arXiv subjects

Teppei Kitahara

Publications and source records attributed to Teppei Kitahara.

71 records · Page 4Linked to original sources

arXiv2017

Protophobic Light Vector Boson as a Mediator to the Dark Sector

The observation of a protophobic 16.7 MeV vector boson has been reported by a $^8$Be nuclear transition experiment. Such a new particle could mediate between the Standard Model and a dark sector, which includes the dark matter. In this paper, we show some simple models of the dark matter which satisfy the thermal relic abundance under the current experimental bounds from the direct and the indirect detections. In a model, it is found that an appropriate self-scattering cross section to solve the small scale structure puzzles can be achieved.

hep-ph↗

arXiv2016

Direct CP violation in $K\to ππ$ decays and supersymmetry

The quantities $ε_K^\prime$ and $ε_K$ measure the amount of direct and indirect CP violation in $K\to ππ$ decays, respectively. Using the recent lattice results from the RBC and UKQCD Collaborations and a new compact implementation of the $ΔS=1$ renormalization group evolution we predict $ \mbox{Re}\, \frac{ε_{K}'}{ε_{K}} = \left(1.06 \pm 5.07 \right) \times 10^{-4}$ in the Standard Model. This value is $2.8\,σ$ below the experimental value of $ \mbox{Re}\, \frac{ε_{K}'}{ε_{K}} = \left(16.6 \pm 2.3 \right) \times 10^{-4}.$ In generic models of new physics the well-understood $ε_K$ precludes large contributions to $ε_K^\prime$, if the new contributions enter at loop level. However, one can resolve the tension in $ε_{K}'/ε_{K}$ within the Minimal Supersymmetric Standard Model. To this end two features of supersymmetry are crucial: First, one can have large isospin-breaking contributions (involving the strong instead of the weak interaction) which enhance $ε_K^\prime$. Second, the Majorana nature of gluinos permits a suppression of the MSSM contribution to $ε_K$, because two box diagrams interfere destructively.

hep-ph↗

arXiv2016

Singularity-free Next-to-leading Order $ΔS= 1$ Renormalization Group Evolution and $ε_{K}^{\prime}/ε_{K}$ in the Standard Model and Beyond

The standard analytic solution of the renormalization group (RG) evolution for the $ΔS = 1$ Wilson coefficients involves several singularities, which complicate analytic solutions. In this paper we derive a singularity-free solution of the next-to-leading order (NLO) RG equations, which greatly facilitates the calculation of $ε_K^{\prime}$, the measure of direct $CP$ violation in $K\to ππ$ decays. Using our new RG evolution and the latest lattice results for the hadronic matrix elements, we calculate the ratio $ε_{K}^{\prime}/ε_{K}$ (with $ε_{K}$ quantifying indirect $CP$ violation) in the Standard Model (SM) at NLO to $ε_{K}^{\prime}/ε_{K} = (1.06 \pm 5.07) \times 10^{-4} $, which is $2.8\,σ$ below the experimental value. We also present the evolution matrix in the high-energy regime for calculations of new physics contributions and derive easy-to-use approximate formulae. We find that the RG amplification of new-physics contributions to Wilson coefficients of the electroweak penguin operators is further enhanced by the NLO corrections: If the new contribution is generated at the scale of 1-10 TeV, the RG evolution between the new-physics scale and the electroweak scale enhances these coefficients by 50-100 %. Our solution contains a term of order $α_{EM}^2/α_s^2$, which is numerically unimportant for the SM case but should be included in studies of high-scale new-physics.

hep-ph↗

arXiv2016

Recent progress on CP violation in $K\to ππ$ decays in the SM and a supersymmetric solution

Using the recent first lattice results of the RBC-UKQCD collaboration for $K \to ππ$ decays, we perform a phenomenological analysis of $ε_K^{\prime}/ε_K$ and find a discrepancy between SM prediction and experiments by $\sim 3\,σ$. We discuss an explanation by new physics. The well-understood value of $ε_K$, which quantifies indirect $CP$ violation, however, typically prevents large new physics contributions to $ε_K^{\prime}/ε_K$. In this talk, we show a solution of the $ε_K^{\prime}/ε_K$ discrepancy in the Minimal Supersymmetric Standard Model with squark masses above 3 TeV without fine-tuning of $CP$ phases. In this solution, the Trojan penguin diagram gives large isospin-breaking contributions which enhance $ε_K^{\prime}$, while the contribution to $ε_K$ is suppressed thanks to the Majorana nature of gluinos.

hep-ph↗

arXiv2016

Supersymmetric Explanation of CP Violation in $K\to ππ$ Decays

Recent progress in the determination of hadronic matrix elements has revealed a tension between the measured value of $ε_K^{\prime}/ε_K$, which quantifies direct $CP$ violation in $K \to ππ$ decays, and the Standard-Model prediction. The well-understood indirect $CP$ violation encoded in the quantity $ε_K$ typically precludes large new-physics contributions to $ε_K^{\prime}/ε_K$ and challenges such an explanation of the discrepancy. We show that it is possible to cure the $ε_K^{\prime}/ε_K$ anomaly in the Minimal Supersymmetric Standard Model with squark masses above 3 TeV without overshooting $ε_K$. This solution exploits two features of supersymmetry: the possibility of large isospin-breaking contributions (enhancing $ε_K^{\prime}$) and the Majorana nature of gluinos (permitting a suppression of $ε_K$). Our solution involves no fine-tuning of $CP$ phases or other parameters.

hep-ph↗

arXiv2016

Gauge invariant Barr-Zee type contributions to fermionic EDMs in the two-Higgs doublet models

We calculate all gauge invariant Barr-Zee type contributions to fermionic electric dipole moments (EDMs) in the two-Higgs doublet models (2HDM) with softly broken Z2 symmetry. We start by studying the tensor structure of h to VV' part in the Barr-Zee diagrams, and we calculate the effective couplings in a gauge invariant way by using the pinch technique. Then we calculate all Barr-Zee diagrams relevant for electron and neutron EDMs. We make bounds on the parameter space in type-I, type-II, type-X, and type-Y 2HDMs. The electron and neutron EDMs are complementary to each other in discrimination of the 2HDMs. Type-II and type-X 2HDMs are strongly constrained by recent ACME experiment's result, and future experiments of electron and neutron EDMs may search O(10) TeV physics.

hep-ph↗

arXiv2016

Prospects for Spin-1 Resonance Search at 13 TeV LHC and the ATLAS Diboson Excess

Motivated by ATLAS diboson excess around 2 TeV, we investigate a phenomenology of spin-1 resonances in a model where electroweak sector in the SM is weakly coupled to strong dynamics. The spin-1 resonances, W' and Z', are introduced as effective degrees of freedom of the dynamical sector. We explore several theoretical constraints by investigating the scalar potential of the model as well as the current bounds from the LHC and precision measurements. It is found that the main decay modes are V' -> VV and V' -> Vh, and the V' width is narrow enough so that the ATLAS diboson excess can be explained. In order to investigate future prospects, we also perform collider simulations at the 13 TeV LHC, and obtain a model independent expected exclusion limit for the process pp -> W' -> WZ -> JJ. We find a parameter space where the diboson excess can be explained, and are within a reach of the LHC at the integrated luminosity of 10 fb-1 and 13 TeV.

hep-ph↗

arXiv2015

ATLAS on-Z Excess via gluino-Higgsino-singlino decay chains in the NMSSM

Recently the ATLAS experiment has reported 3.0 sigma excess in an on-Z signal region in searches for supersymmetric particles. We find that the next-to-minimal supersymmetric standard model can explain this excess by the production of gluinos which mainly decay via $\tilde{g} \to g \tildeχ^0_{2,3} \to g Z \tildeχ^0_{1}$ where $\tildeχ^0_{2,3}$ and $\tildeχ^0_1$ are the Higgsino and the singlino-like neutralinos, respectively. We show that the observed dark matter density is explained by the thermal relic density of the singlino-like neutralino, simultaneously. We also discuss the searches for the Higgs sector of this scenario at the Large Hadron Collider.

hep-ph↗

arXiv2015

Aspects of High-Scale Supersymmetry in a Singlet-Extended Model

The nearly Minimal Supersymmetric Standard Model (nMSSM) is one of the promising models of the new physics, since this model can avoid hierarchy problem, mu problem, cosmological domain wall problem, and tadpole problem simultaneously. In this thesis, we consider the phenomenology of the nMSSM. Especially, we focus on the phenomenology of the dark matter and the baryon asymmetry in the universe generated by the electroweak baryogenesis mechanism. We find that with high-scale supersymmetry breaking the singlino can obtain a sizable radiative correction to the singlino mass, which opens a window for the singlet dark matter scenario with resonant annihilation via the exchange of the Higgs boson. We also propose a new electroweak baryogenesis scenario in the nMSSM with additional vector-like multiplets. If the soft supersymmetry breaking scale is O(10) TeV, these scenarios are compatible with each other and an observed mass of the Higgs boson, constraints by the electric dipole moments measurements and the flavor experiments. As a result of these two studies, we conclude that the nMSSM with a high-scale supersymmetry breaking is valid and can be probed by the direct direction of the singlino dark matter.

hep-ph↗

arXiv2015

Diquark bound states with a completely crossed ladder truncation

The Bethe-Salpeter equation in the diquark channel is investigated by employing the Dyson-Schwinger method together with the Munczek-Nemirovsky model. The novelty of our study is a resummation of completely crossed ladder diagrams in the Bethe-Salpeter kernel. These diagrams are enhanced due to their color factors in the diquark channel, but not in the meson channel. In our analysis, diquark bound-state solutions exist in the Bethe-Salpeter equation.

nucl-th↗

arXiv2015

Towards a Scale Free Electroweak Baryogenesis

We propose a new electroweak baryogenesis scenario in high-scale supersymmetric (SUSY) models. We consider a singlet extension of the minimal SUSY standard model introducing additional vector-like multiplets. We show that the strongly first-order phase transition can occur at a high temperature comparable to the soft SUSY breaking scale. In addition, the proper amount of the baryon asymmetry of the universe can be generated via the lepton number violating process in the vector-like multiplet sector. The typical scale of our scenario, the soft SUSY breaking scale, can be any value. Thus our new electroweak baryogenesis scenario can be realized at arbitrary scales and we call this scenario as a scale free electroweak baryogenesis. This soft SUSY breaking scale is determined by other requirements. If the soft SUSY breaking scale is O(10) TeV, our scenario is compatible with the observed mass of the Higgs boson and the constraints by the electric dipole moments measurements and the flavor experiments. Furthermore, the singlino can be a good candidate of the dark matter.

hep-ph↗

arXiv2014

Singlino Resonant Dark Matter and 125 GeV Higgs Boson in High-Scale Supersymmetry

We consider a singlino Dark Matter (DM) scenario in a singlet extension model of the Minimal Supersymmetric Standard Model, which is so-called the Nearly MSSM (nMSSM). We find that with high-scale supersymmetry breaking the singlino can obtain a sizable radiative correction to the mass, which opens a window for the DM scenario with resonant annihilation via the exchange of the Higgs boson. We show that the current DM relic abundance and the Higgs boson mass can be explained simultaneously. This scenario can be fully probed by XENON1T.

hep-ph↗

arXiv2014

Future Prospects for Stau in Higgs Coupling to Di-photon

We study future prospects of the stau which contributes to the Higgs coupling to di-photon. The coupling is sensitive to new physics and planned to be measured at percent levels in future colliders. We show that, if the excess of the coupling is measured to be larger than 4 %, the lightest stau is predicted to be lighter than about 200 GeV by taking vacuum meta-stability conditions into account. Such a stau can be discovered at ILC. Moreover, we show how accurately the stau contribution to the coupling can be reconstructed from the information that is available at ILC. We also argue that, if the stau mixing angle is measured, the mass of the heaviest stau can be predicted by measuring the Higgs coupling, even when the heaviest stau is not yet discovered at the early stage of ILC.

hep-ph↗

arXiv2013

Reconstructing Supersymmetric Contribution to Muon Anomalous Magnetic Dipole Moment at ILC

We study the possibility to determine the supersymmetric (SUSY) contribution to the muon anomalous magnetic dipole moment by using ILC measurements of the properties of superparticles. Assuming that the contribution is as large as the current discrepancy between the result of the Brookhaven E821 experiment and the standard-model prediction, we discuss how and how accurately the SUSY contribution can be reconstructed. We will show that, in a sample point, the reconstruction can be performed with the accuracy of ~ 13 % with the center-of-mass energy 500 GeV and the integrated luminosity ~ 500-1000 fb-1.

hep-ph↗

arXiv2013

Probing Bino Contribution to Muon g-2

We study SUSY models in which Bino contributions solve the muon g-2 anomaly. The contributions are enhanced by large left-right mixing of the smuons. However, it is constrained by the vacuum stability condition of the slepton--Higgs potential. Therefore, there are upper bounds on masses of sleptons and Bino. When the slepton soft masses are universal, the upper bound on the smuon mass becomes 330 (460)GeV in order to solve the g-2 anomaly at the 1 sigma (2 sigma) level. It is within the reach of LHC and ILC. If the stau is heavier than the smuon, the bound can be as large as 1.4 (1.9)TeV. Such non-universal slepton mass spectrum generically predicts too large LFV/CPV. We show that the models are expected to be probed by LHC/ILC and LFV/CPV complementarily in future.

hep-ph↗

arXiv2013

Stau with Large Mass Difference and Enhancement of the Higgs to Diphoton Decay Rate in the MSSM

The ATLAS and the CMS collaborations have presented results which show an excess of the Higgs to diphoton decay channel. In the Minimal Supersymmetric Standard Model (MSSM), this situation can be achieved by a light stau and a large left-right mixing of the staus. However, this parameter region is severely constrained by vacuum stability. In order to relax the vacuum meta-stability condition, we focus on the parameter region where the mass difference between the two staus is large. This region has not been considered yet. In this paper, we show that staus with a large mass difference can relax the vacuum meta-stability condition sufficiently even if the lighter stau mass is kept light. We find that when the mass difference of two staus is large, the enhancement of the Higgs to diphoton decay rate becomes small in spite of a relaxation of the vacuum meta-stability condition. Because of this feature, an O(70)% enhancement of the Higgs to diphoton decay rate is difficult to achieve in the light stau scenario in the MSSM.

hep-ph↗

arXiv2012

Vacuum Stability Constraints on the Enhancement of the Higgs to Diphoton Rate in the MSSM

The ATLAS and CMS collaborations discovered a new boson particle. If the new boson is the Higgs boson, the diphoton signal strength is 1.5 - 1.8 times larger than the Standard Model (SM) prediction, while the WW and ZZ signal strengths are in agreement with the SM one. In the Minimal Supersymmetric Standard Model (MSSM), overall consistency can be achieved by a light stau and the large left-right mixing of staus. However, a light stau and large left-right mixing of staus may suffer from vacuum instability. We first apply the vacuum meta-stability condition to the Higgs to diphoton decay rate in the MSSM. We show that the vacuum meta-stablity severely constrains the enhancement to the Higgs to diphoton rate. For example, when the lighter stau mass is 100 GeV, the upper bound on the enhancement to the Higgs to diphoton rate becomes 25%.

hep-ph↗