arXiv subjects

Kang Young Lee

Publications and source records attributed to Kang Young Lee.

At least 37 records · Page 2Linked to original sources

arXiv2014

Direct search for heavy gauge bosons at the LHC in the nonuniversal SU(2) model

We study the phenomenology of heavy gauge bosons at the LHC in a nonuniversal gauge interaction model with the separate electroweak SU(2) gauge group for the third generation. Considered are the Drell-Yan processes into the final states of dilepton, dijet, $τ^- τ^+$, and $t \bar{t}$ for $Z'$ boson and those of lepton-neutrino for $W'$ boson. We find that the present LHC data provides lower bounds on the masses of the heavy gauge bosons, $m_{Z'},~m_{W'} > 2$ TeV, more stringent than indirect bounds, when $\sin^2 ϕ> 0.15$ for the mixing angle of two SU(2) gauge groups.We also note that the study of heavy resonances into the third generation fermions may provide some valuable information in the future.

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arXiv2013

Using the Higgs boson to probe the littlest Higgs model with T-parity through $Z_H W_H$ production at the LHC

In the littlest Higgs model with T-parity, the production cross section of the T-odd heavy gauge boson pair $Z_H W_H$ is quite sizable at the LHC. In addition, both the $W_H$ and $Z_H$ bosons have almost exclusively one decay channel into $W A_H$ and $H A_H$ respectively, where the dark matter candidate $A_H$ yields a large missing energy signal. Upon the discovery of the Higgs boson at 125 GeV, we study the discovery sensitivity of the final state $ pp \to b\bar{b} l+\rlap{\,/}{E}_T$ to probe the model at the LHC. We find that the standard model backgrounds are manageable by applying suitable kinematic cuts. The LHC running at $\sqrt{s}=14$ TeV with a 100/fb total luminosity is sensitive to the model if the symmetry breaking scale $f$ is below about 850 GeV.

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arXiv2012

The form factors for hadronic tau decay using background field method

In the two body hadronic tau decays, such as tau->to K pi (eta)nu, vector mesons play important role. Belle and Babar measured hadronic invariant mass spectrum of tau -> K pi nu decay. To compare the spectrum with theoretical prediction, we develop the chiral Lagrangian with vector mesons in Kimura:2012nx. We compute the form factors of the hadronic tau decay taking account of the quantum corrections of Nambu Goldstone bosons. We also show how to renormalizethe divergence of the Feynman diagrams with arbitrary number of loops and determine the counterterms within one loop using background field method.In this report, we discuss the renormalization of Kimura:2012nx by considering the one loop Feynman diagrams.

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arXiv2012

Comprehensive study of two Higgs doublet model in light of the new boson with mass around 125 GeV

The recent discovery of a new boson of mass roughly 125 GeV has been reported by the ATLAS and CMS collaborations. Although its signals in various decay modes resemble those of the standard model (SM) Higgs boson, observed are the combinations of entangled information about the production, decay rates and total decay width of the new boson. In addition, some decay channels show non-negligible deviation from the SM expectation, such as the $2σ$ excess in the diphoton channel. In the four types (I, II, X and Y) of two Higgs doublet models, we perform the global $χ^2$ fit in three scenarios: (i) the new boson is the light CP-even Higgs boson $h^0$; (ii) it is the heavy CP-even Higgs boson $H^0$; (iii) the signals are from degenerate $h^0$ and the pseudoscalar $A^0$. Considering other phenomenological constraints such as flavor physics, electroweak precision data, and the LEP search for the Higgs boson, we find that the the first scenarios in Type II and Type Y models actually provide better or similarly good fit to the data than the SM. All the other models are excluded at 95% C.L..

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arXiv2012

Direct bound on the minimal Universal Extra Dimension model from the $t\bar{t}$ resonance search at the Tevatron

In the minimal Universal Extra Dimension (mUED) model, the second Kaluza-Klein (KK) gluon $g^{(2)}$ has loop-induced vertices with the standard model quarks, mediated by the first KK modes of the quark and the gluon/Higgs boson. With a top quark pair, this vertex is enhanced by the cooperation of the strong coupling of a gluon and the large Yukawa coupling of a top quark, leading to substantial branching ratio of $Br(g^{(2)} \to t \bar{t}) \approx 7%-8%$. As the $g^{(2)}$ coupling with two gluons appears via dimension-6 operator, $q\bar{q}\to g^{(2)}\to t\bar{t}$ is the golden mode for the mUED model. Hence the best channel is the $t\bar{t}$ resonance search in $p\bar{p}$ collisions. The recent Tevatron data at $\sqrt{s}=1.96$ TeV with an integrated luminosity of 8.7 fb$^{-1}$ are shown to give the first direct bound on the $g^{(2)}$ mass above 800 GeV. The implication and future prospect at the LHC are discussed also.

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arXiv2012

Probing axino LSP from diphotons events with large missing transverse energy

In a supersymmetry model with an axino as the lightest supersymmetric particle (LSP) and a Bino as the next LSP (NLSP), supersymmetry particles produced in pair at a high energy collider end up with including two Binos, followed by each Bino's decay into a photon and an axino. Final states are diphoton with large missing energy. We have comprehensively studied the implication of $γγ+$MET data from the ALEPH, CDF II, and recent ATLAS and CMS experiments. No excess over the standard model backgrounds can be explained in this model if the NLSP Bino decays outside the detector, which happens when the axino decay constant $f_a$ is large enough. The ALEPH and CDF II data put a very strong bound on $f_a$ for light Bino case with $m_{\tilde{B}} < 150$ GeV: the narrow hadronic axion window around $f_a \sim 10^6$ GeV is completely closed. In the case where the gluino mass is smaller than about 776 GeV, the recent ATLAS data limit $f_a > 10^5$ GeV for the Bino mass below 700 GeV. This is already stronger than the previous laboratory bounds.

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arXiv2012

CP violation of Extended Higgs sector and Its impact on D^0 -> mu^+ mu^- decay

We study the impact of the CP violation of the extra Higgs sector on $D^0$ decay. The CP even and CP odd neutral Higgs mixing of the two Higgs doublet model is studied and we show how the CP violating effect of the mixing may lead to the longitudinal muon polarization asymmetry of $D^0 \to μ^+ μ^-$. The asymmetry of the short-distance contribution is sensitive to the CP violating phase of the extended Higgs sector.

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arXiv2011

Discrimination of the light CP-odd scalars between in the NMSSM and in the SLHM

The presence of the light CP-odd scalar boson predicted in the next-to-minimal supersymmetric model (NMSSM) and the simplest little Higgs model (SLHM) dramatically changes the phenomenology of the Higgs sector. We suggest a practical strategy to discriminate the underlying model of the CP-odd scalar boson produced in the decay of the standard model-like Higgs boson. We define the decay rate of "the non $b$-tagged jet pair" with which we compute the ratio of decay rates into lepton and jets. They show much different behaviors between the NMSSM and the SLHM.

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arXiv2011

Implications on the mUED model from the early LHC data on $\ell+\met$ signal

Recently the ATLAS and CMS collaborations reported their search for a new heavy gauge boson $W'$ with one lepton plus missing transverse momentum. We find that $W^{(2)}$, the second Kaluza-Kelin (KK) state of the $W$ boson in the minimal Universal Extra Dimension (mUED) model, can be a good candidate for this signal, as its branching ratio into $\ellν$ is sizable. Moreover, nearly degenerate KK mass spectra in the mUED model yield generically very soft SM particle accompanying $W^{(2)}$ from the subsequent decays of the second KK quarks and gluons. In a hadron collider, this indirect $W^{(2)}$ production is difficult to distinguish from the $W^{(2)}$ single production. The involved strong interactions make it more important than the single production. The early LHC data on $\ell+\met$ signal for 1.1 fb$^{-1}$ integrated luminosity is shown insufficient to limit our model even with significant indirect production of $W^{(2)}$. However, the results show that the present LHC 5.6 fb$^{-1}$ data can cover most of the reasonable parameter space of the mUED model.

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arXiv2011

CP violating dimuon charge asymmetry in general left-right models

The recently measured charge asymmetry of like-sign dimuon events by the D0 collaboration at Tevatron shows the 3.9 σ deviation from the standard model prediction. In order to solve this mismatch, we investigate the right-handed current contributions to $B_s-\bar{B}_s$ and $B_d-\bar{B}_d$ mixings which are the major source of the like-sign dimuon events in $b \bar{b}$ production in general left-right models without imposing manifest or pseudo-manifest left-right symmetry. We find the allowed region of new physics parameters satisfying the current experimental data.

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arXiv2011

A like-sign dimuon charge asymmetry at Tevatron induced by the anomalous top quark couplings

We show that the recently measured 3.2 $σ$ deviations of the charge asymmetry of like-sign dimuon events from the standard model prediction by the D0 collaboration at Tevatron can be explained by introducing the anomalous right-handed top quark couplings. Combined analysis with the $\bs$ and $\bd$ mixings and $B \to X_s γ$ decays has been performed. We discuss how to discriminate the effects on the dimuon charge asymmetry of the anomalous $\tsW$ and $\tbW$ couplings.

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arXiv2011

The second Kaluza-Klein neutral Higgs bosons in the minimal Universal Extra Dimension model

Loop-induced decay of a neutral Higgs boson into a pair of gluons or photons has great implications for the Higgs discovery at the LHC. If the Higgs boson is heavy with mass above $\sim 500\gev$, however, these radiative branching ratios are very suppressed in the standard model (SM), as the new decay channels are kinematically open. We note that these radiative decays can be sizable for the heavy CP-odd second Kaluza-Klein (KK) mode of the Higgs boson, $χ^\tw$, in the minimal universal extra dimension model: highly degenerate mass spectrum of the theory prohibits kinematically the dominant KK-number-conserving decays into the first KK modes of the $W$, $Z$ and top quark. We find that the CP-even decay of $h^{(2)} \to g g$ is absent at one-loop level since $h^{(2)}$ couples with different mass eigenstates of $t_{1,2}^\on$ while a gluon does with the same mass eigenstates. The $h^\tw$ production at the LHC is very suppressed. On the contrary, the process $ gg \to χ^\tw \to γγ$ in an optimal scenario can be observed with manageable SM backgrounds at the LHC.

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arXiv2011

Early LHC bound on W' boson in the nonuniversal gauge interaction model

We study the phenomenology of the heavy charged gauge boson and obtain the lower bounds on its mass with the early LHC data at 7 TeV center-of-mass energy in the nonuniversal gauge interaction model, in which the electroweak SU(2) gauge group depends upon the fermion family. We found that the direct bound on the mass of the $W'$ boson is compatible to the indirect bound with only the early data of the LHC.

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arXiv2011

The CDF dijet excess and Z'_{cs} coupled to the second generation quarks

Recently the CDF collaboration has reported the excess in the dijet invariant-mass distribution of the Wjj events, corresponding to a significance of 3.2 standard deviations. Considering the lack of similar excesses in the $γjj$ and $Z jj$ events yet, we propose a new Z' model: Z' couples only to the second generation quarks. Single production of $\zsc$ as well as associated production with $W,γ, Z$ are mainly from the sea quarks. Only $W Z'$ production has additional contribution from one valence quark and one sea quark, which is allowed by CKM mixing. We found that if the new gauge coupling is large enough, marginally permitted by perturbativity, this new model can explain the observed CDF $Wjj$ anomaly as well as the lack of $\gm jj$ and $Z jj$ anomalies. Vanishing coupling of Z'-b-b protects this model from the constraint of p pbar ->WH -> lνb \bar{b}.

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arXiv2010

Lepton flavour violation in a nonuniversal gauge interaction model

The flavour-changing neutral currents (FCNC) are derived at tree level if the electroweak gauge group depends on the fermion family, which are absent in the standard model. We study the lepton flavour violation (LFV) through the FCNC interactions in a nonuniversal gauge interaction model where the third generation fermions are subjected to the separate SU(2)$_L$ gauge group.

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arXiv2009

$SU(4)_L \times U(1)_X$ models with little Higgs

We discuss the aspects of the fermions and gauge bosons in $SU(4)_L \times U(1)_X$ models with little Higgs. We introduce a set of fermions which ensures the cancellation of gauge anomaly, and explicitly show the cancellation of one-loop quadratic divergence to the Higgs mass from all fermion multiplets and all gauge bosons. We present the interactions of the standard model fermions with the physical gauge bosons. We also discuss some phenomenological implications of the model based on recent experimental results.

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arXiv2009

CP violation in tau decays

CP violation of the charged lepton sector has not been found yet. Tau is a unique charged lepton which can decay into hadrons. In this talk, we argue what kind of new physics can be studied with CP violation of tau lepton decays. We also disucuss how to handle hadronic final state interactions for a prediction of the direct CP violation.

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arXiv2008

$B_d-\bar{B}_d$ mixing vs. $B_s-\bar{B}_s$ mixing with the anomalous $Wtb$ couplings

We explore the effects of the anomalous $tbW$ couplings on the $\bd$ mixing and recently measured $\bs$ mixing. The combined analysis of mixings via box diagrams with penguin decays provides strong constraints on the anomalous top quark couplings. We find the bound from the $\bd$ mixing data is stronger than that from the $\bs$ mixing.

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