arXiv subjects

Nobuchika Okada

Publications and source records attributed to Nobuchika Okada.

At least 235 records · Page 13Linked to original sources

arXiv2008

Gravitino dark matter from increased thermal relic particles

We investigate the so-called superWIMP scenario with gravitino as the lightest supersymmetric particle (LSP) in the context of non-standard cosmology, in particular, brane world cosmology. As a candidate of the next-to-LSP (NLSP), we examine slepton and sneutrino. Brane world cosmological effects dramatically enhance the relic density of the slepton or sneutrino NLSP, so that the NLSP with mass of order 100 GeV can provide the correct abundance of gravitino dark matter through its decay. We find that with an appropriate five dimensional Planck mass, this scenario can be realized consistently with the constraints from Big Bang Nucleosynthesis (BBN) for both NLSP candidates of slepton and sneutrino. The BBN constraints for slepton NLSP are more stringent than that for sneutrino, as the result, the gravitino must be rather warm in the slepton NLSP case. The energy density of gravitino produced by thermal scattering is highly suppressed and negligible due to the brane world cosmological effects.

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arXiv2008

A simple SO(10) GUT in five dimensions

A simple supersymmetric SO(10) GUT in five dimensions is considered. The fifth dimension is compactified on the $S^1/(Z_2\times Z_2^\prime)$ orbifold possessing two inequivalent fixed points. In our setup, all matter and Higgs multiplets reside on one brane (PS brane) where the original SO(10) gauge group is broken down to the Pati-Salam (PS) gauge group, SU(4)$_c \timesSU(2)_L \times$ SU(2)$_R$, by the orbifold boundary condition, while only the SO(10) gauge multiplet resides in the bulk. The further breaking of the PS symmetry to the Standard Model gauge group is realized by Higgs multiplets on the PS brane as usual in four dimensional models. Proton decay is fully suppressed. In our simple setup, the gauge coupling unification is realized after incorporating threshold corrections of Kaluza-Klein modes. When supersymmetry is assumed to be broken on the other brane, supersymmetry breaking is transmitted to the PS brane through the gaugino mediation with the bulk gauge multiplet.

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arXiv2008

Unparticle Dark Matter

Once a parity is introduced in unparticle physics, under which unparticle provided in a hidden conformal sector is odd while all Standard Model particles are even, unparticle can be a suitable candidate for the cold dark matter (CDM) in the present universe through its coupling to the Standard Model Higgs doublet. We find that for Higgs boson mass in the range, 114.4 GeV < m_h < 250 GeV, the relic abundance of unparticle with mass 50 GeV < m_U < 80 GeV can be consistent with the currently observed CDM density. In this scenario, Higgs boson with mass m_h < 160 GeV dominantly decays into a pair of unparticles and such an invisible Higgs boson may be discovered in future collider experiments.

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arXiv2008

Higgs Mass Bounds, Type II SeeSaw and LHC

In type II seesaw utilized to explain the observed neutrino masses and mixings, one extends the Standard Model (SM) by introducing scalar fields which transform as a triplet under the electroweak gauge symmetry. New scalar couplings involving the Higgs doublet then appear and, as we show, these have important implications for the Higgs boson mass bounds obtained using vacuum stability and perturbativity arguments. We identify, in particular, regions of the parameter space which permit the SM Higgs boson to be as light as 114.4 GeV, the LEP2 bound. The triplet scalars include doubly charged particles whose masses could, in principle, be in the few hundred GeV range, and so they may be accessible at the LHC.

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arXiv2008

Higgs Boson Mass Bounds in the Standard Model with Type III and Type I Seesaw

In type III seesaw utilized to explain the observed solar and atmospheric neutrino oscillations the Standard Model (SM) particle spectrum is extended by introducing three SU(2)_L triplet fermion fields. This can have important implications for the SM Higgs boson mass (M_H) bounds based on vacuum stability and perturbativity arguments. We compute the appropriate renormalization group equations for type III seesaw, and then proceed to identify regions of the parameter space such that the SM Higgs boson mass window is enlarged to 125 GeV < M_H < 174 GeV, with the type III seesaw scale close to TeV. We also display regions of the parameter space for which the vacuum stability and perturbativity bounds merge together for large neutrino Yukawa couplings. Comparison with type I seesaw is also presented.

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arXiv2008

Unparticle physics at the photon collider

Recently, a conceptually new physics beyond the Standard Model (SM), unparticle, has been proposed, where a hidden conformal sector is coupled to the SM sector through higher dimensional operators. In this setup, we investigate unparticle physics at the photon collider, in particular, unparticle effects on the gamma gamma to gamma gamma process. Since this process occurs at loop level in the SM, the unparticle effects can be significant even if the cutoff scale is very high. In fact, we find that the unparticle effects cause sizable deviations from the SM results. The scaling dimension of the unparticle d_U reflects the dependence of the cross section on the final state photon invariant mass, so that precision measurements of this dependence may reveal the scaling dimension of the unparticle.

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arXiv2008

Dynamical Supersymmetry Breaking from Meta-stable Vacua in an N=1 Supersymmetric Gauge Theory

We investigate supersymmetry breaking meta-stable vacua in N=2, SU(2)\times U(1) gauge theory with N_f=2 massless flavors perturbed by the addition of small N=1 preserving mass terms in a presence of a Fayet-Iliopoulos term. We derive the low energy effective theory by using the exact results of N=2 supersymmetric QCD and examine the effective potential. At the classical level, the theory has supersymmetric vacua on Coulomb and Higgs branches. We find that supersymmetry on the Coulomb branch is dynamically broken as a consequence of the strong dynamics of SU(2) gauge symmetry while the supersymmetric vacuum on the Higgs branch remains. We also estimate the lifetimes of the local minima on the Coulomb branch. We find that they are sufficiently long and therefore the local vacua we find are meta-stable.

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arXiv2008

Effective Potential of Higgs Field in Warped Gauge-Higgs Unification

The gauge-Higgs unification is one of influential scenarios to solve the hierarchy problem in the Standard Model. Recently, the scenario on the warped background attracts many attentions due to a large possibility to construct a realistic model naturally in this framework. It is, however, well known that the effective potential for the Higgs field, which is the most important prediction of the scenario, is not easy to calculate on the warped background, because masses of Kaluza-Klein particles are not obtained analytically. In this article, we derive useful formulae for the effective potential. The formulae allow us to calculate the Higgs mass easily, thus to construct a realistic model in the gauge-Higgs unification scenario on the warped background. Using obtained formulae, we calculate contributions from bulk fermions with several boundary conditions. We also show bulk fermions, which have boundary conditions not allowed in the orbifold picture, do not contribute to the effective potential.

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arXiv2008

New contribution to dimension five operators on proton decay in anomaly mediation scenario

In supergravity, effective superpotential relevant to dimension five operators on proton decay processes also leads to supersymmetry breaking terms among sfermions, dimension four operators. These dimension four operators induce the dimension five operators through 1-loop diagrams dressed by gauginos. We find that, in a class of models with the anomaly mediation, the 1-loop contributions can be comparable to those at the tree level. Therefore, such operators have a great impact on proton decay rate. Depending on a universal phase of gaugino masses and soft mass spectrum, the proton decay rate can be enhanced or suppressed.

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arXiv2008

Unparticle physics and Higgs phenomenology

Recently, conceptually new physics beyond the Standard Model has been proposed, where a hidden conformal sector provides ``unparticle'' which couples to the Standard Model sector through higher dimensional operators in low energy effective theory. Among several possibilities, we focus on operators involving unparticle, the Higgs doublet and the gauge bosons. Once the Higgs doublet develops the vacuum expectation value, the conformal symmetry is broken and as a result, the mixing between unparticle and Higgs boson emerges. We find that this mixing can cause sizable shifts for the couplings between Higgs boson and a pair of gluons and photons, because these couplings exist only at the loop level in the Standard Model. These Higgs couplings are the most important ones for the Higgs boson search at the CERN Large Hadron Collider, and the unparticle physics effects may be observed together with the discovery of Higgs boson.

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arXiv2008

Gauge-Higgs Unification at LHC

Higgs boson production by the gluon fusion and its decay into two photons at the LHC are investigated in the context of the gauge-Higgs unification scenario. The qualitative behaviors for these processes in the gauge-Higgs unification are quite distinguishable from those of the Standard Model and the universal extra dimension scenario because of the overall sign difference for the effective couplings induced by one-loop corrections through Kaluza-Klein (KK) modes. For the KK mode mass smaller than 1 TeV, the Higgs productions cross section and its branching ratio into two photons are sizably deviated from those in the Standard Model. Associated with the discovery of Higgs boson, this deviation may be measured at the LHC.

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arXiv2008

A 3 X 2 texture for neutrino oscillations and leptogenesis

In an economical system with only two heavy right handed neutrinos, we postulate a new texture for $3 \times 2$ Dirac mass matrix $m_D$. This model implies one massless light neutrino and thus displays only two patterns of mass spectrum for light neutrinos, namely hierarchical or inverse-hierarchical. Both the cases can correctly reproduce all the current neutrino oscillation data with a unique prediction $m_{ν_e ν_e} = \frac{\sqrt{Δm^2_{solar}}}{3}$ and $\sqrt{Δm^2_{atm}}$ for the hierarchical and the inverse-hierarchica cases, respectively, which can be tested in next generation neutrino-less double beta decay experiments. Introducing a single physical CP phase in $m_D$, we examine baryon asymmetry through leptogenesis. Interestingly, through the CP phase there are correlations between the amount of baryon asymmetry and neutrino oscillation parameters. We find that for a fixed CP phase, the hierarchical case also succeeds in generating the observed baryon asymmetry in our universe, plus a non-vanishing $U_{e3}$ which is accessible in future baseline neutrino oscillation experiments.

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arXiv2007

Meta-stable Vacuum in Spontaneously Broken N=2 Supersymmetric Gauge Theory

We consider an N=2 supersymmetric SU(2) \times U(1) gauge theory with N_f=2 massless flavors and a Fayet-Iliopoulos (FI) term. In the presence of the FI term, supersymmetry is spontaneously broken at tree level (on the Coulomb branch), leaving a pseudo-flat direction in the classical potential. This vacuum degeneracy is removed once quantum corrections are taken into account. Due to the SU(2) gauge dynamics, the effective potential exhibits a local minimum at the dyon point, where not only supersymmetry but also U(1)_R symmetry is broken, while a supersymmetric vacuum would be realized toward infinity with the runaway behavior of the potential. This local minimum is found to be parametrically long-lived. Interestingly, from a phenomenological point of view, in this meta-stable vacuum the massive hypermultiplets inherent in the theory play the role of the messenger fields in the gauge mediation scenario, when the Standard Model gauge group is embedded into their flavor symmetry.

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arXiv2007

Supersymmetry Breaking by Type II Seesaw Assisted Anomaly Mediation

Anomaly mediated supersymmetry breaking (AMSB), when implemented in MSSM is known to suffer from the problem of negative slepton mass squared leading to breakdown of electric charge conservation. We show however that when MSSM is extended to explain small neutrino masses by including a pair of superheavy Higgs triplet superfields (the type II seesaw mechanism), the slepton masses can be deflected from the pure AMSB trajectory and become positive. In a simple model we present in this paper, the seesaw scale is about $10^{13}-10^{14}{\rm GeV}$. Gauge coupling unification can be maintained by embedding the triplet to SU(5) {\bf 15}-multiplet. In this scenario, bino is the LSP and its mass is nearly degenerate with NLSP slepton when the triplet mass is right around the seesaw scale.

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arXiv2007

Diquark Higgs at LHC

Existence of color sextet diquark Higgs fields with TeV masses will indicate a fundamentally different direction for unification than conventional grand unified theories. There is a class of partial unification models based on the gauge group $SU(2)_L\times SU(2)_R\times SU(4)_c$ that implement the seesaw mechanism for neutrino mass with seesaw scale around $10^{11}$ GeV, where indeed such light fields appear naturally despite the high gauge symmetry breaking scale. They couple only to up-type quarks in this model. We discuss phenomenological constraints on these fields and show that they could be detected at LHC via their decay to either $tt$ or single top + jet. We also find that existing Tevatron data gives a lower bound on its mass somewhere in the 400-500 GeV, for reasonable values of its coupling.

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arXiv2007

Unparticle Physics And Gauge Coupling Unification

Unparticle physics from a hidden conformal sector can alter the evolution of the Standard Model (SM) gauge couplings via TeV scale threshold corrections. We discuss how this may lead to gauge coupling unification at M_{GUT}\approx 2 \times 10^{15} GeV -- 5 \times 10^{17} GeV without introducing new particles in the SM sector.

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arXiv2007

Higgs Boson Mass From Gauge-Higgs Unification

In certain five dimensional gauge theories the Standard Model Higgs doublet is identified, after compactification on the orbifold S^1/Z_2, with the zero mode of the fifth component of the gauge field. An effective potential for the Higgs field is generated via quantum corrections, triggered by the breaking of the underlying gauge symmetry through boundary conditions. The quartic Higgs coupling can be estimated at low energies by employing the boundary condition that it vanishes at the compactification scale Λ, as required by five dimensional gauge invariance. For Λ\gtrsim 10^{13}-10^{14} GeV, the Standard Model Higgs boson mass is found to be m_H = 125 \pm 4 GeV corresponding to a top quark pole mass M_t = 170.9 \pm 1.8 GeV. A more complete (gauge-Higgs-Yukawa) unification can be realized for Λ~ 10^8 GeV, which happens to be the scale at which the SU(2) weak coupling and the top quark Yukawa coupling have the same value. For this case, m_H = 117\pm 4 GeV.

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arXiv2007

Window For Higgs Boson Mass From Gauge-Higgs Unification

We consider six dimensional gauge models compactified on the orbifold T^2/Z_N (N=2,3,4,6) such that the Standard Model (SM) Higgs doublet arises from the extra-dimensional components of the gauge field. For Λ\leq 10^{19} GeV, where Λdenotes the compactification scale, we obtain 114.4 GeV \leq m_H \leq 164 GeV for the SM Higgs boson mass. We also consider gauge-Higgs-top and gauge-Higgs-bottom Yukawa unification which respectively yield m_H = 131^{+4}_{-5} GeV and m_H = 150^{+2}_{-2} GeV for a top quark pole mass M_t =170.9^{+1.8}_{-1.8} GeV. As a special case we recover the result m_H \leq 132 GeV previously obtained for five dimensional models.

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