arXiv subjects

Gi-Chol Cho

Publications and source records attributed to Gi-Chol Cho.

53 records · Page 3Linked to original sources

arXiv2002

The stau exchange contribution to muon g-2 in the decoupling solution

We study the possibility that the lepton-flavor changing process can induce the suitable magnitude of the muon anomalous magnetic moment (g_μ-2) in the decoupling solution to the flavor problem in the minimal supersymmetric standard model. Our analyses introduce the flavor mixings of left- and right-handed stau and smuon phenomenologically. It is found that if both the left- and right-handed sleptons have sizable flavor mixings, the correction to g_μ-2 from the lighter slepton can reach to 10^{-9} while the correction to the branching ratio of τ\to μγsatisfies the current experimental bound. On the other hand, when only the left-handed or right-handed sleptons have the large flavor mixing, the suitable magnitude of the correction to g_μ-2 is not realized owing to the experimental bound of τ\to μγ.

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arXiv2001

Mixing Matrix of Quarks Having Natural Twisting in E_6 Grand Unified Model

Mass matrix of quarks is studied in the Supersymmetric $E_6$ Grand Unified Theory (GUT). The fundamental representation $\textbf{27}$ in $E_6$ which corresponds to one generation contains two sets of $\textbf{5}^{*}$'s in SU(5), so that there are six flavors of lepton doublets and right-handed down-quark triplets. It is known that the twisting (interchange) among the $\textbf{5}^{*}$ representations may reproduce the observed quark and lepton mixing matrices. If $E_6$ is directly broken down to $SU(3)_C \times SU(2)_L \times U(1)_Y$ and two extra U(1)'s, the extra 3 sets of down-type quarks do not necessarily decouple from the quark mass matrix, under the appropriate choice of the U(1) flavor charges on the quark and Higgs fields. Then, by diagonalizing the 6 $\times$ 6 down-quark mass matrix, we find a certain set of parameters, in which the twisting between a pair of the right-handed down-quarks occurs naturally, reproducing the reasonable values for the $d$-, $s$- and $b$-quark masses, and for $V_{ud}$, $V_{us}$, $V_{cd}$, $V_{cs}$ and $V_{tb}$ of the CKM matrix elements. As a by-product, one vector-like down-quark appears at the experimentally accessible TeV scale.

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arXiv2001

Large effects on $\BsBs$ mixing by vector-like quarks

We calculate the contributions of the vector-like quark model to $\BsBs$ mixing, taking into account the constraints from the decay $B\to X_sγ$. In this model the neutral bosons mediate flavor-changing interactions at the tree level. However, $\BsBs$ mixing is dominated by contributions from the box diagrams with the top quark and the extra up-type quark. In sizable ranges of the model parameters, the mixing parameter $x_s$ is much different from the standard model prediction.

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arXiv2001

Muon g-2 and Minimal Supersymmetric Standard Model

The supersymmetric contribution to the muon g-2 is studied in light of the finalization of the LEP electroweak precision data. The recent precise measurement of the muon g-2 of E821 experiment is explained well by the relatively light chargino and sleptons. We find that such the MSSM parameter space is also favored from the electroweak precision data, in which the fit to the data is better than that of the SM (Δχ^2_{min} \sim -2), if the lighter chargino has a mixed higgsino-wino character (μ/M_2 \sim 1). The models with light gauginos (μ/M_2 > 10) or light higgsinos (μ/M_2 < 0.1) also show the better fit over the SM, but the improvement is marginal as compared to the case of the mixed higgsino-wino case (μ/M_2 \sim 1).

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arXiv2001

Supersymmetric contributions to muon g-2 and the electroweak precision measurements

In view of the recent measurement of the muon g-2 and finalization of the LEP electroweak data, we re-examine all the new particle contributions to those observables in the MSSM. The SM fits the latest electroweak data excellently with the observed top-quark mass and the Higgs-boson mass of around 100 GeV, and so does the MSSM in the decoupling limit. The MSSM gives a slightly better fit to the data than the SM (Δχ^2_{min} \sim -2) when relatively light left-handed sleptons of mass \sim 200 GeV and a light chargino of mixed higgsino-wino character (μ/M_2 \sim 1) with mass \sim 100 GeV co-exist. The improvement in the fit diminishes quickly for wino- or higgsino- dominant charginos, for heavier charginos, and for lighter sleptons. We find that the MSSM contributions to the muon g-2 is most efficient when the light chargino has a mixed character. If \tanβ< 10, the set of a light mixed chargino and light sleptons that is favored by the electroweak data is also favored by the g-2 data for μ>0. Models with light gauginos (μ/M_2 > 10) or light higgsinos (μ/M_2 < 0.1) give significant contributions to muon g-2 only for large \tanβ(\simgt 15).

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arXiv2000

Supersymmetry versus precision experiments revisited

We study constraints on the Minimal Supersymmetric Standard Model from electroweak experiments. We find that the light sfermions always make the fit worse than the Standard Model, while the light chargino generally make the fit slightly better through the oblique corrections. The best overall fit to the precision measurements are found when the mass of lighter chargino is about 100 GeV and the SU(2)_L doublet sfermions are all much heavier. We find the slight improvement of the fit over the SM, where the total χ^2 of the fit decreases by about one unit.

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arXiv2000

Muon g-2 and precision electroweak physics in the MSSM

The minimal supersymmetric extension of standard model (MSSM) is examined by analyzing its quantum effects on the precision electroweak measurements and the muon g-2. We examine carefully the effects of light charginos and neutralinos that are found to improve the fit to the electroweak data. We identify two distinct regions on the $(μ, M_2)$-plane that fit well to the electroweak data and give significant contribution to muon g-2.

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arXiv2000

Looking for Z' bosons in Supersymmetric E_6 Models through Electroweak Precision Data

We review constraints on additional Z' bosons predicted in supersymmetric (SUSY) E_6 models from electroweak experiments - Z-pole experiments, mW measurements and the low-energy neutral current (LENC) experiments. Four representative models - χ,ψ, η, νmodels - are studied in some detail. We find that the improved data of parity violation in cesium atom, which is 2.2-σaway from the Standard Model (SM) prediction, could be explained by the exchange of the heavy mass eigenstate Z_2 in the intermediate state. The improvement over the SM can be found in χ, η, νmodels, where the total χ^2 of the fit to the 26 data points decreases by about five units, owing to the better fit to the atomic parity violation. Impacts of the kinetic mixing between the U(1)_Y and U(1)' gauge bosons on the χ^2-analysis are studied. We find that the Z' model with (β_E, δ)=(-π/4,0.2), where β_E is the mixing angle between Z_χand Z_ψbosons and δdenotes the kinetic mixing, shows the most excellent fit to the data: the total χ^2 decreases by about seven units as compared to the SM. We introduce the effective mixing parameter ζ, a combination of the mass and the kinetic mixing parameters. The 95% CL lower mass bound of Z_2 can be shown as a function of ζ. A theoretical prediction on ζand the U(1)'gauge coupling g_E is studied for the χ,ψ,ηand νmodels by assuming the minimal particle content of the SUSY E_6 models.

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arXiv2000

Supersymmetry versus precision experiments revisited

We study constraints on the supersymmetric standard model from the updated electroweak precision measurements --- the Z-pole experiments and the $W$-boson mass measurements. The supersymmetric-particle contributions to the universal gauge-boson-propagator corrections are parametrized by the three oblique parameters Sz, Tz and mw. The oblique corrections, the Zqq and Zll vertex corrections, and the vertex and box corrections to the μ-decay width are separately studied in detail. We first study individual contribution from the four sectors of the model, the squarks, the sleptons, the supersymmetric fermions (charginos and neutralinos), and the supersymmetric Higgs bosons, to the universal oblique parameters, where the sum of individual contributions gives the total correction. We find that the light squarks or sleptons, whose masses just above the present direct search limits, always make the fit worse than that of the Standard Model (SM), whereas the light charginos and neutralinos generally make the fit slightly better. The contribution from the supersymmetric Higgs sector is found small. We then study the vertex/box corrections carefully when both the supersymmetric fermions (-inos) and the supersymmetric scalars (squarks and sleptons) are light, and find that no significant improvement over the SM fit is achieved. The best overall fit to the precision measurements are found when charginos of mass $\sim 100\gev$ with a dominant wino-component are present and the doublet squarks and sleptons are all much heavier. The improvement over the SM is marginal, however, where the total χ^2 of the fit to the 22 data points decreases by about one unit, due mainly to a slightly better fit to the $Z$-boson total width.

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arXiv1999

CP asymmetry for radiative B-meson decay in the supersymmetric standard model

We study a rate asymmetry in the inclusive decay $\Bsgamma$, assuming the supersymmetric standard model based on N=1 supergravity. A complex coefficient $A$ for scalar-trilinear couplings is the source of CP violation, which is contained in the mass-squared matrices for squarks. The model parameters are nontrivially constrained by experimental results for the branching ratio of the radiative decay and for the electric dipole moments of the neutron and the electron. The decay rate asymmetry is predicted to be much larger than that by the standard model in a wide region of the parameter space compatible with the experiments. Its magnitude can be maximally around 0.1, which will be well accessible at B factories.

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arXiv1998

Decay rate asymmetry in $B \to X_s γ$ as a signature of supersymmetry

In the supersymmetric standard model, the radiative $B$-meson decay $B \to X_s γ$ could receive a large contribution from a new source of flavor-changing neutral current, which also violates CP invariance. The CP-conjugate processes then may sizably differ in decay width. If a new CP-violating phase is not suppressed, this decay rate asymmetry can be significantly larger than the prediction by the standard model, possibly detectable at B factories. Such a large asymmetry may be implied by the measured branching ratio which is consistent with the standard model.

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arXiv1998

Constraints on leptophobic Z' models from electroweak experiments

We study the constraints from updated electroweak data on the three leptophobic Z' models, the $η$ model with an appropriate $U(1)'-U(1)_Y$ kinetic mixing, a Z' model motivated by the flipped SU(5) \times U(1) unification, and the phenomenological Z' model of Agashe, Graesser, and Hinchliffe. The $Z-Z'$ mixing effects are parametrized in terms of a positive contribution to the $T$ parameter, $T_{new}$, and the effective mass mixing parameter, $\barξ$. All the theoretical predictions for the $Z$ boson parameters, the $W$ boson mass and the observables in low-energy neutral current experiments are presented together with the standard model radiative corrections. The allowed region in the ($\barξ,T_{new}$) plane is shown for the three models. The 95% CL lower limit on the heavier mass eigenstate $Z_2$ is given as a function of the effective $Z-Z'$ mixing parameter $ζ$.

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arXiv1998

$Z'$ bosons in supersymmetric $E_6$ models confront electroweak data

We study constraints on additional $Z'$ bosons predicted in the supersymmetric (SUSY) $E_6$ models by using the updated results of electroweak experiments -- $Z$-pole experiments, $m_W$ measurements and low-energy neutral current (LENC) experiments. We find that the effects of $Z$-$Z'$ mixing are parametrized by (i) a tree-level contribution to the $T$-parameter, (ii) the effective $Z$-$Z'$ mass mixing angle $\xibar$. In addition, the effect of the direct exchange of the heavier mass eigenstate $Z_2$ in the LENC processes is parametrized by (iii) a contact term $\contact$. We give the theoretical predictions for the observables in the electroweak experiments together with the standard model radiative corrections. Constraints on $T_{\rm new}$ and $\xibar$ from the $Z$-pole and $m_W$ experiments and those on $\contact$ from the LENC experiments are separately shown. Impacts of the kinetic mixing between the ${\rm U(1)}_Y$ and ${\rm U(1)'}$ gauge bosons on the $χ^2$-analysis are studied. We show the 95% CL lower mass limit of $Z_2$ as a function of the effective $Z$-$Z'$ mixing parameter $ζ$, a combination of the mass and kinetic mixings. Theoretical prediction on $ζ$ and $g_E$ is found for the $χ, ψ, η$ and $ν$ models by assuming the minimal particle content of the SUSY $E_6$ models. In a certain region of the parameter space, the $Z_2$ boson mass in the detectable range of LHC is still allowed.

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arXiv1998

Impacts on searching for signatures of new physics from $K^+ \to π^+ ν\barν$ decay

Impacts on new physics search from $\bbbar, \kkbar$ mixings and the rare decay $\kplusdecay$ are discussed. We show that, in a certain class of new physics models, the extra contributions to those processes can be parametrized by its ratio to the standard model (SM) contribution with the common CKM factors. We introduce two ratios to measure the new physics contributions, $R_1$ for $\xd$ and $\ek$ parameters, and $R_2$ for $\kplusdecay$ decay. Then, the experimentally allowed region for the new physics contributions can be given in terms of $R_1, R_2$ and the CP violating phase $δ$ of the CKM matrix. We find constraints on $R_1$ and $\cosd$ by taking account of current experimental data and theoretical uncertainties on $\bbbar$ and $\kkbar$ mixings. We also study impacts of future improved measurements on ($R_1,R_2,\cosd$) basis. As typical examples of new physics models, we examine contributions to those processes in the minimal supersymmetric SM and the two Higgs doublet model.

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arXiv1998

Impacts on searching for signatures of new physics from $K^+ \to π^+ ν\barν$ decay

We study impacts on new physics search from the rare decay $K^+ \to π^+ ν\barν$. In a certain class of new physics models, the extra contributions to FCNC processes can be parametrized by its ratio to the standard model (SM) contribution with the common CKM factors. The ratio $R_1$ has been used in the analysis of $\xd$ and $ε_K$ parameters. In the above class of models, the $K^+ \to π^+ ν\barν$ decay amplitude can be parametrized by the ratio $R_2$. Then the experimentally allowed region for new physics contributions can be given in terms of $R_1,R_2$ and the CP violating phase $δ$ of the CKM matrix. Constraints on $R_1$ and $\cosd$ are obtained by taking account of current experimental data and theoretical uncertainties on $\bbbar$ and $\kkbar$ mixings. We study impacts of future improved measurements by using $(R_1, R_2, \cosd)$ basis. We discuss contributions in the minimal supersymmetric SM and the two Higgs doublet model as examples.

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arXiv1997

Constraints on four-Fermi contact interactions from low-energy electroweak experiments

We investigate the constraints on four-Fermi contact interactions from low-energy lepton-quark and lepton-lepton scattering experiments --- polarization asymmetries in electron (muon)-nucleon scattering experiments, cesium and thallium atom parity violation measurements, neutrino-nuclei and neutrino-electron scattering experiments. These constraints are then combined by assuming the lepton and quark universalities and SU(2)*U(1) gauge invariance of the contact interaction, which leave independent six lepton-quark and three pure-leptonic interactions.Impacts of these constraints on models with an additional Z-boson are briefly discussed. We also present updates of the low-energy constraints on the S and T parameters.

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arXiv1997

String unification scale and the hyper-charge Kac-Moody level in the non-supersymmetric standard model

The string theory predicts the unification of the gauge couplings and gravity. The minimal supersymmetric Standard Model, however, gives the unification scale $\sim 2\times 10^{16}$ GeV which is significantly smaller than the string scale $\sim 5\times 10^{17}$ GeV of the weak coupling heterotic string theory. We study the unification scale of the non-supersymmetric minimal Standard Model quantitatively at the two-loop level. We find that the unification scale should be at most $\sim 4\times 10^{16}$ GeV and the desired Kac-Moody level of the hyper-charge coupling should be $1.33 \lsim \ky \lsim 1.35$ in the absence of significant string threshold effects.

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