, and examine physical implications concerning hidden particles on an extension of the standard model coexisting different types of boundary conditions. A model with conjugate boundary conditions is applied on a gauge-Higgs inflation scenario.

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arXiv2016

Quantum gravity corrections to the standard model Higgs in Einstein and $R^2$ gravity

We evaluate quantum gravity corrections to the standard model Higgs potential $V(ϕ)$ a la Coleman-Weinberg and examine the stability question of $V(ϕ)$ at scales of Planck mass $M_{\rm Pl}$. We compute the gravity one-loop corrections by using the momentum cut-off in Einstein gravity. The gravity corrections affect the potential in a significant manner for the value of $Λ= (1 - 3)M_{\rm Pl}.$ In view of reducing the UV cut-off dependence we also make a similar study in the $R^2$ gravity.

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arXiv2015

Inflation from radion gauge-Higgs potential at Planck scale

We study whether the inflation is realized based on the radion gauge-Higgs potential obtained from the one-loop calculation in the 5-dimensional gravity coupled to a $U(1)$ gauge theory. We show that the gauge-Higgs can give rise to inflation in accord with the astrophysical data and the radion plays a role in fixing the values of physical parameters. We clarify the reason why the radion dominated inflation and the hybrid inflation cannot occur in our framework.

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arXiv2014

Radion stabilization in the presence of Wilson line phase

We study the stabilization of an extra-dimensional radius in the presence of a Wilson line phase of an extra $U(1)$ gauge symmetry on a five-dimensional space-time, using the effective potential relating both the radion and the Wilson line phase at the one-loop level. We find that the radion can be stabilized by the introduction of a small number of fermions.

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