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Ken-ichi Okumura

Publications and source records attributed to Ken-ichi Okumura.

At least 19 recordsLinked to original sources
arXiv2019

Hide and Seek with Massive Fields in Modulus Mediation

We study the modulus mediation of supersymmetry breaking motivated by superstring theory. We show that the renormalization group running of the soft supersymmetry breaking parameters due to the interactions of massive fields is canceled by the threshold corrections at one-loop order, if their mass is given by nonperturbative dynamics controlled by the same modulus that mediates supersymmetry breaking and a sum rule of the modular weights holds for the Yukawa couplings. As an example, we discuss order reduction of lepton flavor violation in the supersymmetric seesaw mechanism, which revives the parameter space already excluded.

hep-ph↗
arXiv2017

A dilaton-pion mass relation

Recently, Golterman and Shamir presented an effective field theory which is supposed to describe the low-energy physics of the pion and the dilaton in an $SU(N_c)$ gauge theory with $N_f$ Dirac fermions in the fundamental representation. By employing this formulation with a slight but important modification, we derive a relation between the dilaton mass squared~$m_τ^2$, with and without the fermion mass~$m$, and the pion mass squared~$m_π^2$ to the leading order of the chiral logarithm. This is analogous to a similar relation obtained by Matsuzaki and~Yamawaki on the basis of a somewhat different low-energy effective field theory. Our relation reads $m_τ^2=m_τ^2|_{m=0}+KN_f\hat{f}_π^2m_π^2/(2\hat{f}_τ^2)+O(m_π^4\ln m_π^2)$ with~$K=9$, where $\hat{f}_π$ and~$\hat{f}_τ$ are decay constants of the pion and the dilaton, respectively. This mass relation differs from the one derived by Matsuzaki and~Yamawaki on the points that $K=(3-γ_m)(1+γ_m)$, where $γ_m$ is the mass anomalous dimension, and the leading chiral logarithm correction is~$O(m_π^2\ln m_π^2)$. For~$γ_m\sim1$, the value of the decay constant~$\hat{f}_τ$ estimated from our mass relation becomes $\sim50\%$ larger than $\hat{f}_τ$ estimated from the relation of Matsuzaki and~Yamawaki.

hep-lat↗
arXiv2016

Fermion number anomaly with the fluffy mirror fermion

Quite recently, Grabowska and Kaplan presented a 4-dimensional lattice formulation of chiral gauge theories based on the chiral overlap operator. We study this formulation from the perspective of the fermion number anomaly and possible associated phenomenology. A simple argument shows that the consistency of the formulation implies that the fermion with the opposite chirality to the physical one, the "fluffy mirror fermion" or "fluff", suffers from the fermion number anomaly in the same magnitude (with the opposite sign) as the physical fermion. This immediately shows that if at least one of the fluff quarks is massless, the formulation provides a simple viable solution to the strong CP problem. Also, if the fluff interacts with gravity essentially in the same way as the physical fermion, the formulation can realize the asymmetric dark matter scenario.

hep-lat↗
arXiv2016

TeV scale mirage mediation in NMSSM and precision Higgs measurement

We propose that the TeV scale mirage mediation in the Next-to-Minimal Supersymmetric Standard Model (NMSSM) is a novel solution for the little hierarchy problem in supersymmetry. The 125 GeV Higgs boson can be accommodated with the new quartic coupling or the mixing with the singlet. The fine-tuning measures are estimated numerically and found as low as 10\% or below for $1.5$ TeV gluino and $1$ TeV stop. The higgsino can be as heavy as 500 GeV without deteriorating the fine-tuning. An undesirable singlet-doublet mixing in the Higgs sector is suppressed due to approximate scale symmetries. We investigate the couplings of the Higgs bosons and discuss the prospects for studying them at LHC and ILC.

hep-ph↗
arXiv2016

Phenomenology of NMSSM in TeV scale mirage mediation

We study the next-to-minimal supersymmetric standard model (NMSSM) with the TeV scale mirage mediation, which is known as a solution for the little hierarchy problem in supersymmetry. Our previous study showed that 125 GeV Higgs boson is realized with O(10) % fine-tuning for 1.5 TeV gluino (1 TeV stop) mass. The $μ$ term could be as large as 500 GeV without sacrificing the fine-tuning thanks to a cancellation mechanism. The singlet-doublet mixing is suppressed by $\tanβ$. In this paper, we further extend this analysis. We argue that approximate scale symmetries play a role behind the suppression of the singlet-doublet mixing. They reduce the mixing matrix to a simple form that is useful to understand the results of the numerical analysis. We perform a comprehensive analysis of the fine-tuning including the singlet sector by introducing a simple formula for the fine-tuning measure. This shows that the singlet mass of the least fine-tuning is favored by the LEP anomaly for moderate $\tanβ$. We also discuss prospects for the precision measurements of the Higgs couplings at LHC and ILC and direct/indirect dark matter searches in the model.

hep-ph↗
arXiv2012

TeV scale mirage mediation in NMSSM

We study the next-to-minimal supersymmetric standard model. We consider soft supersymmetry breaking parameters, which are induced by the mirage mediation mechanism of supersymmetry breaking. We concentrate on the mirage mediation, where the so-called mirage scale is the TeV scale. In this scenario, we can realize the up-type Higgs soft mass of O(200) GeV, while other masses such as gaugino masses and stop masses are heavy such as 1 TeV or more. Cancellation between the effective μ-term and the down-type Higgs soft mass ameliorates the fine-tuning in the electroweak symmetry breaking even for μ=O(500) GeV. The mixing between the doublet and singlet Higgs bosons is suppressed by (λ/κ)/tanβ. Then the lightest doublet Higgs mass naturally reaches 125 GeV lifted by the new quartic coupling. The higgsino and singlino are light and their linear combination is the lightest superparticle.

hep-ph↗
arXiv2008

Axionic Mirage Mediation

In this talk, we propose a model of mirage mediation, in which Peccei-Quinn symmetry is incorporated. In this \textit{axionic mirage mediation}, it is shown that the Peccei-Quinn symmetry breaking scale is dynamically determined around $10^{10}$ GeV to $10^{12}$ GeV due to the supersymmetry breaking effects. The problems in the original mirage mediation such as the $μ$-problem and the moduli problem can be solved simultaneouly. Furthermore, in our model the axino, which is the superpartner of the axion, is the lightest sparticle.

hep-ph↗
arXiv2008

Axionic Mirage Mediation

Although the mirage mediation is one of the most plausible mediation mechanisms of supersymmetry breaking, it suffers from two crucial problems. One is the μ-/B μ-problem and the second is the cosmological one. The former stems from the fact that the B parameter tends to be comparable with the gravitino mass, which is two order of magnitude larger than the other soft masses. The latter problem is caused by the decay of the modulus whose branching ratio into the gravitino pair is sizable. In this paper, we propose a model of mirage mediation, in which Peccei-Quinn symmetry is incorporated. In this axionic mirage mediation, it is shown that the PQ symmetry breaking scale is dynamically determined around 10^{10-12} GeV due to the supersymmetry breaking effects, and the μ-problem can be solved naturally. Furthermore, in our model, the lightest supersymmetric particle (LSP) is the axino, that is the superpartner of the axion. The overabundance of the LSPs due to decays of modulus/gravitino, which is the most serious cosmological difficulty in the mirage mediation, can be avoided if the axino is sufficiently light. The next-LSPs (NLSPs) produced by the gravitino decay eventually decay into the axino LSPs, yielding the dominant component of the axinos remaining today. It is shown that the axino with the mass of O(100) MeV is naturally realized, which can constitute the dark matter of the Universe, with the free-streaming length of the order of 0.1 Mpc. The saxion, the real scalar component of the axion supermultiplet, can also be cosmologically harmless due to the dilution of the modulus decay. The lifetime of NLSP is relatively long, but much shorter than 1 sec., when the big-bang nucleosynthesis commences. The decay of NLSP would provide intriguing collider signatures.

hep-ph↗
arXiv2007

TeV Scale Mirage Mediation and Natural Little SUSY Hierarchy

TeV scale mirage mediation has been proposed as a supersymmetry breaking scheme reducing the fine tuning for electroweak symmetry breaking in the minimal supersymmetric extension of the standard model. We discuss a moduli stabilization set-up for TeV scale mirage mediation which allows an extra-dimensional interpretation for the origin of supersymmetry breaking and naturally gives an weak-scale size of the Higgs B-parameter. The set-up utilizes the holomorphic gauge kinetic functions depending on both the heavy dilaton and the light volume modulus whose axion partners are assumed to be periodic fields. We also examine the low energy phenomenology of TeV scale mirage mediation, particularly the constraints from electroweak symmetry breaking and FCNC processes.

hep-ph↗
arXiv2006

New Constraints on SUSY Flavour Mixing in Light of Recent Measurements at the Tevatron

We discuss the implications, for general flavour mixing, of the recent results regarding the new allowed range for mixing in the $B_s$ meson system, as well as the new improved bounds on $BR(\bar{B}_s\toμ^+μ^-)$. Constraints on right handed insertions, in particular, improve considerably in the large $\tanb$ regime where the contributions to $B_s$ mixing are dominated by double Higgs penguins. Similarly, the allowed regions of parameter space when varying multiple insertions also decrease significantly.

hep-ph↗
arXiv2005

Current and Future Limits on General Flavour Violation in b-->s Transitions in Minimal Supersymmetry

We discuss the current and prospective limits that can be placed on supersymmetric contributions to $b\to s$ transitions in the Minimal Supersymmetric Standard Model with general flavour mixing amongst the squarks. We consider three processes: $\bar{B}\to X_sγ$, $\bar{B}_sμ^+μ^-$ and $\bar{B}_s-B_s$ mixing, and pay particular attention to the large $\tanβ$ regime and beyond leading order contributions where the difference between our analysis and previous analyses is most pronounced. We find that even present limits on BR$(\bar{B}_sμ^+μ^-)$ and $Δm_{B_s}$ often provide additional constraints on the amount of flavour violation still allowed by BR$(\bar{B}\to X_sγ)$. Limits on supersymmetric and Higgs mass parameters can also be strongly dependent on the flavour violation. In particular, the case $μ<0$ can still be allowed, even for light sparticle masses. We discuss how future measurements at the Tevatron, the LHC and elsewhere can improve present limits but also provide unique signatures for the existence of flavour violation.

hep-ph↗
arXiv2005

Sparticle spectrum and EWSB of mixed modulus-anomaly mediation in fluxed string compactification models

We examine low energy sparticle mass-spectrum and electroweak symmetry breaking (EWSB) in the mixed modulus-anomaly mediation which is naturally realized in KKLT flux compactification. We find that the anomaly-mediation effectively lowers messenger scale in the moduli-mediation and leads to 'squeezed' mass-spectrum distinct from any known mediation mechanisms. The lightest neutralino typically becomes a mixed state of bino and higgsino or pure higgsino, which has a considerable impact on the cold dark matter physics.

hep-ph↗
arXiv2005

Little SUSY hierarchy in mixed modulus-anomaly mediation

Motivated by the KKLT string compactification involving a supersymmetry-breaking uplifting potential, we examine 4D effective supergravity with a generic form of uplifting potential, focusing on the possibility that the resulting mixed modulus-anomaly mediated soft terms realize the little hierarchy between the Higgs boson masses $m_H$ and the sparticle masses $m_{SUSY}$. It is noted that for some type of uplifting potential, the anomaly-mediated contribution to $m_H^2$ at $M_{GUT}$ can cancel the subsequent renormalization group evolution of $m_H^2$ down to TeV scale, thereby the model can naturally realize the little hierarchy $m_H^2\sim m_{SUSY}^2/8π^2$ which is desirable for the lightest Higgs boson mass to satisfy the experimental bound. In such models, the other Higgs mass parameters $μ$ and $B$ can have the desirable size $μ\sim B \sim m_H$ without severe fine-tuning of parameters, although the gravitino is much heavier than the Higgs boson. Those models for the little hierarchy avoid naturally the dangerous SUSY flavor and CP violations, and predict nearly degenerate low energy gaugino masses, pure Higgsino LSP, and also a specific relation between the stop and gaugino masses.

hep-ph↗
arXiv2005

Phenomenology of Mixed Modulus-Anomaly Mediation in Fluxed String Compactifications and Brane Models

In some string compactifications, for instance the recently proposed KKLT set-up, light moduli are stabilized by nonperturbative effects at supersymmetric AdS vacuum which is lifted to a dS vacuum by supersymmetry breaking uplifting potential. In such models, soft supersymmetry breaking terms are determined by a specific mixed modulus-anomaly mediation in which the two mediations typically give comparable contributions to soft parameters. Similar pattern of soft terms can arise also in brane models to stabilize the radion by nonperturbative effects. We examine some phenomenological consequences of this mixed modulus-anomaly mediation, including the pattern of low energy sparticle spectrum and the possibility of electroweak symmetry breaking. It is noted that adding the anomaly-mediated contributions at $M_{GUT}$ amounts to replacing the messenger scale of the modulus mediation by a mirage messenger scale $(m_{3/2}/M_{Pl})^{α/2}M_{GUT}$ where $α=m_{3/2}/[M_0\ln(M_{Pl}/m_{3/2})]$ for $M_0$ denoting the modulus-mediated contribution to the gaugino mass at $M_{GUT}$. The minimal KKLT set-up predicts $α=1$. As a consequence, for $α={\cal O}(1)$, the model can lead to a highly distinctive pattern of sparticle masses at TeV scale, particularly when $α= 2$.

hep-ph↗
arXiv2005

Probing the Flavour Structure of Supersymmetry Breaking With Rare B-Processes - A Beyond Leading Order Analysis

In the framework of minimal supersymmetry with general flavour mixing in the squark sector we consider dominant beyond leading order (BLO) effects in the rare processes $\bar{B}\to X_sγ$, $\bar{B}_sμ^+μ^-$ and $\bar{B}_s-B_s$ mixing. We present analytic expressions for corrected vertices, which are applicable in general, and provide a recipe for the inclusion of the dominant and subdominant BLO effects in existing LO calculations. We also derive similar expressions in the mass insertion approximation. We investigate in more detail the focusing effect pointed out in our earlier work, which, at large $\tanβ$ and $μ>0$, leads to a reduced supersymmetric contribution to the above processes. We also find that, in some cases, flavour dependence, that accidentally cancels at leading order, can reappear at BLO. We further include electroweak corrections, which, while generally subdominant, in some cases may have a substantial effect. For example, their contribution to the charged Higgs vertex in $\bar{B}\to X_sγ$ can be of the order of 20% at BLO. They can also reduce the contribution of LL insertions to $\bar{B}_s\toμ^+μ^-$ and $\bar{B}_s-B_s$ mixing by up to 20%, even at the LO. We also analyse radiative generation of CKM elements and find the possibility that the CKM matrix elements $K_{ts}$ and $K_{cb}$ can be generated entirely by LR insertions. This work constitutes the first complete analysis of dominant BLO effects in the GFM scenario.

hep-ph↗
arXiv2005

New Higgs Effects in B--Physics in Supersymmetry with General Flavour Mixing

We investigate the effect of general flavour mixing among squarks on the rare decays $\bar{B}\to X_sγ$, $\bar{B}_s\toμ^+μ^-$ and $\bar{B}_s-B_s$ mixing beyond the leading order in perturbation theory. We include all large $\tanβ$--enhanced corrections whilst also taking into account the effects of general flavour mixing on the uncorrected quark mass matrix and ${\rm SU}(2)_L\times{\rm U}(1)_Y$ breaking. For $\bar{B}_s\toμ^+μ^-$ and $\bar{B}_s-B_s$ mixing we find that, in analogy to $\bar{B}\to X_sγ$, there appears a focusing effect which can reduce the contribution due to the $δ_{RR}$ (and the $δ_{LL}$) insertion by up to a factor of two at large $\tanβ$ and $μ>0$. A dependence on $δ_{LR}$ and $δ_{RL}$, that otherwise cancels to first order in the mass insertion approximation, is also reintroduced. Taking into account the current experimental bounds on $ΔM_{B_s}$ and BR($\bar{B}_s\toμ^+μ^-$), we find that the insertions $δ_{RL}$ and $δ_{RR}$ can be significantly constrained compared to bounds obtained from $\bar{B}\to X_sγ$ only.

hep-ph↗