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Yasuhiro Okada

Publications and source records attributed to Yasuhiro Okada.

At least 19 recordsLinked to original sources
arXiv2019

The International Linear Collider. A Global Project

A large, world-wide community of physicists is working to realise an exceptional physics program of energy-frontier, electron-positron collisions with the International Linear Collider (ILC). This program will begin with a central focus on high-precision and model-independent measurements of the Higgs boson couplings. This method of searching for new physics beyond the Standard Model is orthogonal to and complements the LHC physics program. The ILC at 250 GeV will also search for direct new physics in exotic Higgs decays and in pair-production of weakly interacting particles. Polarised electron and positron beams add unique opportunities to the physics reach. The ILC can be upgraded to higher energy, enabling precision studies of the top quark and measurement of the top Yukawa coupling and the Higgs self-coupling. The key accelerator technology, superconducting radio-frequency cavities, has matured. Optimised collider and detector designs, and associated physics analyses, were presented in the ILC Technical Design Report, signed by 2400 scientists. There is a strong interest in Japan to host this international effort. A detailed review of the many aspects of the project is nearing a conclusion in Japan. Now the Japanese government is preparing for a decision on the next phase of international negotiations, that could lead to a project start within a few years. The potential timeline of the ILC project includes an initial phase of about 4 years to obtain international agreements, complete engineering design and prepare construction, and form the requisite international collaboration, followed by a construction phase of 9 years.

hep-ex↗
arXiv2018

The LHC potential of Vector-like quark doublets

The existence of new vector-like quarks is often predicted by models of new physics beyond the Standard Model, and the development of discovery strategies at colliders is the object of an intense effort from the high-energy community. Our analysis aims at identifying the constraints on and peculiar signatures of simplified scenarios containing \textit{two} vector-like quark doublets mixing with \textit{any} of the SM quark generations. This scenario is a necessary ingredient of a broad class of theoretically motivated constructions. We focus on the two charge $2/3$ states $t_{1,2}^\prime$ that, due to their peculiar mixing patterns, feature new production and decay modes that are not searched for at the LHC: single production of the heavier state can dominate over the light one, while pair production via electroweak interactions overcomes the QCD one for masses at the TeV scale.

hep-ph↗
arXiv2015

Interplay of vector-like top partner multiplets in a realistic mixing set-up

The ATLAS and CMS collaborations at the LHC have performed analyses on the existing data sets, studying the case of one vector-like fermion or multiplet coupling to the standard model Yukawa sector. In the near future, with more data available, these experimental collaborations will start to investigate more realistic cases. The presence of more than one extra vector-like multiplet is indeed a common situation in many extensions of the standard model. The interplay of these vector-like multiplet between precision electroweak bounds, flavour and collider phenomenology is a important question in view of establishing bounds or for the discovery of physics beyond the standard model. In this work we study the phenomenological consequences of the presence of two vector-like multiplets. We analyse the constraints on such scenarios from tree-level data and oblique corrections for the case of mixing to each of the SM generations. In the present work, we limit to scenarios with two top-like partners and no mixing in the down-sector.

hep-ph↗
arXiv2014

Lepton flavor violation in the supersymmetric seesaw model after the LHC 8 TeV run

We study the lepton flavor violation in the supersymmetric seesaw model taking into account recent experimental improvements, especially for the Higgs boson mass measurement, direct searches of superpartners and the rare decay of B_s -> mu+ mu- at the LHC, the neutrino mixing angle of theta_{13} at the neutrino experiments, and the search of mu -> e gamma at the MEG experiment. We obtain the latest constraints on the parameters in the supersymmetry breaking terms and study the effect on the lepton flavor violating decays of tau -> mu gamma and mu -> e gamma. In particular, we consider two kinds of assumption on the structures in the Majorana mass matrix and the neutrino Yukawa matrix. In the case of the Majorana mass matrix proportional to the unit matrix, allowing non-vanishing CP violating parameters in the neutrino Yukawa matrix, we find that the branching ratio of tau -> mu gamma can be larger than 10^{-9} within the improved experimental limit of mu -> e gamma. We also consider the neutrino Yukawa matrix that includes the mixing only in the second and third generations, and find that a larger branching ratio of tau -> mu gamma than 10^{-9} is possible with satisfying the recent constraints.

hep-ph↗
arXiv2012

LHC signatures of vector-like quarks

This work provides an overview on the current status of phenomenology and searches for heavy vector-like quarks, which are predicted in many models of new physics beyond the Standard Model. Searches at Tevatron and at the LHC, here listed and shortly described, have not found any evidence for new heavy fermionic states (either chiral or vector-like), and have therefore posed strong bounds on their masses: depending on specific assumptions on the interactions and on the observed final state, vector-like quarks with masses up to roughly 400-600 GeV have been excluded by all experiments. In order to be as simple and model-independent as possible, the chosen framework for the phenomenological analysis is an effective model with the addition of a vector-like quark representation (singlet, doublet or triplet under SU(2)) which couples through Yukawa interactions with all SM families. The relevance of different observables for the determination of bounds on mixing parameters is then discussed and a complete overview of possible two-body final states for every vector-like quark is provided, including their subsequent decay into SM particles. A list and short description of phenomenological analyses present in literature is also provided for reference purposes.

hep-ph↗
arXiv2012

Structure of dimension-six derivative interactions in pseudo Nambu-Goldstone N Higgs doublet models

We derive the general structure of dimension-six derivative interactions in the N Higgs doublet models, where Higgs fields arise as pseudo Nambu-Goldstone modes of a strongly interacting sector. We show that there are several relations among the dimension-six operators, and therefore the number of independent operators decreases compared with models on which only SU(2)_L x U(1)_Y invariance is imposed. As an explicit example, we derive scattering amplitudes of longitudinal gauge bosons and Higgs bosons at high energy on models involving two Higgs doublets, and compare them with the amplitudes in the case of one Higgs doublet.

hep-ph↗
arXiv2012

Heavy Vector-like Top Partners at the LHC and flavour constraints

We consider the phenomenology at the Large Hadron Collider of new heavy vector-like quarks which couple mainly to the third generation quarks via Yukawa interactions, with special emphasis on non-standard doublet representations which are less constrained from present data. We also discuss in detail the flavour limits at tree level and loop level and implications of a generalised CKM mixing matrix to these cases.

hep-ph↗
arXiv2011

Indirect bounds on heavy scalar masses of the two-Higgs-doublet model in light of recent Higgs boson searches

We study an upper bound on masses of additional scalar bosons from the electroweak precision data and theoretical constraints such as perturbative unitarity and vacuum stability in the two Higgs doublet model taking account of recent Higgs boson search results. If the mass of the Standard-Model-like Higgs boson is rather heavy and is outside the allowed region by the electroweak precision data, such a discrepancy should be compensated by contributions from the additional scalar bosons. We show the upper bound on masses of the additional scalar bosons to be about 2 $(1)$ TeV for the mass of the Standard-Model-like Higgs boson to be 240 $(500)$ GeV.

hep-ph↗
arXiv2011

Tau and muon lepton flavor violations in the littlest Higgs model with T-parity

Lepton flavor violation in tau and mu processes is studied in the littlest Higgs model with T-parity. We consider various asymmetries defined in polarized tau and mu decays. Correlations among branching ratios and asymmetries are shown in the following lepton flavor violation processes: mu+ --> e+ gamma, mu+ --> e+ e+ e-, mu- A --> e- A (A = Al, Ti, Au and Pb), tau+ --> mu+ gamma, tau+ --> mu+ mu+ mu-, tau+ --> mu+ e+ e-, tau+ --> mu+ P (P = pi0, eta and eta'), tau+ --> mu+ V (V = rho0, omega and phi), tau+ --> e+ gamma, tau+ --> e+ e+ e-, tau+ --> e+ mu+ mu-, tau+ --> e+ P, tau+ --> e+ V, tau+ --> mu+ mu+ e- and tau+ --> e+ e+ mu-. It is shown that large parity asymmetries and time-reversal asymmetries are allowed in mu+ --> e+ e+ e-. For tau lepton flavor violation processes, sizable asymmetries are possible reflecting characteristic chirality structure of lepton flavor violating interactions in this model.

hep-ph↗
arXiv2010

Higgs boson pair production in new physics models at hadron, lepton, and photon colliders

We study Higgs boson pair production processes at future hadron and lepton colliders including the photon collision option in several new physics models; i.e., the two-Higgs-doublet model, the scalar leptoquark model, the sequential fourth generation fermion model and the vector-like quark model. Cross sections for these processes can deviate significantly from the standard model predictions due to the one-loop correction to the triple Higgs boson coupling constant. For the one-loop induced processes such as $gg \to hh$ and $γγ\to hh$, where $h$ is the (lightest) Higgs boson and $g$ and $γ$ respectively represent a gluon and a photon, the cross sections can also be affected by new physics particles via additional one-loop diagrams. In the two-Higgs-doublet model and scalar leptoquark models, cross sections of $e^+e^-\to hhZ$ and $γγ\to hh$ can be enhanced due to the non-decoupling effect in the one-loop corrections to the triple Higgs boson coupling constant. In the sequential fourth generation fermion model, the cross section for $gg\to hh$ becomes very large because of the loop effect of the fermions. In the vector-like quark model, effects are small because the theory has decoupling property. Measurements of the Higgs boson pair production processes can be useful to explore new physics through the determination of the Higgs potential.

hep-ph↗
arXiv2010

Bounds and Decays of New Heavy Vector-like Top Partners

We study the phenomenology of new heavy vector-like fermions that couple to the third generation quarks via Yukawa interactions, covering all the allowed representations under the standard model gauge groups. We first review tree and loop level bounds on these states. We then discuss tree level decays and loop-induced decays to photon or gluon plus top. The main decays at tree level are to W b and/or Z and Higgs plus top via the new Yukawa couplings. The radiative loop decays turn out to be quite close to the naive estimate: in all cases, in the allowed perturbative parameter space, the branching ratios are mildly sensitive on the new Yukawa couplings and small. We therefore conclude that the new states can be observed at the LHC and that the tree level decays can allow to distinguish the different representations. Moreover, the observation of the radiative decays at the LHC would suggest a large Yukawa coupling in the non-perturbative regime.

hep-ph↗
arXiv2010

Proceedings of the ILC Physics Working Group Meeting at KEK in the period from May 2007 to June 2009

The ILC physics working group is a mixture of experimentalists and theorists mainly working in Japan. It has its origin in the previous LC physics study group and has been reformed with the initiative of a JSPS Creative Scientific Research project: "Research and Development of a Novel Detector System for the International Linear Collider". The working group is, however, formally independent of the JSPS project and is open to everybody who is interested in ILC physics. The primary task of the working group is to reexamine the ILC physics in the context of the expected LHC outcome and to further strengthen the physics case for the ILC project. The topics covered in the working group activities range from key measurements such as those of the Higgs self-coupling and the top Yukawa coupling to uncover the secrets of the electroweak symmetry breaking to various new physics scenarios like supersymmetry, large extra dimensions, and other models of terascale physics. The working group has held ten General Meetings in the period of May 2007 to June 2009 to discuss the topics mentioned above. This report ummarizes the progress made in this period and sets a milestone for future developments in ILC physics.

hep-ph↗
arXiv2010

Higgs boson pair production at the Photon Linear Collider in the two Higgs doublet model

We calculate the cross section of the lightest Higgs boson pair production at the Photon Linear Collider in the two Higgs doublet model. We focus on the scenario in which the lightest Higgs boson has the standard model like couplings to gauge bosons. We take into account the one-loop correction to the $hhh$ coupling as well as additional one-loop diagrams due to charged bosons to the $γγ\to hh$ helicity amplitudes. We discuss the impact of these corrections on the $hhh$ coupling measurement at the Photon Linear Collider.

hep-ph↗
arXiv2010

Universality test of the charged Higgs boson couplings at the LHC and at B factories

Many extensions of the Standard Model (SM) of particle physics predict the existence of charged Higgs bosons with substantial couplings to SM particles, which would render them observable both directly at the LHC and indirectly at B-factories. For example, the charged Higgs boson couplings to fermions in two doublet Higgs models of type II, are proportional to the ratio of the two Higgs doublet vacuum expectation values (tan(beta)) and fermionic mass factors and could thus be substantial at large tan(beta) and/or for heavy fermions. In this work we perform a model-independent study of the charged Higgs boson couplings at the LHC and at B-factories for large values of tan(beta). We have shown that at high luminosity it is possible to measure the couplings of a charged Higgs boson to the third generation of quarks up to an accuracy of 10%. We further argue that by combining the possible measurements of the LHC and the B-factories, it is possible to perform a universality test of charged Higgs boson couplings to quarks.

hep-ph↗
arXiv2010

Lepton electric dipole moments in supersymmetric type II seesaw model

We study the lepton electric dipole moments in the framework of the supersymmetric type II seesaw model where the exchange of heavy SU(2)_W triplets generates small neutrino masses. We show that the CP violating phase of the bilinear soft supersymmetry breaking term associated with the SU(2)_W triplets contributes to lepton electric dipole moments mainly through threshold corrections to the gaugino masses at the seesaw scale. As a consequence, the ratio of the electric dipole moments of the muon and the electron is the same as the ratio of their masses in a wide region of parameter space.

hep-ph↗
arXiv2009

On the model discriminating power of mu -> e conversion in nuclei

We assess the model discriminating power of a combined phenomenological analysis of mu -> e gamma and mu -> e conversion on different target nuclei, including the current hadronic uncertainties. We find that the theoretical uncertainties can be largely reduced by using input from lattice QCD and do not constitute a limiting factor in discriminating models where one or at most two underlying operators (dipole, scalar, vector) provide the dominant source of lepton flavor violation. Our results show that a realistic discrimination among underlying mechanisms requires a measurement of the ratio of conversion rates at the 5% level (two light nuclei) or at the 20% level (one light and one heavy nucleus). We have illustrated these main conclusions also in the context of a supersymmetric model.

hep-ph↗
arXiv2009

Higgs boson pair production at a photon-photon collision in the two Higgs doublet model

We calculate the cross section of Higgs boson pair production at a photon collider in the two Higgs doublet model. We focus on the scenario in which the lightest CP even Higgs boson ($h$) has the standard model like couplings to the gauge bosons. We take into account the one-loop correction to the $hhh$ coupling as well as additional one-loop diagrams due to charged Higgs bosons to the $γγ\to hh$ helicity amplitudes. It is found that the full cross section can be enhanced by both these effects to a considerable level. We discuss the impact of these corrections on the $hhh$ coupling measurement at the photon collider.

hep-ph↗