arXiv subjects

Shohei Okawa

Publications and source records attributed to Shohei Okawa.

29 records · Page 2Linked to original sources

arXiv2021

TeV-scale vector leptoquark from Pati-Salam unification with vectorlike families

In this paper, we show an explicit way to realize a TeV-scale vector leptoquark from the Pati-Salam (PS) unification with extra vectorlike families. The leptoquark mass is constrained to be heavier than PeV-scale by the measurement of a flavor violating kaon decay, $K_L \to μe$, in conventional models. This strong constraint can be avoided by introducing vectorlike families consistently with the quark and lepton masses and CKM and PMNS matrices. The other flavor violating processes are also suppressed. In this model, the vector leptoquark can be sufficiently light to explain the recent $b\to sμμ$ anomaly, while the $b\to cτν$ anomaly is difficult to be explained due to the strong constraints from the $Z^\prime$ boson and vectorlike quark searches at the LHC. When the $b\to sμμ$ anomaly is explained, we show that $\mathcal{O}({0.2})$ % tuning is required in the fermion matrix, the future experiments in $μ\to eγ$ and $μ$-$e$ conversions will cover the most available parameter space, and sizable neutral meson mixings, induced by the extra Higgs doublets, are unavoidable.

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arXiv2021

Search for U(1)$_{L_μ-L_τ}$ charged Dark Matter with neutrino telescope

We study a simple Dirac fermion dark matter model in U(1)$_{L_μ-L_τ}$ theory. The new light gauge boson $X$ plays important roles in both dark matter physics and the explanation for the muon $g-2$ anomaly. The observed dark matter relic density is realized by a large U(1)$_{L_μ-L_τ}$ charge without introducing a resonance effect of the $X$ boson. As a by-product of the model, characteristic neutrino signatures from sub-GeV dark matter $ψ$ are predicted depending on the mass spectrum. We formulate the analysis of $ψ\barψ\to ν\barν$, and of $ψ\barψ\to XX$ followed by $X\toν\barν$, in a model independent way. The energy spectrum of neutrinos in the former process is monochromatic while in the latter process is bowl-shape. We also evaluate sensitivity at Super-Kamiokande and future Hyper-Kamiokande detectors. The analysis is finally applied to the U(1)$_{L_μ-L_τ}$ dark matter model.

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arXiv2020

Current status and muon $g-2$ explanation of lepton portal dark matter

In this paper, we summarize phenomenology in lepton portal dark matter (DM) models, where DM couples to leptons and extra leptons/sleptons. There are several possible setups: complex/real scalar DM and Dirac/Majorana fermion DM. In addition, there are choices for the lepton chirality that couples to DM. We discuss the prediction of each model and compare it with the latest experimental constraints from the DM, the LHC, and the flavor experiments. We also propose a simple setup to achieve the discrepancy in the anomalous magnetic moment of muon.

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arXiv2019

Electric Dipole Moments From Dark Sectors

We examine the sensitivity of electric dipole moments (EDMs) to new $CP$-violating physics in a hidden (or dark) sector, neutral under the Standard Model (SM) gauge groups, and coupled via renormalizable portals. In the absence of weak sector interactions, we show that the electron EDM can be induced purely through the gauge kinetic mixing portal, but requires five loops, and four powers of the kinetic mixing parameter $ε$. Allowing weak interactions, and incorporating the Higgs and neutrino portals, we show that the leading contributions to $d_e$ arise at two-loop order, with the main source of $CP$-violating being in the interaction of dark Higgs and heavy singlet neutrinos. In such models, EDMs can provide new sensitivity to portal couplings that is complementary to direct probes at the intensity frontier or high energy colliders.

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arXiv2019

WIMP dark matter in the parity solution to the strong CP problem

We extend the Standard Model (SM) with parity symmetry, motivated by the strong CP problem and dark matter. In our model, parity symmetry is conserved at high energy by introducing a mirror sector with the extra gauge symmetry, $ SU(2)_R \times U(1)_R$. The charges of $SU(2)_R \times U(1)_R$ are assigned to the mirror fields in the same way as in the SM, but the chiralities of the mirror fermions are opposite to respect the parity symmetry. The strong CP problem is resolved, since the mirror quarks are also charged under the $SU(3)_c$ in the SM. In the minimal setup, the mirror gauge symmetry leads to stable colored particles which would be inconsistent with the observed data, so that we introduce two scalars in order to deplete the stable colored particles. Interestingly, one of the scalars becomes stable because of the gauge symmetry and therefore can be a good dark matter candidate. We especially study the phenomenology relevant to the dark matter, i.e. thermal relic density, direct and indirect searches for the dark matter. The bounds from the LHC experiment and the Landau pole are also taken into account. As a result, we find that a limited region is viable: the mirror up quark mass is around [600\,GeV, 3\,TeV] and the relative mass difference between the dark matter and the mirror up quark or electron is about ${\cal O}$(1-10\,\%). We also discuss the neutrino sector and show that the right-handed neutrinos in the mirror sector can increase the effective number of neutrinos or dark radiation by $0.14$.

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arXiv2017

Impact of the $b \to s ll$ anomalies on dark matter physics

Recently, the LHCb collaboration has reported the excesses in the $b \to s ll$ processes. One of the promising candidates for new physics to explain the anomalies is the extended Standard Model (SM) with vector-like quarks and leptons. In that model, Yukawa couplings between the extra fermions and SM fermions are introduced, adding extra scalars. Then, the box diagrams involving the extra fields achieve the $b \to s ll$ anomalies. It has been known that the excesses require the large Yukawa couplings of leptons, so that this kind of model can be tested by studying correlations with other observables. In this paper, we consider the extra scalar to be a dark matter (DM) candidate, and investigate DM physics as well as the flavor physics and the LHC physics. The DM relic density and the direct-detection cross section are also dominantly given by the Yukawa couplings, so that we find some explicit correlations between DM physics and the flavor physics. In particular, we find the predictions of the $b \to s ll$ anomalies against the direct detection of DM.

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arXiv2017

Hidden sector behind the CKM matrix

The small quark mixing, described by the Cabibbo-Kobayashi-Maskawa (CKM) matrix in the Standard Model, may be a clue to reveal new physics around the TeV scale. We consider a simple scenario that extra particles in a hidden sector radiatively mediate the flavor violation to the quark sector around the TeV scale and effectively realize the observed CKM matrix. The lightest particle in the hidden sector, whose contribution to the CKM matrix is expected to be dominant, is a good dark matter (DM) candidate, so we focus on the contribution, and discuss the DM physics. In this scenario, there is an explicit relation between the CKM matrix and flavor violating couplings, such as four-quark couplings, because both are radiatively induced by the particles in the hidden sector. Then, we can explicitly find the DM mass region and the size of Yukawa couplings between the DM and quarks, based on the study of flavor physics and DM physics. In conclusion, we show that DM mass in our scenario is around the TeV scale, and the Yukawa couplings are between ${\cal O}(0.01)$ and ${\cal O}(1)$. The spin-independent DM scattering cross section is estimated as ${\cal O}(10^{-9})$ [pb]. An extra colored particle is also predicted at the ${\cal O}(10)$ TeV scale.

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arXiv2017

Scalegenesis via dynamically induced multiple seesaws

We propose a model which accounts for the dynamical origin of the electroweak symmetry breaking (EWSB), directly linking to the mass generation of dark matter (DM) candidate and active neutrinos. The standard model (SM) is weakly charged under the $U(1)_{\rm B-L}$ gauge symmetry, in conjunction with newly introduced three right-handed Majorana neutrinos and the $U(1)_{\rm B-L}$ Higgs. The model is built on the classical scale invariance, that is dynamically broken by a new strongly coupled sector, what is called the hypercolor (HC) sector, which is also weakly coupled to the $U(1)_{\rm B-L}$ gauge. At the HC strong scale, the simultaneous breaking of the EW and $U(1)_{\rm B-L}$ gauge symmetries is triggered by dynamically induced multiple seesaw mechanisms, namely bosonic seesaw mechanisms. Thus, all the origins of masses are provided singly by the HC dynamics: that is what we call the dynamical scalegenesis. We also find that an HC baryon, with mass on the order of a few TeV, can be stabilized by the HC baryon number and the $U(1)_{\rm B-L}$ charge, so identified as a DM candidate. The relic abundance of the HC-baryon DM can be produced dominantly via the bosonic-seesaw portal process, and the HC-baryon DM can be measured through the large magnetic moment coupling generated from the HC dynamics, or the $U(1)_{\rm B-L}$-gauge boson portal in direct detection experiments.

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arXiv2017

Dark matter physics, flavor physics and LHC constraints in the dark matter model with a bottom partner

In the scenario that a dark matter (DM) is a weakly interacting massive particle, there are many possibilities of the interactions with the Standard Model (SM) particles to achieve the relic density of DM. In this paper, we consider one simple DM model where the DM candidate is a complex scalar and interacts with the SM particles via exchange of the Higgs particle and an extra quark, named bottom partner. The extra quark carries the same quantum number as the right-handed down-type quarks and has Yukawa couplings with the DM candidate and the right-handed down-type quarks. The Yukawa interactions are not only relevant to the thermal relic density of the DM, but also contribute to the flavor physics, such as the $ΔF=2$ processes. In addition, the flavor alignment of the Yukawa couplings is related to the decay modes of the extra quark. Then, we can find the explicit correlations among the physical observables in DM physics, flavor physics and the signals at the LHC. Based on the numerical analyses of the thermal relic density, the direct detection of the DM and the current LHC bounds using the latest results, we survey our predictions for the $ΔF=2$ processes. We investigate the perturbative bound on the Yukawa coupling, as well. Study of a fermionic DM model with extra scalar quarks is also given for comparison.

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arXiv2016

Relic Abundance in a Secluded Dark Matter Scenario with a Massive Mediator

The relic abundance of the dark matter (DM) particle $d$ is studied in a secluded DM scenario, in which the $d$ number decreasing process dominantly occurs not through the pair annihilation of $d$ into the standard model particles, but via the $dd \to mm$ scattering process with a subsequently decaying mediator particle $m$. It is pointed out that the cosmologically observed relic abundance of DM can be accomplished even with a massive mediator having a mass $m_m$ non-negligibly heavy compared with the DM particle mass $m_d$. In the degenerated $d$-$m$ case ($m_d=m_m$), the DM relic abundance is realized by adjusting the $dd \to mm$ scattering amplitude large enough and by choosing an appropriate mediator particle life-time. The DM evolution in the early universe exhibits characteristic "terrace" behavior, or two-step number density decreasing behavior, having a "fake" freeze-out at the first step. Based on these observations, a novel possibility of the DM model buildings is introduced in which the mediator particle $m$ is unified with the DM particle $d$ in an approximate dark symmetry multiplet. A pionic DM model is proposed to illustrate this idea in a renormalizable field theory framework.

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arXiv2015

Unitarity sum rules, three site moose model, and the ATLAS 2 TeV diboson anomalies

We investigate $W'$ interpretations for the ATLAS 2 TeV diboson anomalies. The roles of the unitarity sum rules, which ensure the perturbativity of the longitudinal vector boson scattering amplitudes, are emphasized. We find the unitarity sum rules and the custodial symmetry are powerful enough to predict various nontrivial relations among $WWZ'$, $WZW'$, $WWh$, $WW'h$ and $ZZ'h$ coupling strengths in a model independent manner. We also perform surveys in the general parameter space of $W'$ models and find the ATLAS 2 TeV diboson anomalies may be interpreted as a $W'$ particle of the three site moose model, i.e., a Kaluza-Klein like particle in a deconstructed extra dimension model. It is also shown that the non standard-model-like Higgs boson is favored by the present data to interpret the ATLAS diboson anomalies as the consequences of the $W'$ and $Z'$ bosons.

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