arXiv subjects

S. Y. Choi

Publications and source records attributed to S. Y. Choi.

At least 91 records · Page 5Linked to original sources

arXiv1999

CP Phases in Supersymmetric Tri-lepton Signals at the Tevatron

We have analyzed the supersymmetric tri-lepton signals for sparticle searches at the Tevatron in the minimal supersymmetric standard model with general CP phases without generational mixing. The CP phases may affect very strongly the chargino and neutralino mass spectrums and $σ(p\bar{p}\to\tildeχ^-_1\tildeχ^0_2)$ as well as ${\cal B}(\tildeχ^-_1\to\tildeχ^0_1\ell^-ν)$ and ${\cal B}(\tildeχ^0_2\to\tildeχ^0_1\ell^+\ell^-)$. Even under the stringent constraints from the electron electric dipole moment the CP phases can lead to a minimum of the tri-lepton event rate for their non-trivial values.

hep-ph↗

arXiv1999

Neutrino Oscillations and R-parity Violating Collider Signals

R-parity and L violation in the MSSM would be the origin of the neutrino oscillation observed in Super-Kamiokande. A distinctive feature of this framework is that it can be tested in colliders by observing decay products of the destabilized LSP. We examine all the possible decay processes of the neutralino LSP assuming the bilinear contribution to neutrino masses dominates over the trilinear one which gives rise to the solar neutrino mass. We find that it is possible to probe neutrino oscillations through colliders in most of the R-parity conserving MSSM parameter space.

hep-ph↗

arXiv1999

Single-photon events in e^+ e^- collisions

We provide a detailed investigation of single-photon production processes in $e^+e^-$ collisions with missing momenta carried by neutrinos or neutralinos. The transition amplitudes for both processes can be organized into a generic simplified, factorized form; each neutral V$\pm$A vector current of missing energy carriers is factorized out and all the characteristics of the reaction is solely included in the electron vector current. Firstly, we apply the generic form to give a unified description of a single-photon production with a Dirac-type or Majorana-type neutrino-pair and to confirm their identical characteristics as suggested by the so-called Practical Dirac-Majorana Confusion Theorem. Secondly, we show that the generic amplitude form is maintained with the anomalous P- and C-invariant WW$γ$ couplings in the neutrino-associated process and it enables us to easily understand large contributions of the anomalous WW$γ$ couplings at higher energies and, in particular, at the points away from the Z-resonance peak. Finally, the neutralino-associated process, which receives modifications in both the left-handed and right-handed electron currents due to the exchanges of the left-handed and right-handed selectrons, can be differentiated from the neutrino-associated ones through the left-right asymmetries and/or the circular polarization of the outgoing photon.

hep-ph↗

arXiv1998

Determining SUSY Parameters in Chargino Pair-Production in $e^+e^-$ Collisions

In most supersymmetric theories, charginos $\tildeχ^\pm_{1,2}$, mixtures of charged color-neutral gauginos and higgsinos, belong to the class of the lightest supersymmetric particles. They are easy to observe at $e^+e^-$ colliders. By measuring the total cross sections and the left-right asymmetries with polarized electron beams in $e^+e^-\to\tildeχ_i^-\tildeχ_j^+ [i,j=1,2]$, the chargino masses and the gaugino-higgsino mixing angles can be determined. From these observables the fundamental SUSY parameters can be derived: the SU(2) gaugino mass $M_2$, the modulus $|μ|$ and $\cos Φ_μ$ of the higgsino mass parameter, and $\tanβ= v_2/v_1$, the ratio of the vacuum expectation values of the two neutral Higgs doublet fields. The solutions are unique; the CP-violating phase $Φ_μ$ can be determined uniquely by analyzing effects due to the normal polarization of the charginos.

hep-ph↗

arXiv1998

Phenomenological Impacts of the CP-odd Rephase-Invariant Phase of the Chargino Mass Matrix in the Production of Light Chargino-Pair in $e^+e^-$ Collisions

One CP--odd rephase-invariant phase appears in the chargino mass matrix in the minimal Supersymmetric Standard Model. We investigate in detail the phenomenological impacts of the CP-odd complex phase in the production of light charginos in $e^+e^-$ annihilation. The values of the chargino masses and the mixing angles, determining the size of the wino and higgsino components in the chargino wave functions, are so sensitive to the CP-odd phase that the constraints on the supersymmetric parameters based on the conventional assumptions for the parameters are recommended to be re-evaluated including the CP-odd phase.

hep-ph↗

arXiv1998

Chargino Pair Production in $e^+e^-$ Collisions

Charginos $χ_1^\pm$ are expected to be the lightest observable supersymmetric particles in many supersymmetric models. We present a procedure which will allow to determine the chargino mixing angles and, subsequently, the fundamental SUSY parameters $M_2, μ$ and $\tanβ$ by measurements of the total cross section and the spin correlations in $e^+e^-$ annihilation to $χ_1^+ χ_1^-$ chargino pairs.

hep-ph↗

arXiv1998

CP Violation in the Cabbibo-suppressed Decay Tau->K Pi Nu_tau with Polarized Tau Leptons

CP violation from physics beyond the Standard Model (SM) is investigated in the Cabbibo-suppressed decay $τ\to Kπν_τ$ with polarized $τ$ leptons, to which both the ${\rm J}^{P}=1^-$ resonance $K^*$ and the $ J^{P}=0^+$ resonance $K^*_0$ contribute. In addition to the CP-odd rate asymmetry, $τ$ polarization enables us to construct three additional CP-odd polarization asymmetries that can be enhanced due to the interference between the $K^*$ and $K^*_0$, and whose magnitudes depend crucially on the $K^*_0$ decay constant, $f_{K^*_0}$. Taking a QCD sum rule estimate of $f_{K^*_0}=45$ MeV and the present experimental constraints on the CP-odd parameters into account, we estimate quantitatively the maximally-allowed values for the CP-odd rate and polarization asymmetries in the multi-Higgs-doublet (MHD) model and the scalar-leptoquark (SLQ) models consistent with the SM gauge symmetry where neutrinos are massless and left-handed as in the SM. We find that the CP-odd rate and polarization asymmetries are of a similar size for highly-polarized $τ$ leptons and, for their maximally-allowed values, CP violation in the MHD model and two SLQ models may be detected with about $10^6$ and $10^7$ $τ$'s at the $1σ$ level.

hep-ph↗

arXiv1998

Chargino Pair Production at CERN LEP II

Charginos are expected to be the lightest observable supersymmetric particles in many of the supersymmetric models. In the scenario that lighter charginios are pair-produced at CERN LEP II, we present a straightforward procedure for determining the SUSY parameters, $\tanβ$, $M_2$, $μ$, and the electron sneutrino mass $m_{\tildeν}$ up to a four-fold discrete ambiguity, although a large number of unknown SUSY parameters are involved in chargino decays.

hep-ph↗

arXiv1998

T-Odd Gluon-Top-Quark Couplings at LHC

The T-odd top-quark chromoelectric dipole moment, $d_t$, is probed through top-quark-pair production via gluon fusion at the CERN LHC with the possibility of having polarized proton beams in account. At 1-$σ$ level, the typical CP-odd lepton energy and tensor correlations enable us to measure $Re(d_t)$ and $Im(d_t)$ up to the order of 10$^{-18}$ ($g_scm$) in the unpolarized case and the initial CP-odd gluon spin correlation allows us to probe $Im(d_t)$ up to the order of 10$^{-20}$ ($g_scm$) for $\sqrt{s}=14$ TeV and the integrated luminosity $L_{pp}=10 fb^{-1}$.

hep-ph↗

arXiv1997

CP Violation at Electron-Positron Colliders

A detailed, model-independent study of CP violation at present and future electron-positron colliders is reported. Firstly, we investigate CP violation effects in $W$ boson and $t$ quark pair production in $e^+e^-$ annihilation and in two-photon fusion at a next $e^+e^-$ linear collider, where Compton-backscattered laser light off the electrons or positrons are employed as a powerful polarized photon source. Secondly, noting that there do not exist any direct measurements for the tau-lepton electric dipole moment (EDM), we address the importance of performing its direct measurements at $e^+e^-$ collisions off the Z-boson pole at TRISTAN, LEPII and CLEOII. We present a rough comparison of the potential of these experiments in the $τ$ EDM measurements. Finally, we report on our recent works for probing CP violation in the semileptonic decays of the tau lepton, which involve two different intermediate resonances with large ratios of widths to masses, and which can be most efficiently identified at the planned $B$ and proposed $τ$-charm factories. All the CP-violation phenomena in the processes under consideration, if discovered, imply new CP-violation mechanisms completely different from the Standard Model CP violation mechanism through the complex phase of the Cabibbo-Kobayashi-Maskawa matrix.

hep-ph↗

arXiv1997

T-odd Gluon-Top-Quark Effective Couplings at the CERN Large Hadron Collider

The T-odd top-quark chromoelectric dipole moment (tCEDM) is probed through top-quark-pair production via gluon fusion at the CERN Large Hadron Collider (LHC) by considering the possibility of having polarized protons. The complete analytic expressions for the tree-level helicity amplitudes of gg-> ttbar is also presented. For the derived analytic results we determine the 1-sigma statistical sensitivities to the tCEDM form factor for (i) typical CP-odd observables composed of lepton and anti-lepton momenta from t and tbar semileptonic decays for unpolarized protons, and (ii) a CP-odd event asymmetry for polarized protons by using the so-called Berger-Qiu (BQ) parametrization of polarized gluon distribution functions. We find that at the CERN LHC, the CP-odd energy and angular correlations can put a limit of 10^{-18} to 10^{-17} g_scm on the real and imaginary parts of the tCEDM, while the simple CP-odd event asymmetry with polarized protons could put a very strong limit of 10^{-20} g_scm on the imaginary part of the tCEDM.

hep-ph↗

arXiv1997

CP Violation in the Top-Quark Pair Production at a Next Linear Collider

We provide a detailed, model-independent, study for CP violation effects due to the T-odd top-quark electric dipole moment (EDM) and weak dipole moment (WDM) in the top-quark pair production via $e^+e^-$ and two-photon annihilation at a next $e^+e^-$ linear collider (NLC). There are two methods in detecting CP violation effects in these processes. One method makes use of measurements of various spin correlations in the final decay products of the produced top-quark pair, while the other is to measure various CP-odd polarization asymmetry effects of the initial states. In the $e^+e^-$ case only the first method can be used, and in the $γγ$ case both methods can be employed. We provide a complete classification of angular correlations of the $t$ and $\bar{t}$ decay products under CP and CP\tilde{T} which greatly faciliate CP tests in the $e^+e^-$ mode. Concentrating on the second method with the Compton back-scattered high-energetic laser light off the electron or positron beam in the two-photon mode, we construct two CP-odd and CP\tilde{T}-even initial polarization configurations and apply them to investigating CP-violating effects due to the top-quark EDM. With a typical set of experimental parameters at the NLC, we compare the 1-σsensitivities to the top-quark EDM and WDM in the $e^+e^-$ mode and the two-photon mode. Some model expectation values of the T-odd parameters are compared with the results.

hep-ph↗

arXiv1996

Probing CP Violation in $γγ\to W^+W^-$ with Polarized Photon Beams

We demonstrate in a general framework that polarized photons by backscattered laser beams of adjustable frequencies at a TeV linear $e^+e^-$ collider provide us with a very efficient mechanism to probe CP violation in two-photon collisions. CP violation in the process $γγ\rightarrow W^+W^-$ is investigated in detail with linearly polarized photon beams. There are two useful CP-odd asymmetries that do not require detailed information on $W$ decay products. The sensitivity to the CP-odd form factors are studied quantitatively by assuming a perfect $e$-$γ$ conversion and the 20 fb$^{-1}$ $e^+e^-$ integrated luminosity at the $e^+e^-$ c.m. energies $\sqrt{s}=0.5$ and $1.0$ TeV. The sensitivity is so high that such experiments will allow us to probe new CP violation effects beyond the limits from some specific models with reasonable physics assumptions. We find that a counting experiment of $W^+W^-$ events in the two-photon mode with adjustable laser frequencies can have much stronger sensitivity to the CP-odd $γ(γ) WW$ form factors than can a $W^+W^-$ decay-correlation experiment with a perfect detector achieve in the $e^+e^-$ mode. PACS number(s): 14.70.Bh, 14.70.Hp, 13.88.+e

hep-ph↗

arXiv1995

Form Factors of Semileptonic $M_{l3}$ Decays

We study in detail the kinematics of semileptonic $M_{l3}$ ($M \rightarrow M^\prime l ν$) decay, considering only the new models preserving all the gauge symmetries of the Standard Model (SM), and assuming that neutrinos are massless and left-handed as in the SM. And we present a brief review of the recent theoretical progress on model independent constraints on the form factors of $M_{l3}$ decays using dispersion relations. Finally we review a new parton model approach for semileptonic $B \rightarrow D(D^*) l ν$ decays, by extending the inclusive parton model and by combining with the results of the heavy quark effective theory. We also obtain the slope of the Isgur-Wise function.

hep-ph↗

arXiv1995

Probing the Top-Quark Electric Dipole Moment at a Photon Linear Collider

We probe the top-quark electric dipole moment (EDM) in top-quark pair production via photon-photon fusion at a photon linear collider. We show how linearly-polarized photon beams can be used to extract information on the top-quark EDM without the use of complicated angular correlations of top-quark decay products. If the luminosity of the laser back-scattered photon-photon collisions is comparable to that of the $e^+e^-$ collisions, then the measurement of the top-quark EDM obtained by counting top-quark-production events in photon fusion can be as accurate as the measurement obtained by studying the $t\bar{t}$ decay correlations in $e^+e^-$ collisions using a perfect detector.

hep-ph↗

arXiv1994

Probing the Weak Boson Sector in $γe\rightarrow Ze$

We study possible deviations from the standard model in the reaction $γe\rightarrow Ze$ at a 500 GeV $e^+e^-$ collider. As a photon source we use a laser backscattered photon beam. We investigate the most general $γZγ$ and $γZZ$ vertices including operators up to energy-dimension-six which are Lorentz invariant. These vertices require four extra parameters; two are CP-conserving, $h^γ_1$ and $h^Z_1$, and two are CP-violating, $h^γ_2$ and $h^Z_2$. We present analytical expressions of the helicity amplitudes for the process $γe\rightarrow Ze$ for arbitrary values of anomalous couplings. Assuming Standard Model values are actually measured we present the allowed region in the ($h^γ_1,h^Z_1$) plane at the 90\% confidence level. We then show how the angular correlation of the $Z$ decay products can be used to extract detailed information on the anomalous (especially CP-violating) $γZγ$ and $γZZ$ couplings.

hep-ph↗

arXiv1994

CP Violation in $τ\rightarrow 3πν_τ$

We consider CP violating effects in the decays $τ\rightarrow (3π)ν_τ$ where both the ${\rm J}^{\rm P}=1^+$ resonance, $a_1$, and ${\rm J}^{\rm P}=0^-$ resonance, $π^\prime$, can contribute. The interference between the $a_1$ and $π^\prime$ resonances can lead to enhanced CP-violating asymmetries whose magnitudes depend crucially on the $π^\prime$ decay constant, $f_{π^\prime}$. We make an estimate of $f_{π^\prime}$ with a simplified chiral Lagrangian coupled to a massive pseudoscalar field, and we compare the estimates from the non-relativistic quark model and from the QCD sum rule with the estimate from the `mock' meson model. We then estimate quantitatively the size of CP-violating effects in a multi-Higgs-doublet model and scalar-leptoquark models. We find that, while CP-violating effects in the scalar-leptoquark models may require more than $10^{10}$ $τ$ leptons, CP-violating effects from the multi-Higgs-doublet model can be seen at the $2σ$ level with about $10^7$ $τ$ leptons using the chiral Lagrangian estimate of $f_{π^\prime}=(1\sim 5)\times 10^{-3}$ GeV.

hep-ph↗

arXiv1994

Factorization and polarization in linearized gravity

We investigate all the four-body graviton interaction processes: $gX\rightarrow γX$, $gX\rightarrow gX$, and $gg\rightarrow gg$ with $X$ as an elementary particle of spin less than two in the context of linearized gravity except the spin-3/2 case. We show explicitly that gravitational gauge invariance and Lorentz invariance cause every four-body graviton scattering amplitude to be factorized. We explore the implications of this factorization property by investigating polarization effects through the covariant density matrix formalism in each four-body graviton scattering process.

hep-th↗