arXiv subjects

Yeong Gyun Kim

Publications and source records attributed to Yeong Gyun Kim.

53 records · Page 3Linked to original sources

arXiv2000

Scrutinizing LSP Dark Matter at the LHC

We show that LHC experiments might well be able to determine all the parameters required for a prediction of the present density of thermal LSP relics from the Big Bang era. If the LSP is an almost pure bino we usually only need to determine its mass and the mass of the SU(2) singlet sleptons. This information can be obtained by reconstructing the cascade $\tilde{q}_L \to \tildeχ_2^0 q \to \tilde{\ell}_R \ell q \to \tildeχ_1^0 \ell^+ \ell^- q$. The only requirement is that $m_{\tilde{\ell}_R} < m_{\tildeχ_2^0}$, which is true for most of the cosmologically interesting parameter space. If the LSP has a significant higgsino component, its predicted thermal relic density is smaller than for an equal--mass bino. We show that in this case squark decays also produce significant numbers of $\tildeχ_4^0$ and $\tildeχ_2^\pm$. Reconstructing the corresponding decay cascades then allows to determine the higgsino component of the LSP.

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arXiv2000

Hadronic Invariant Mass Spectrum in B -> X_u l nu Decay with Lepton Energy Cut

We discuss the implications of charged lepton energy cut to the hadronic invariant mass spectrum in charmless semileptonic B decays. Charged-lepton energy cut is inevitable in order to remove secondary leptonic events such as b -> c, tau -> l, and to identify the chaged leptons at detectors experimentally. We consider three possible lepton energy cuts, E_l^{cuts} = 0.6,1.5,2.3 GeV, and found that with the most probable cuts E_l^{cut} = 1.5 GeV and M_X^{max} = 1.5~(1.86) GeV, 45 ~ 60 % ~(58 ~ 67 %) of decay events survive. Therefore, B -> X_u l nu decay events can be efficiently distinguished from B -> X_c l nu decay events. We also discuss the possible model dependence on the results.

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arXiv2000

Azimuthal Angle Distribution in $B \to K^* (\to K π) \ell^+ \ell^- $ at low invariant $m_{\ell^+ \ell^-}$ Region

We present the angular distribution of the rare B decay, $B \to K^* (\to K π) \ell^+ \ell^-$. By studying the azimuthal angle distribution in the low invariant mass region of dileptons, we can probe new physics effects efficiently. In particular, this distribution is found to be quite sensitive to the ratio of the contributions from two independent magnetic moment operators, which also contribute to $B \to K^* γ$. Therefore, our method can be very useful when new physics is introduced without changing the total decay rate of the $b \to s γ$. The angular distributions are compared with the predictions of the standard model, and are shown for the cases when the afore-mentioned ratio is different from the standard model prediction.

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arXiv1999

$b \to s γ$ decays in the Left-Right Symmetric Model

We consider $b \to s γ$ decays in the Left-Right Symmetric Model. Values of observables sensitive to chiral structure such as the $Λ$ polarization in the $Λ_b \to Λγ$ decays and the mixing-induced CP asymmetries in the $B_{d,s} \to M^0 γ$ decays can deviate in the LRSM significantly from the SM values. The combined analysis of $P_Λ$ and $A_{CP}$ as well as ${\cal BR}(b \to s γ)$ can be used to determine the model parameters.

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arXiv1999

Constraints on the R-parity Violating Couplings from $B^\pm \to l^\pm ν$ Decays

We derive the upper bounds on certain products of R-parity- and lepton-flavor-violating couplings from $B^\pm \to l^\pm ν$ decays. These modes of B-meson decays can constrain the product combinations of the couplings with one or more heavy generation indices which are comparable with or stronger than the present bounds. And we investigate the possible effects of R-parity violating interactions on $B_c \to l ν$ decays. These decay modes can be largely affected by R-parity violation.

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arXiv1999

New Physics Effects in $B \to K^(*) νν$ Decays

We present a model-independent analysis of rare B decays, $B \to K^{(*)} νν$. The effect of possible new physics is written in terms of dimension-6 four-fermi interactions. The lepton number violating scalar- and tensor-type interactions are included, and they induce $B \to K^{(*)} νν({\bar ν} {\bar ν})$ decays. We show systematically how the branching ratios and missing mass-squared spectrum depend on the coefficients of the four-fermi interactions.

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arXiv1999

Possible New Physics Signals in b -> s gamma and b -> s l^+ l^-

We consider possible new physics contributions to b\to s l^+ l^- assuming the new physics modifies (chromo)magnetic and electric form factors in b\to s γ^* and b\to s g with the same chirality structure as in the standard model. Parametrizing the new physics effects on b\to s γ^* and b\to s g in terms of four real parameters, one finds that there are enough region of parameter space in which the measured branching ratio for B \to X_s γcan be accomodated, and the predicted CP violation effect could be as large as \sim 30%. Moreover, the branching ratio and the forward-backward asymmetry of a lepton in B \to X_s l^+ l^- and the tau polarization asymmetry in B \to X_s τ^+ τ^- can be deviated from the SM predictions by a factor of \sim 2, which can be accessible at B factories. We also discuss these observables in a specific class of supersymmetric models with gluino-mediated flavor changing neutral current (FCNC).

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arXiv1998

Complete analysis of photino-mediated lepton flavor violaitons in generalized supersymmetric models

We consider lepton flavor violations (LFV) mediated by photino as a result of the nondiagonal slepton mass matrices in general supersymmetric models. Using the experimental upper bounds on l \to l' + gamma and mu^- + Ti \to e^- + Ti as constraints on the flavor changing slepton mass insertions, we predict the possible ranges of the upper limits on the branching ratios of other LFV processes such as l \to 3l', tau \to l_{i\neq 3} + pi^0 (or eta, rho^0, phi), Z^0 \to l_i bar{l}_{j\neq i}, and muonium \to antimuonium conversion. Most of these decays are expected to occur with small branching ratios far below the current or future experimental search limits. We also derive constraints on the flavor conserving mass insertions from the anomalous magnetic moments of leptons.

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arXiv1998

B decays into two light pseudoscalar mesons and possible effects of enhanced b --> s g

We consider the branching ratios and CP asymmetry in the B meson decays into two pseudoscalar mesons in the generalized factorization approximation. We also investigate the possible effects of the enhanced chromomagnetic interaction b --> s g on these exclusive B meson decays that was suggested as a possible solution to the semileptonic branching ratio of B mesons, the missing charm puzzle and the large B --> η^{'} + X_s. We finds that such enhanced b --> s g interaction degrades the agreement between the data and the model predictions for B meson decays into two light mesons in the generalized factorization approximation.

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arXiv1997

B -> Xs l_i^+ l_j^+ Decays with R-parity Violation

We derive the upper bounds on certain products of R-parity- and lepton-flavor-violating couplings from $B \ra X_s {l_i}^+ {l_j}^-$ decays. These modes of B-meson decays can constrain the product combinations of the couplings with one or more heavy generation indices which are comparable with or stronger than the present bounds. From the studies of the invariant dilepton mass spectrum and the forward backward asymmetry of the emitted leptons we note the possibility of detecting R-parity-violating signals even when the total decay rate due to R-parity violating couplings is comparable with that in the standard model and discriminating two types of R-parity-violating signals. The general expectation of the enhancement of the forward backward asymmetry of the emitted leptons in the minimal supersymmetric standard model with R-parity may be corrupted by R-parity violation.

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arXiv1997

R-parity Violation and Semileptonic Decays of B-meson

We investigate the effects of R-parity violation on the semileptonic decays of B-meson in the minimal supersymmetric standard model with explicit R-parity violation and discuss its physical implications. We find that the semileptonic decays of B-meson can be largely affected by the R-parity violation.

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arXiv1996

Form Factors for Exclusive Semileptonic $B$--Decays

We investigate the form factors for exclusive semileptonic decays of $B$-meson to $D,~D^*$, based on the parton picture and helped by the results of the HQET. We obtain the numerical results for the slope of the Isgur-Wise function, which is consistent with the experimental results, and we extracte the dependences of hadronic form factors on $q^2$ by varying input heavy quark fragmentation function without the nearest pole dominance ansätze.

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arXiv1995

Strategy for detecting b polarization effects

We suggest a strategy for detecting $b$ polarization effects in $e^+e^-$ annihilation on $Z$ resonance. Using two types of inclusive leptonic samples with different ${Λ_b}$ fractions in $Z \ra b\bar{b}$ events, the $b$ polarization effects may be detected without large uncertainties of $b$ fragmentation and ${Λ_b}$ decay model.

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arXiv1995

$α_s(M_Z^2)$ and $R_b$ discrepancy with nonuniversal interactions

We implement global fits to LEP data with the \non interactions. Consistent $R_b$ with experimental value and consistent $\as(M_Z^2)$ with that from low energy experiments are obtained. We also find that the $\xx$ is better than the Standard Model. And we argue that other kinds of new physics are needed to explain the difference between the values of $\as(M_Z^2)$ from low energy experiments and from the 3-jet ratio.

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arXiv1995

Form Factors for Exclusive Semileptonic $B$--Decays

We developed the new parton model approach for exclusive semileptonic decays of $B$-meson to $D,~D^*$ by extending the inclusive parton model, and by combining with the results of the HQET, motivated by Drell-Yan process. Without the nearest pole dominance ansätze, we {\bf derived} the dependences of hadronic form factors on $q^2$. We also calculated numerically the slope of the Isgur-Wise function, which is consistent with the experimental results.

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arXiv1995

$B \rightarrow τνX$ decays in a two Higgs doublet model

We calculate the decay rate for inclusive $B \rightarrow τνX$ decays in a two Higgs doublet model using heavy quark expansion and operator product expansion. Combined with the recent measurement of Br($B \ra τνX$), we find a limit $\tan β< 0.58 m_H/\mbox{GeV}$ at 90\%\ C.L..

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arXiv1994

Tests of the Parton Model for Inclusive Semileptonic B Decays with the Heavy Quark Effective Theory

In order to study the end-point spectrum of inclusive semileptonic $b \rightarrow u$ decays, we investigate the parton model motivated by deep inelastic scattering process. The reliability of the model is carefully examined by comparing the decay rates and the lepton spectra for $b \rightarrow c$ decay with those of the heavy quark effective theory. We also compute the decay rates of the inclusive semileptonic decay into tau lepton in terms of the parton model, which give another test of the model independently. We conclude that one can make the parton model consistent with the heavy quark effective theory, and it would be phenomenologically useful in extracting $|V_{ub}|$.

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