Toshifumi Yamashita

A general formula of the effective potential in 5D SU(N) gauge theory on orbifold

We show a general formula of the one loop effective potential of the 5D SU(N) gauge theory compactified on an orbifold, S1/Z2. The formula shows the case when there are fundamental, (anti-)symmetric tensor and adjoint representational bulk fields. Our calculation method is also applicable when there are bulk fields belonging to higher dimensional representations. The supersymmetric version of the effective potential with Scherk-Schwarz breaking can be obtained straightforwardly. We also show some examples of effective potentials in SU(3), SU(5) and SU(6) models with various boundary conditions, which are reproduced by our general formula.

Minimal Supersymmetric SU(5) Model with Nonrenormalizable Operators Seesaw Mechanism and Violation of Flavour and CP

Flavour and CP violations that the neutrino-seesaw couplings of types I, II, and III induce radiatively in the soft massive parameters of the minimal supersymmetric SU(5) model, made realistic by nonrenormalizable operators, are analyzed. Effective couplings are used to parametrize the couplings of renormalizable operators and of the corrections that nonrenormalizable ones provide at the tree level. It is found that for a limited, but sufficient accuracy in the calculations of such violations, it is possible to extend the picture of effective couplings to the quantum level, all the way to the cutoff scale. The arbitrariness introduced by nonrenormalizable operators is analyzed in detail. It is shown that it can be drastically reduced in the Yukawa sector if the effective Yukawa couplings involving colored triplet Higgs bosons are tuned to suppress the decay rate of the proton. In the supersymmetrybreaking sector, the usual requirement of independence of flavour and field type for the mechanism of mediation of supersymmetry breaking is not sufficient to forbid arbitrary flavour and CP violations at the tree level. Special conditions to be added to this requirement, under which such violations can be avoided, are identified. Depending on how and whether these conditions are implemented, different phenomenological scenarios emerge. Flavour and CP violations of soft massive parameters induced by neutrino-seesaw couplings are discussed explicitly for the simplest scenario, in which no such violations are present at the tree level. Guidelines for studying them in other, less simple scenarios are given. Lists of all renormalization group equations needed for their calculations are provided for each of the three types of seesaw mechanism, at all energies between the TeV scale and Planck scale. Subject Index: 113, 132, 140, 142

E6 Unification, Doublet-Triplet Splitting and Anomalous U(1)A Symmetry

We propose a natural Higgs sector in E 6 grand unified theory (GUT) with anomalous U(1) A gauge symmetry. In this scenario, the doublet-triplet splitting can be realized, while proton decay via dimension 5 operators is suppressed. Gauge coupling unification is also realized without fine-tuning. The GUT scale obtained in this scenario is generally lower than the usual GUT scale, 2 x 10 1 6 GeV, and therefore it should be possible to observe proton decay via dimension 6 operators in near future experiments. The lifetime of a nucleon in this model is roughly estimated as T p (p → e + π 0 ) ∼ 3 x 10 3 3 years. It is shown that the Higgs sector is compatible with the matter sector proposed by one of the present authors, which reproduces realistic quark and lepton mass matrices, including a bi-large neutrino mixing angle. Combining the Higgs sector and the matter sector, we can obtain a completely consistent E 6 GUT. The input parameters for this model are only eight integer anomalous U(1) A charges (+3 for singlet Higgs) for the Higgs sector and three (half) integer charges for the matter sector.

Two Loop Finiteness of Higgs Mass and Potential in the Gauge-Higgs Unification

The zero mode of an extra-dimensional component of gauge potentials serves as a 4D Higgs field in the gauge-Higgs unification. We examine QED on M 4 × S 1 and determine the mass and potential of a 4D Higgs field (the A5 component) at the two loop level with gauge invariant regularization. It is seen that the mass is free from divergences and independent of the renormalization scheme.

Effective theoretical approach of Gauge-Higgs unification model and its phenomenological applications

We derive the low energy effective theory of Gauge-Higgs unification (GHU) models in the usual four dimensional framework. We find that the theories are described by only the zero-modes with a particular renormalization condition in which essential informations about GHU models are included. We call this condition “Gauge-Higgs condition” in this letter. In other wards, we can describe the low energy theory as the SM with this condition if GHU is a model as the UV completion of the Standard Model. This approach will be a powerful tool to construct realistic models for GHU and to investigate their low energy phenomena. E-mail: haba@ph.tum.de E-mail: smatsu@post.kek.jp E-mail: okadan@post.kek.jp E-mail: tyamashi@post.kek.jp

Dichotomy between the Hole and Electron Behavior in Multiband Superconductor FeSe Probed by Ultrahigh Magnetic Fields.

Magnetoresistivity ρ(xx) and Hall resistivity ρ(xy) in ultrahigh magnetic fields up to 88 T are measured down to 0.15 K to clarify the multiband electronic structure in high-quality single crystals of superconducting FeSe. At low temperatures and high fields we observe quantum oscillations in both resistivity and the Hall effect, confirming the multiband Fermi surface with small volumes. We propose a novel approach to identify from magnetotransport measurements the sign of the charge carriers corresponding to a particular cyclotron orbit in a compensated metal. The observed significant differences in the relative amplitudes of the quantum oscillations between the ρ(xx) and ρ(xy) components, together with the positive sign of the high-field ρ(xy), reveal that the largest pocket should correspond to the hole band. The low-field magnetotransport data in the normal state suggest that, in addition to one hole and one almost compensated electron band, the orthorhombic phase of FeSe exhibits an additional tiny electron pocket with a high mobility.

Two-loop Calculation of Higgs Mass in Gauge-Higgs Unification: 5D Massless QED Compactified on S**1

Abstract We calculate the quantum corrections to the mass of the zero mode of the fifth component of the gauge field at two-loop level in a five-dimensional massless QED compactified on S 1 . We discuss in detail how the divergences are exactly canceled and the mass becomes finite. The key ingredients to obtain the result are the shift symmetry and the Ward–Takahashi identity. We also evaluate the finite part of corrections.

Grand Gauge-Higgs Unification

We propose a novel way to break grand unified gauge symmetries via the Hosotani mechanism in models that can accommodate chiral fermions. Adjoint scalar fields are realized through the so-called diagonal embedding method which is often used in the heterotic string theory. We calculate the one-loop effective potential of the adjoint scalar field in a five dimensional model compactified on an S^1/Z_2 orbifold, as an illustration. It turns out that the potential is basically the same as the one in an S^1 model, and thus the results in literatures, in addition to the chiral fermions, can be realized easily.

Simple E6 Unification with Anomalous U(1)A Symmetry

We propose Higgs sectors in E6 grand unified theory (GUT) with anomalous U(1)A gauge symmetry that are simpler than that of a previous model [N. Maekawa and T. Yamashita, Prog. Theor. Phys. 107 (2002), 1201]. In these new Higgs sectors, as in the previous one, the doublet-triplet (DT) splitting can be realized in a natural way, while proton decay via dimension 5 operators is suppressed, and gauge coupling unification is also realized without fine-tuning. Combining the matter sector, simple complete GUTs can be obtained. Since the Higgs sector is simpler, it is possible for the gauge coupling constant at the cutoff scale to be in the perturbative region, and therefore, the estimated value of the lifetime of a nucleon in this model, τp(p → eπ) ∼ 5× 10 years, is more reliable.

Giant superconducting fluctuations in the compensated semimetal FeSe at the BCS–BEC crossover

The physics of the crossover between weak-coupling Bardeen–Cooper–Schrieffer (BCS) and strong-coupling Bose–Einstein condensate (BEC) limits gives a unified framework of quantum-bound (superfluid) states of interacting fermions. This crossover has been studied in the ultracold atomic systems, but is extremely difficult to be realized for electrons in solids. Recently, the superconducting semimetal FeSe with a transition temperature Tc=8.5 K has been found to be deep inside the BCS–BEC crossover regime. Here we report experimental signatures of preformed Cooper pairing in FeSe, whose energy scale is comparable to the Fermi energies. In stark contrast to usual superconductors, large non-linear diamagnetism by far exceeding the standard Gaussian superconducting fluctuations is observed below T*∼20 K, providing thermodynamic evidence for prevailing phase fluctuations of superconductivity. Nuclear magnetic resonance and transport data give evidence of pseudogap formation at ∼T*. The multiband superconductivity along with electron–hole compensation in FeSe may highlight a novel aspect of the BCS–BEC crossover physics.